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Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges Keywords: IA, DigiOne-IAP, DOIAP, DORPIA, upgrade, ASCII, Rockwell, Modbus Abstract: This document provides tutorial information on grounding, isolation, surge management and shielding, as well as recommendations related to specific Digi products, especially the DigiOne-IA, DigiOne-IAP, and DigiOne-SP. 1 Introduction “Industrial grounding is a little-understood topic. There are really only 4 experts in the whole world who really understand it – and those other 3 guys don’t know what they are talking about!” Digi provides some of the world’s best Terminal Server (TS) and Device Server (DS) products. These function as - Speaker’s opening joke Ethernet to serial converters. with some truth to it But even the ultimate TS/DS design won’t perform well or survive long, if improperly installed. Issues of grounding, isolation, surge management, and shielding must be properly matched to your situation. The goal is to have a system you install once and essentially forget about. There is no one right way to install a communications product, though; what works at one site does not necessarily work at another. Different sites have different combinations of needs or dependencies. Some vendors may supply incorrect information because they presume all sites follow the same modern office grounding philosophies. You need to understand the basics of grounding, isolation, surge management, and shielding to properly install your products. This document provides basic information on grounding, isolation, surge management and shielding, plus recommendations related to specific Digi products. The solutions discussed here are conservative and well proven by the author in over a decade of work in heavy industry (including oil refineries and water treatment plants) in lightning-prone areas of South-East Asia. Following conservative design practices can lead to many years of trouble-free operation. PN: 90000641_B http://www.digi.com/ support/ia Page 1 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 2 Common Serial Media 2.1 EIA/RS-232 2.1.1. EIA verse RS RS may stand for “Recommended Standard”, but who is recommending it? The answer is an organization called the “Electronic Industries Alliance” (http://www.eia.org). They want users to stop referring to standards as RS-232 and use TIA/EIA-232 (considered the standard’s “proper” name) instead. Many people consider the terms to be interchangeable, but most people tend to use the term RS-232 more loosely than EIA allows. Reference to RS-232 defines the application of TIA/EIA-232 parts that may or may not follow the standard strictly. Reference to TIA/EIA-232 implies that all parts of the system are strictly “per the standard”. For example, the original EIA/RS-232-C requires 25-pin connectors and a maximum speed of 9600 baud. This makes virtually all products on the market not EIA-232 compliant. The letter at the end (‘A’ to ‘F’) defines the version of the standard. The most come reference is EIA-232-C, which was the first version to reach widespread usage back in the 1980’s. Fortunately the electrical characteristics of RS-232 have not changed significantly between “C”, “D”, “E”, or “F”. Most changes affected the behavior and naming of control signals. Unless you are involved in international telephony or use specialized modem devices, you can consider RS-232C support interoperable with TIA/EIA-232-F support. 2.1.1. What is RS-232? RS-232 is an interface standard, not a data V+ Gnd communication standard. RS-232 was designed to allow the old US phone monopoly to create a single modem and enable all Vcomputer makers to connect to it. RS-232 can only go short distances and has very limited driving power; it was assumed the modem would sit in the same room as the ASCII "I" computer. RS-232 was not designed to connect an industrial controller to a computer 100 feet away in another room and on another power circuit. 0 1 0 1 0 0 1 1 RS-232 provides full duplex, point-to-point data transfer between two devices. The signal ground is included as one of the wires, making it very susceptible to damaging ground loops. Data is transmitted as a voltage polarity relative to the common signal ground. The diagram above shows the signal for an ASCII character 'I'. • When signal voltage is greater than +3v, the data is a binary 0. • When signal voltage is less than -3v, the data is a binary 1. • A voltage signal between -3v and +3v is undefined. PN: 90000641_B http://www.digi.com/ support/ia Page 2 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges For normal asynchronous serial lines, an idle line will be in the binary 1 state. This voltage signal is referenced to a ground that MUST be shared between both devices. This shared ground makes RS-232 very susceptible to noise, ground and surge problems. 2.1.2 RS-232 and Cabling RS-232 has no specific distance limitation. Instead the standard defines a capacitance limitation of 2500pF per transmitters. To change from a binary 0 to a binary 1, the transmitter must literally charge or discharge the wire to change polarity; the higher the capacitance, the slower this change occurs. Because capacitance is accumulative with length, longer cables mean more capacitance. The often-quoted 45 foot or 15 meter distance limitation is only found in an appendix to the specification that explains this distance is a good rule of thumb when users do not know the specification of their cable. Using a quality lowcapacitance cable with 42pF/meter (such as the Belden 1421A) means you can professionally run RS-232 over 55 meters. Note another rule of thumb: the thinner the overall RS-232 cable is, the higher its capacitive rating due to cross coupling between wires. You can expect a low-capacitance cable to appear fatter than the cables you are accustomed to using. Shielding is critical for RS-232. Never use unshielded cables for anything but bench-top trial runs. Unless your communications includes error-detection codes, unshielded Ethernet Cat 5 cable is not suggested for professional RS-232 use. Using good quality cabling saves you future costs and problems. 2.1.3 RS-232 and Grounding Look at the direct RS-232 connection at right; note the direct ground connection through the RS-232 cable. Although most engineers may be trained to avoid multiple ground paths, most engineers will be shocked by this drawing. Yet you can take your multi-meter and measure your own computer. Very likely you’ll find less than 1 ohm of resistance between the RS-232 signal ground and the computer chassis ground. By definition, RS-232 requires this common ground to function. This would not be a problem with a computer and modem sharing a common power source. So it is critical for EIA/RS-232 that 1 of the following 2 situations be true: 1) Both devices MUST share a common ground with no ground potential difference. 2) Or one device must isolate its RS-232 port to break any path to local ground. PN: 90000641_B http://www.digi.com/ support/ia Page 3 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges The next sections discuss how this grounding assumption affects your selection and use of Digi products. A good rule-of-thumb is to plug both RS-232 devices into the same power strip to assure that there will be no grounding problems. If you cannot plug both into the same power strip, you should consider using another signal standard such as EIA-485. 2.1.4 Surge Management Tutorial The term surge protection can be a bit misleading, as there is no way to completely block a surge. Yet this is the perception most people have: slap on a surge protector and surges are gone. Surge protection, however, is really the art of surge management. Surges occur because a higher voltage potential wants to dissipate to a lower voltage potential. There are no magic loopholes in the laws of physics that allow surges to evaporate or just go away. Instead, you need to provide paths for the surge to bypass your equipment. You need to understand where surges come from, where they want to go, and how to get them there without going through your equipment. Surge management, as the “art of bypass,” includes understanding the three forms of surges: • Static discharge or ESD protection. ESD is the shock you get when walking on carpet in the dry winter months. While of a very high-voltage, the ESD surge has very tiny energy content. Most modern serial communication devices include ESD protection built into the actual receiver/driver chips, but more to prevent failures from human handling during production and installation than an effort to provide surge protection for customers. The telltale signs of ESD protection masquerading as surge protection are the rating of 15,000 volts with no mention of current or energy (watts or joules). True surge devices require an energy rating. All Digi devices include 15,000-volt ESD protection. • Transient or spike surge protection. This protection handles highvoltage, modest energy spikes or noise surges induced in any long wire. These normal-mode surges are caused by the same phenomena as the pops and clicks you hear on your radio as you turn devices on or off. These can be identified in products with surge protection ratings in the 400 to 1500 watt range. The exact voltage rating isn’t as important as the combined voltage and current (or energy) rating. In general, presume that transient surge protection is not included in a product unless the vendor mentions it, and remember that a 15,000-volt rating implies ESD protection. The Digi industrial products, such as the Digi One IA and Digi One IAP, do include transient surge protection on the serial port. Other Digi products may require adding external surge protection if transient surges are expected. • Heavy or lightning surge protection. This protection can handle the very large energy content of common-mode ground surges. The most common forms of protection are gas-discharge tubes or spark gaps. Think of them as small neon tubes that convert much of the surge energy into heat and light. These surge suppressors are commonly rated in amperes or joules; for example, 20,000A or 10,000J. This sounds like a lot of power, but the surge PN: 90000641_B http://www.digi.com/ support/ia Page 4 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges is of very short duration. One of the more common product tests for lightning protection is IEC 801.5, which uses a 6000-volt, 3000 amp surge with a specific pattern that peaks in 8 micro-seconds and largely dissipates after 20 micro-seconds. Most surge management solutions involve a mixture of fine and coarse protection. They use heavy surge devices to burn and bypass the majority of the surge energy. These devices, however, are coarse; that is, not very accurate. They may fire at any voltage between 150 to 400 volts, which means that the attached device still needs to handle a low-energy 400-volt surge. Low-energy surge diodes are added after the heavy devices, as fine protection to clamp the remaining energy to within a few volts of the desired maximum. There is a fourth class of device, often called the varistor or MOV. Think of this device as tens of thousands of surge diodes in parallel, which handle thousands of times more energy than a lone diode and have a much more accurate clamp range than gas discharge tubes or spark gaps. But the MOV also has a very high capacitance that makes it more suitable for power circuits or DC signals The RS232 limitation of 2500pF is quickly passed, for example, when a 15,000pF MOV is added to the line. Avoid using the MOV in data communications where it affects the signal. MOV also has a tendency to progressively leak more current after every surge handled. 2.1.5 RS-232 and Surge Management Since RS-232 runs the ground directly through the cable, proper surge protection is difficult if not impossible. Major surge protection of RS-232 is possible only if the RS-232 port is isolated from the local ground. This isolation causes the path into your RS-232 port to appear as very high impedance (that is, a very bad path to ground). The surge then uses your surge device and you effectively “herd” it to ground. The TIA/EIA-232 standards require components to handle ±25 volts. Virtually all RS-232 circuits work between +12v and –12v, and ±15 is the standard’s limit for normal operating voltages. It is recommended that you plan for ±15v or ±16v surge devices, which start protecting in the 18-20v range. The selection of surge devices for RS-232 is limited by the need to keep capacitance below 2500pF per wire. Surge diodes add between 500 and 1000pF each, while varistors (or MOV) have as much as 15,000pF each. Adding a surge device shortens your overall permitted distance and greatly lowers the permitted baud rate. Lightning protection and RS-232 shouldn’t be mentioned in the same sentence; if you really need to protect from lightning surges, it is best to use fiber optics or RS-485 with heavy surge protection. 2.1.6 RS-232 and Everyday Data Errors When used properly, RS-232 allows virtually error-free communications. This is not a feature of RS-232, just a side effect of the rather strict limitations on its usage. The RS-232 limitations are: PN: 90000641_B http://www.digi.com/ support/ia Page 5 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges • A well-shielded, low capacitance cable • Two devices that must be physically located close together • No measurable ground potential differences or EMI disturbances If you cannot meet the RS-232 limitations, consider switching to RS-422 or RS485. It is worth repeating: Unshielded Ethernet Cat 5 cable is suitable only for short RS-232 cables or for use in applications that include error-detection and gracefully handle data errors. 2.2 EIA/RS-422 2.2.1 What is RS-422? RS-422 and companion RS-423 were defined as a fix or replacement for RS-232. The standard is designed as a real data communication standard, rather than interface standard, that includes realistic cable lengths and features to counter line noise. Instead of a single wire and voltage measured to a shared notion of ground, RS-422 introduces a pair of signals for a differential signal. The binary data value is defined by the polarity between the 2 wires without respect to ground. Electrical ground is still iimportant for RS422 to function, however; the laws of physics still require a complete circuit to operate. The 2 wires in a pair are not a current loop. The RS-422 driver sources current on both wires and the receiver sinks current on both. Either a 5th signal return wire is required or both devices must indirectly share the same ground through earth. See the EIA/RS-485 section, later in this document, for more information. True TIA/EIA-422 devices are rare these days Most vendors advertising RS-422 support (Digi included) instead use chips supporting RS-485 specification. Some vendors use the term RS-422 to imply a point-to-point link and the term RS-485 for multi-point (or multi-drop) link. Still others use the term RS-422 to mean a multi-point RS-485 link with 4-wires and the use the term RS-485 for a 2-wire half-duplex link. 2.3 EIA/RS-423 2.3.1 What is RS-423? RS-422 requires 2 wires for each signal. A traditional RS-232 cable with 25 wires, then, would now require 50 wires. The RS-423 companion standard was devised using an unbalanced design (like PN: 90000641_B http://www.digi.com/ support/ia Page 6 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges RS-232) but with more robust features to cover realistic distances and the removal of the RS-232’s assumption for a shared ground reference. RS-423 treats the remote ground as a pure voltage reference for data detection and does not mix the remote ground with it’s own local ground. The RS-423 standard simplified the creation of devices that could support both RS-232 and RS-423 on the same port. Most people encountered RS-423 in the world of minicomputers, which have evolved into today’s work-station. A simple cable change allows connecting locally to an RS-232 device or up to 1000 meters away to another RS-423 device. Digi doesn’t directly support RS-423. You may come across the standard when integrating mini-computer or workstation technology in SCADA or DCS systems. With Digi products, you’ll need to use RS-232 or add third-party RS-232 to RS423 converters. 2.4 EIA/RS-485 2.4.1 What is RS-485? RS-485 is a half-duplex data communication standard that can be use for point-to-point or multi-drop applications. It uses twisted wire pairs. Data is transmitted by a differential voltage signal. The two wires in a pair are not a loop; both wires are '+' (positive) signals sourcing current to a third virtual ground conductor. This example shows the ASCII "I" differential signal for an ASCII character 'I'. Though labels vary from vendor to vendor, one wire of the pair is often labeled A and the other B. Data is represented by the relative voltage of A to B. A B 0 1 0 1 0 0 1 1 • • • When VA < VB, the data is a binary 1. When VA > VB, the data is a binary 0. An idle line without data is in the binary 1 state. This differential voltage signal on a twisted wire pair robust and less susceptible to noise or minor shifts in signal reference ground. The diagram at right shows how minor noise on one wire induces a comparable noise on its mate. The noise does not affect the relationship between the voltage of wire A and B, so it does not lead to false data bits. In contrast, if this was an unbalanced signal such as RS-232, the noise on the solid line, VB, may have approached 0v/ground enough to create a false data bit. PN: 90000641_B http://www.digi.com/ support/ia Page 7 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 2.4.2 What is RS-422/485? RS-422/485 is a technical definition. Digi supports both RS-422 and RS-485 using TIA/EIA-485 compatible chips. This document uses the term RS-422/485 to refer to any of these three connection designs: • RS-422 as full-duplex 4-wire point-to-point • RS-485 as half-duplex 2-wire multi-drop • RS-485 as full-duplex 4-wire multi-drop. 2.4.3 2-wire vs. 4-wire 2-wire RS-485 is strictly half-duplex. One wire pair is used as a bi-directional bus. Most devices use a master-slave design, where a "request" is transmitted and then a "response" is received. Other systems use complex token passing to allow multiple masters (or peers) to share the same half-duplex wire pair. Examples of such RS-485 “networks” are DH485 and ProfiBus. Many industrial products support both 2- and 4-wire RS-485. Providing terminals for 4-wireallows external jumpers to short the two A signals and two B signals for 2-wire. PN: 90000641_B http://www.digi.com/ support/ia Page 8 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 4-wire uses two twisted wire pairs, one pair for transmit and one pair for receive. Masters use the TX-pair to communicate with the slaves. Slaves use the RX-pair to respond. 4-wire RS-485 is more robust than 2-wire with low quality cable or high environmental noise. 4-wire RS485 also reduces the data communication interrupt load on the slave devices since slaves only receive messages from the master and do not see other slaves’ responses. Note that there is a special form of 2-wire RS-485 that allows an optional 2nd wire pair to be used as a control (RTS) signal to manage repeaters in the system. 2.4.4 RS-422/485 and Grounding Grounding and RS-422/485 is one of the most misunderstood issues of serial communications. Many people believe that RS-422/485 does not require a ground. One myth is that the plus (+) and minus (-) signals act like a current loop and complete the electrical circuit by themselves. Another myth is that a differential signal just compares 2 voltages without reference to common ground. But the biggest myth is that RS-422/485 withed (WHAT IS THIS WORD SUPPOSED TO BE?)without proper grounding appears to work. The truth is that both plus (+) and minus (-) signals source current during transmission and sink current when receiving. While differential voltage signals can be compared without direct ground reference, an electrical circuit still requires a complete loop to operate reliably. Per the TIA/EIA-485A standard: "Proper operation of the generator and receiver circuits requires the presence of a signal return path between the circuit grounds of the equipment at each end of the interconnection. The circuit reference may be established by a third conductor connecting the common leads of devices, or it may be established by connections in each using equipment to an earth reference." RS-422/485 appears to work without proper ground consideration because either the RS-422/485 devices are sharing a common earth reference through their power supplies (TIA/EIA-485A point #2) or there is enough leakage through the RS-422/485 on-chip protection circuits to approximate a common signal return (TIA/EIA-485A point #1). You need to understand how RS-422/485 works and select an appropriate ground design. PN: 90000641_B http://www.digi.com/ support/ia Page 9 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 2.4.5 Grounded RS-422/485 Circuits & Grounding (EIA-485A Option #2) In an office environment, most RS-422/485 devices work with the notion of a grounded power supply. Within a residential or commercial environment, all devices must reference themselves to the one common “safety earth” or “physical Earth” (PE). The drawing at the right shows that as device A transmits to Grounded by Shared Earth device B (that is, sources a tiny current), the signal return completes the circuit through the shared notion of ground. This works only if both devices ground themselves to the common earth. If either or both devices have a floating power supply, the circuit is not complete and functions because of phantom currents imposed on the RS422/485 signals as each device transmits or receives. Confusion is added here because connecting the two grounds of these grounded (IS “GROUNDED” NECESSARY HERE?)devices creates a ground loop, which can lead to catastrophic device failure. For example, communication wires can literally unsolder themselves from the connectors or melt, and/or the “magic smoke” may leave either or both devices and they will no longer function. 2.4.6 Isolated RS-422/485 Circuits & Grounding (EIA-485A Option #1) When either device in the RS422/485 link has an isolated ground, then “a third conductor connecting the common leads of devices” is required. This is when the “signal ground” pins or terminals of the devices are connected. This drawing Grounded by Signal Return shows optical isolation in the RS-422/485 port, but the same concept applies if the power supply is isolated. When in doubt, TIA/EIA-485A recommends you include a 100-ohm 1watt resister in the ground wire at each attached device. This prevents common damage should a serious ground potential difference develop between devices with a ground wire. For safer connections, add this to an RS-232 cable. 2.4.7 RS-422/485 and Surge Management RS-422/485 is surge-management friendly. The chips use the signal ground indirectly, and are designed to handle minor ground potential differences of ±7 PN: 90000641_B http://www.digi.com/ support/ia Page 10 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges volt. RS-422/485 is not much affected by capacitance, so better surge protection devices can be used with less impact on normal data communications. The TIA/EIA-485 standards require components to handle voltages of ±25 volt. Digi recommends you plan for ±15 or 18v surge devices, whichfunction in the 18-20v range. Some vendors suggest ±6v surge protection – do NOT use these. Because the RS-422/485 chips used by Digi normally communicate in the range of –7v to +12v, clamping at ±6v can interfere with normal operations and overload or damage power supplies. For the ultimate in surge protection, you can convert RS-422/485 to fiber optics. This eliminates grounding problems and can increase the overall distance you can cover. 2.4.8 RS-422/485 and Everyday Data Errors Running RS-422/485 a few dozen feet over quality cable with good shielding and ground can be virtually error-free. That’s not what people use RS-422/485 for, however. Any electrical signal running over 1000 meters of wire (that is, 3300+ feet of antenna) will pick up noise. The differential twisted-pair nature of RS422/485 helps reduce the impact of this noise, but realize that a few communications errors occur daily. Running long RS-422/485 lines is suitable only for protocols that include proper error-detect (a CRC or block-checksum). If you run, for example, RS-485 for 700 meters for applications such as aan ASCII console or an event log printer, these are the types of errors that occur: 2.5 RS-422/485 users should hope for 0% error, but be able to function with 0.5% error! • With an ASCII console, byte errors could change the remote device configuration and put it off-line. • The event log printer might see control characters that affect printing. Comparison Table of RS-232, RS-422, RS-485 You want to use the right tool for the right job. Each standard has qualities that better apply to certain jobs than the other standards 2.5.1 When to use RS-232 RS-232 is ideal for two “nearby” devices powered by the same power circuit. RS-232 is a very low-power signal, which helps your devices run cooler. This standard is more consistently used by vendors; you may struggle to make weird cables, but the result will work. 2.5.2 When to use RS-422/485 RS-422/485 is better for devices more than 50 feet apart or when the quality of all grounds and powers is not known; for example, different vendors’ scope of supply or different functional areas in a plant. RS-422/485 can consume up to 100 times more power than RS-232, so be sure you have adequate power available. The downside to RS-422/485 is that vendors use conflicting jargon and designs, and interoperability can become a headache complete with trialand-error testing. Issues of biasing, line-control, and grounding can make some PN: 90000641_B http://www.digi.com/ support/ia Page 11 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges vendor combinations nearly impossible. You may need to add external resistors or purchase signal converters to have interoperability between products from different vendors. TIA/EIA-232 TIA/EIA-422 TIA/EIA-485 Unbalanced Differential Differential Distance per Standard 15m 1000m 1000m Practical Distance with good cable 50m 2000m 2000m Maximum Speed per Standard 19200 bps 1Mbps 1Mbps Number of Devices Connected Point-to-Point Point-to-Point Up to 32 3, 5, 7 or 9 4 (5 with gnd) 2 (3 with gnd) 4 (5 with gnd) Multi-core Twisted pair Twisted pair +/- 15v 0v to +6v -7v to +12v Permitted Ground Potential Difference None None +/- 7v Receiver Threshold +/- 3v +/- 1.5v +/- 200mV Max Short-Circuit Current Permitted 100mA 150mA 250mA Min Driver Current Expected 1mA 22mA 65mA Over-voltage Protection per Standard +/- 25v +/- 25v +/- 25v Recommended Surge Device Rating +/- 15v +/- 15v +/- 15v Signal Type Number of Conductors Cable Type (all must be shielded) Normal Voltage Range PN: 90000641_B http://www.digi.com/ support/ia Page 12 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 3 How to Know … 3.1 Is your supply isolated or floating? You can measure your supply with a common resistance or digital multi-meter. With your power supply powered off, measure the resistance from the 0v of the DC output to the neutral and ground (N and PE respectively) of the AC power side. Small AC/DC “wall-warts” tend to be isolated because they use a simple transformer. The AC/DC power supplies commonly used with notebook computers often have a grounded design to prevent ground discharges when connecting serial devices to the notebook. More expensive switching AC/DC power supplies often have some way to enable or disable connecting the “frame ground” to the DC 0-volt ground. (WHAT DENOTES ISOLATED AND WHAT DENOTES FLOATING?) 3.2 Is your device’s serial port isolated or floating? You can measure this (WHAT?) with a common resistance or digital multi-meter. With your device disconnected and powered off, measure the resistance from the serial port’s ground pin to the ‘+’ and ‘–’ terminals for power. If the resistance is infinite, you most likely have an isolated serial port. It takes a thorough understanding of device design and the intended installation details to completely measure and interpret the results Installation becomes a critical factor in keeping or losing the benefits of isolation. The next sections of the document cover various Digi products and how they relate to issues of isolation, grounding, and surge protection. 3.3 What about using Cat-5 cable? IEEE Category 5 cable is fast becoming the duct-tape of the computer world. There are a few requirements you need to consider, though, for trouble-free data communications. 3.3.1 Cat-5 and Shielding RS-232/422/485 all work best with well-shielded cables. • Using UTP cable results in a higher probability of noise and surge problems. • STP cable uses a thin foil that makes it error-prone when attempting to make well-shielded cables onsite. STP is most reliable, though, when pre-made cables are used. • Formal RS-232/422/485 cables are available with single or multiple layers of shielding. These cables can include the bare “shield drain” wire required for reliable, hand-formed cables. 3.3.2 Cat-5 and Flexibility Cat-5 cable usually comes as a solid core, small-gauge wire. This is susceptible to damage by bending, kinking, or repeated flexing. All standard RS- PN: 90000641_B http://www.digi.com/ support/ia Page 13 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 232/422/485 cables are stranded. If you must use Cat-5, consider one of the more industrial versions that include stranded wires. 3.3.3 Cat-5 and Capacitance Cat-5 cable is often poorly described. Better quality Cat-5 cable may have a capacitance of about 15pF/foot. If you ignore the noise and line surges caused by lack of shielding, you can use normal UTP Cat-5 cable to run RS-232 about 175 feet and still remain within the 2500pF limitations. But at that distance, grounding potentials are more likely to occur and damage your devices. PN: 90000641_B http://www.digi.com/ support/ia Page 14 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 4 DigiOneIA and DigiOne-IAP (DIN-Rail products) 4.1 Overview The DigiOneIA (DigiOne-IA or DigiOneIA??) is the only product Digi makes that includes specific DC-to-DC isolation. The DigiOneIA accepts a 10v to 30vDC supply and internally isolate that DC power by at least 500 volts. With the inherent isolation of IEEE mandated UTP Ethernet added to the mix, the DigiOneIA line is a product with three distinct grounds. If you use the 802.3af powered Ethernet option, this is also isolated internally and remains related to ‘Ethernet Ground #1’ (shown in the upper left corner). 4.2 Using the Direct DC Power Source The DigiOneIA line has floating-ground serial ports. The serial ports (ground #3) in RS-232, RS-422, or RS-485 mode all float without reference to the local power supply (ground #2). Rather, the serial ports try to reference themselves to the ground of the attached serial device(s). Even if the remote device has a ground-potential difference of a few hundred volts, a DigiOneIA will connect and communicate. Ground #3 (see the previous drawing) would have a similar large ground-potential difference from Power Ground #2. Operating with such ground-potential differences long-term, however, represents a shock hazard to employees. This ability is meant to allow full system operation during short-term ground-potential disturbances; think of it as a “flex-coupler” between ground systems. The shock hazard exists because portions of the DigiOneIA circuitry and shield shells may have a large voltage potential compared to the local metal cabinets. If serious ground-potential differences are expected long-term, the only safe design is to add fiber optics to the serial link. This completely and safely decouples the DigiOneIA from the remote ground potential, eliminating the shock hazard. Since fiber optics are immune to noise and lightning surges, they should be considered whenever you run serial cable between buildings. PN: 90000641_B http://www.digi.com/ support/ia Page 15 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 4.2.1 Grounding and RS-232 The DigiOneIA’s floating RS-232 circuit Has Ground Ethernet UTP Wire!! readily references itself to the remote DC Power RS-232 device’s ground. This Attached DigiOneIA is ideal for complex Device DigiOneIA-RealPort or multi-vendor DORPIA Drawing #1 (Good) systems. DORPIA drawing #1 shows three separate grounds. How they relate is not important. In most situations the grounds will have identical voltage potentials, but it is not significant even if fault conditions disturb this. 4.2.2 Grounding and RS-422/485 The DigiOneIA’s Has Ground floating RS-422/485 Ethernet UTP Wire!! port complicates DC Power cabling a bit. By RS-485 definition, the Attached DigiOneIA isolated DigiOneIA Device DigiOneIA-RealPort does not meet DORPIA Drawing #2 (Good) TIA/EIA-485A’s Option #2 requirements for signal return; that is, “connections in each using equipment to an earth reference”. Rather, you must meet TIA/EIA-485A’s Option #1 requirement for signal return: “a third conductor connecting the common leads of devices”. This means 3 wires are required for RS-485 “2-wire” operation and 5 wires are required for RS-422 or RS-485 “4-wire” operation. Will the DigiOneIA still work without this Lacks Ground Ethernet UTP Wire!! connection? Most likely it will, but its DC Power long-term reliability RS-485 Attached DigiOneIA and survivability will Device DigiOneIA-RealPort be reduced. The DORPIA Drawing #3 (Bad!) problem is that you have added a fourth ground –the floating DigiOneIA – and are linking it improperly by RS-485 to the remote RS-485 device. You have not followed TIA/EIA-485A requirements and included a signal return. 4.2.3 Grounding and Ethernet The IEEE specification for UTP Ethernet requires 1500v isolation transformers. PN: 90000641_B http://www.digi.com/ support/ia Page 16 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 4.3 Shielding 4.3.1 Serial Cables RS-232, 422, and 485 all will perform better with proper cable shielding. Select a cable with a bare drain wire included; trying to properly connect shields directly to foil or braided shields is error-prone. Shields must be connected (grounded) to something as a floating shield can create more noise problems than no shield at all. Shields must be connected at one end only. Connecting a shield at both ends allows large current flows through the shield that also add unwanted noise to the serial line. The D-Sub Shell connector shell is grounded to the RS-232/485 signal ground. This means you can passively connect the shield through the cable shell. Note, however, that should you also ground the shield externally, you have shorted your floating RS-232/485 signal ground to that ground. Be careful you don’t accidentally defeat the DigiOneIA’s isolation this way. To maximize the value of the DigiOneIA’s isolation, Digi recommends you tie your serial shield to ground at the remote device and not at the DigiOneIA. 4.3.2 Ethernet Cables The Ethernet shield is also tied to the DigiOneIA’s floating RS-232/485 signal ground. Since most users use unshielded (UTP) Ethernet cables this won’t be an issue. If you decide to use shielded (STP) Ethernet cables, you need to consider this issue carefully. Digi recommends you connect the STP shield at the remote end only. If you do connect the shield at the DigiOneIA end, be aware that you have tied the shield to the digital ground of the DigiOneIA’s floating RS-232/485 signal ground. This can lead to unexpected consequences. Also be aware that a “floating shield” generally causes more problems than no shield at all; if you select an STP design, you must include a detailed plan for how shields are connected and grounded. Buying STP cables instead of UTP cables to appear “robust” without a serious ground design is a waste of money or worse. 4.4 Surge management 4.4.1 Serial Cables The DigiOneIA’s serial ports already include transient surge protection. You will not need to add external surge protection for normal-mode spikes and line noise. If you do need extra surge protection, the range of ±15v to 18v is the best choice as it works for RS-232, RS-422, and RS-485. Transient protection can be grounded to the serial signal ground or to a proper surge ground. Remember that the goal is “surge herding” or bypass. The small level of energy included in line spikes is not dangerous in itself, as long as it is allowed to bypass your RS232/422/485 chips. Connecting the surge output of these devices to a local surge ground, impacts the isolation inherent in the DigiOneIA. Make sure you take this into account in your design. Also, grounding surge devices near the Digi (the DigiOneIA??) increases the likelihood of surge energy coming your way. PN: 90000641_B http://www.digi.com/ support/ia Page 17 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges [The surge is just trying to get to a lower potential and adding earth-grounded surge devices is a good way to attract them toward your equipment.](Is this last sentence necessary?) If DigiOneIA’s serial lines run between buildings, between power systems, or between vendors’ scope of supply, consider adding heavy common-mode lightning surge protection or converting to fiber optics. This is because, at any moment in time, you cannot really know how the remote device is grounded or the quality of that ground. What if a 100kw motor faults all of its 480vac triplephase power to the remote earth creating a major ground-potential disturbance? What if a lightning strike or the design of their lightning protection system creates a large common-mode surge on your serial line? What if “they” accidentally put 110vac on your serial line? You can wait until such a surge damages your device or you can add the proper protection from the start, to avoid the repairs and save time and effort. The high energy involved in common-mode lightning or ground surges requires them to be properly grounded to a local surge earth ground. The earth connection will affect your isolation, and providing a good path to earth at your devices requires the remote device to take proper surge precautions as well. Heavy surge protection is all or nothing -- either all devices involved have comparable protection or none do. While a local surge earth connection reduces the isolation of the DigiOneIA, it is not wasted. The isolation inherent in the DigiOneIA makes any surge path through the DigiOneIA appear very high-impedance. That is, a large surge entering your serial line will reach the surge device and have two potential paths to continue on: • The surge can exit through the surge device, with it’s low-impedance path to earth. • The surge can continue on into the DigiOneIA and try to punch through the high-impedance isolation barrier. This leads us back to the herding notion: the isolation in the DigiOneIA virtually guarantees all surge energy avoids the Digi and uses your surge protection investment to pass harmlessly to earth. 4.4.2 Ethernet Cables Presuming that the DigiOneIA’s Ethernet connects directly to a local hub, switch, or router in the same cabinet, no special surge protection is required on the Ethernet port. If you are running the cable between buildings, panels, or power systems, it is recommended that you add fiber optic or wireless media rather than Ethernet Surge Devices. A surge device must appear as high-impedance and have minimum impact on the signal during normal operations. And, it must rapidly detect a surge and change from “open circuit” to “short circuit” to route the surge energy away from the data line. A strong surge protection design may adversely affect the signal during normal operations, while a weak surge protection design may not act fast enough to protect your equipment. What is considered a fast protection PN: 90000641_B http://www.digi.com/ support/ia Page 18 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges response for a 9600 baud RS-232 signal (that is, 10kHz is not fast to a 100Mhz Ethernet (that is, 100,000KHz). Using fiber optics or wireless Ethernet, rather than an Ethernet surge device, allows the surge protection to operate as required, and reduces grounding and noise issues while greatly increasing the permitted end-to-end distance to many kilometers. 4.5 Issues of Ground Potential Problems with 2 Devices The DigiOne-IAP, unlike the DigiOneIA, allows the 2 serial connectors (one DShell and one screw terminals) to act as one or two ports, which in turn allows 2 serial devices to share the DigiOne-IAP. A unique (is it unique??) Passthru port allows a serial master (such as PC/HMI or local Operator Panel) to share a single-port serial slave with the network. Special firmware helps manage the priority of these shared masters to prevent the serial master from being starved by heavy network master demands. Unfortunately, the pass-thru port opens the DigiOne-IAP to a classic killer ground loop problem. Remember that the DigiOne-IAP has 3 separate grounds internal. The isolation between the Ethernet port, power supply, and serial ports is still valid. Both serial ports still share a common ground, however, which is not a problem when both serial devices share a common ground externally. But if the serial ports do not share a common ground externally, the DigiOne-IAP becomes an expensive fuse. This drawing shows a classic “Killer Ground Loop”. An industrial or building-automation system with many remote sensors uses a filtered or floating earth for safety and ease of troubleshooting. Any surge energy entering the system is presumed to exit through specific inductors and surge protection paths. If, however, there is a non-isolated RS-232 port, the floating RS-232 earth of a DigiOne-IAP will reference itself to this filtered earth. By itself, this is not a problem. Connecting the second RS-232 cable to a common PC computer is the source of the “Killer Ground Loop”. The PC computer assumes the metal chassis is ground, which in turn connects directly to the safety or PE ground in your building’s AC mains (is this supposed to be plural?) or power system. This is the third ground conductor added to most modern devices. Now, any surge energy entering the alarm system will see a lower impedance path to ground by exiting the RS-232 port, passing through the DigiOne-IAP and the second RS-232 into the PC computer. A low-energy surge most likely will not damage the DigiOne-IAP, but may destroy the PC computer’s RS-232 port and possibly it’s CPU and motherboard. A high-energy surge taking this path most likely will kill the RS- PN: 90000641_B http://www.digi.com/ support/ia Page 19 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 232 ports on both connected devices and could melt the PCB board in the DigiOne-IAP (this is why it becomes an “expensive fuse”). There is an easy solution to this problem. One of the two external devices (either the alarm panel or the PC) must have an isolated port. Because the alarm panel uses a filtered earth, it should have an isolated RS-232 port even if this option costs extra. But because a modern multi-GHz computer is a very sensitive device, one could argue that purchasing isolated serial ports for it is also a must. If either of these options is not available or too expensive, many third-party suppliers sell RS-232 isolators that can be used for any RS-232 connection. Regardless of which one is isolated, one or both devices must be, to make the DigiOne-IAP pass-thru feature a long-term, robust solution. Adding surge management devices may partially compensate for this ground loop problem, but it is only a partial solution. Adding RS-232 isolation is a complete solution. 4.6 “But I want RS-485 on BOTH ports of the DigiOne-IAP in pass-thru” The DigiOne-IAP only supports RS-232 on the DB9 in pass-through mode. The primary reason is lowering overall cost; many people do not need the 2nd port or pass-thru mode. Having a 2nd port or pass-thru mode, 1st RS-485 Device however, it does have Ethernet UTP a secondary benefit. Because the two 2nd RS-485 Device DC Power ports of the DigiOneIsolated RS-232 DigiOneIA in IAP are NOT isolated to RS-485 Converter pass-thru mode from each other, DOIARP for 2 RS-485 (Good) running RS-485 direct to two very different field devices has a fairly high probability of ground damage to the DigiOne-IAP. To support dual RS-485 ports, then, the DigiOne-IAP should support full optical/galvanic isolation between the two, which might increase the overall cost for all users. Those requiring a second RS-485 port should invest in a good isolated RS-232 to RS-485 converter to gain not only a second RS-485 port, but also to achieve full isolation between the two RS-485 systems. PN: 90000641_B http://www.digi.com/ support/ia Page 20 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 5 The DigiOneSP (Single Port product) The following section presumes you have read Section 4 of this document. It refers to and contrasts issues for the DigiOne SP to those of the DigiOne-IAP. 5.1 Overview The DigiOneSP does not include internal DC-to-DC isolation. (This is not a defect but a normal design for small single-port Device Servers.) The DigiOneSP accepts a 9 to 30vDC supply, but the DC ground (or 0-volts) is assumed to be the same as the shield and serial ground. Ground isolation is included in the Digi-supplied AC/DC power pack, which creates a third ground external to the DigiOneSP. Add to this the inherent isolation of IEEE mandated UTP Ethernet and the DigiOneSP is a product with 2 distinct grounds internal, plus a third implied ground if isolated DC power is supplied. If you use the 802.3af powered Ethernet option, this (WHAT?) is also isolated internally so remains related to the Ethernet Ground #1. 5.2 Using the Digi supplied AC/DC Power Source Unless you understand ground system design, Digi highly recommends you use the isolated AC/DC power pack supplied with the DigiOne SP. 5.2.1 Grounding and RS-232 with floating ground Using the DigiOneSp Ethernet UTP with an isolated Has Ground Wire!! AC/DC supply allows "Other" Power the floating RS-232 circuit to readily link Attached Digi-Suppied DigiOneSP and reference itself to Device AC/DC Pack DigiTS1 the remote device’s SP Drawing #1 (Good) notion of ground. Given the sensitivity of RS-232 to ground loops, using an isolated DC supply with the DigiOneSP is always the preferred solution. DigiSP Drawing #1 shows this situation. Three distinct grounding systems are created. You do not need to be concerned with how these systems are linked externally. • Ethernet UTP ground. This ground references itself elsewhere. PN: 90000641_B http://www.digi.com/ support/ia Page 21 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges • AC mains power ground. This ground references to the safety earth as part of the electrical codes. • Other Ground. Other ground is defined by the attached device, which likely is the same as the AC mains power ground. But, DigiSP works fine even if the “Other Ground” is floating, filtered or tied to another power system. Changes done at the attached device do NOT affect the DigiOneSP. 5.2.2 Grounding and RS-422/485 with floating ground Using DigiOneSp with Ethernet UTP an isolated AC/DC Has Ground Wire!! supply creates a RS422/485 circuit with "Other" Power floating ground, so Attached Digi-Suppied DigiOneSP the ideal wiring Device AC/DC Pack DigiTS1 includes an explicit SP Drawing #2 (Good) ground wire to tie the DigiOneSP to the “Other” ground. Your RS-485 2-wire becomes 3-wire, and 4-wire becomes 5wire. Discussion of the 3 grounds shown is the same as for SP Drawing #1. Do not run a pure 2Ethernet UTP wire or 4-wire RSLacks Ground Wire!! 485. The problem is shown in DigiSP "Other" Power Drawing #3. The Attached Digi-Suppied DigiOneSP floating RS-422/485 Device AC/DC Pack DigiTS1 of the DigiOneSP has SP Drawing #3 (BAD!) created a fourth ground that is being improperly linked by the RS-485 chips to the “Other” ground. The DigiOneSp still works without proper serial grounding, but you’ll find that long-term reliability and survivability is reduced. A common symptom of RS-422/485 with improper grounding is that is it works great --99.9% of the time. But you may lose communications a few hours each week or month. See Section 3 in this document for drawings of how to properly ground your RS-422/485. 5.2.3 Grounding and Ethernet with floating ground The IEEE specification for UTP Ethernet requires 1500v isolation transformers. Thus the DigiOne SP’s Ethernet port is always isolated and not affected by use of an isolated or non-isolated power. 5.3 Using the Direct DC Power Source Since the DigiOne SP accepts DC power supply of 9v to 30vDC, many people are tempted to share existing central 12v or 24v DC supplies. You can do this, but be sure you understand the consequences. Using a non-isolated DC power causes the DigiOne SP’s serial ground to be tied to the power ground. PN: 90000641_B http://www.digi.com/ support/ia Page 22 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 5.3.1 Grounding and RS-232 with grounded power Using a shared DC power Ethernet UTP Has Ground supply instead of the Wire!! AC/DC supply from Digi "Other" DC Power Supply means the DigiOneSP’s RS-232 circuit is Attached DigiOneSP Non-Isolated Device DigiTS1 DC Supply referenced to the power supply ground, creating SP Drawing #4 (Warning – makes Assumption) the potential for ground loop currents entering the DigiOneSP’s RS-232 port to reach the power supply ground. Note that the DC Power Supply Ground is thought to be the same as the Other Ground – you have built a dependency or assumption into the system that may not be true today or after any future changes to the system. Because the RS-232 signal ground has less than 1 ohm of resistance to the power supply ground, any RS-232 port can easily be damaged by ground currents should the DC Power Supply Ground and the Other Ground be different even for a fraction of a second during a system fault. Using the DigiOneSP with Ethernet UTP Has Ground non-isolated DC power is Wire!! fine when you connect a DC Power Supply device with a floating or isolated serial port, or one Non-Isolated DigiOneSP Attached Device DC Supply DigiTS1 with Floating Ground that uses the same DC power supply. Most small SP Drawing #5 (Good) devices with external ACpower-packs (bar-code scanners, weight scales, etc) will have an RS-232 port with a floating ground. Of course you have built a dependency or assumption into the system that may not be true after any future changes to the system (where? How? What did I miss?). You need to make sure future device replacement doesn’t introduce a ground loop such as that shown in DigiOneSP drawing #4. Ethernet UTP The ideal design when using a non-isolated DC "Other" DC Power Supply power supply with the RS-232 Attached DigiOneSP DigiOneSP is to add an Isolator Device DigiTS1 external RS-232 isolator. SP Drawing #6 (Good) These isolators break the ground path and prevent ground loop currents through the RS-232 cable. Cost for external isolators ranges from cheap to expensive, depending on what added features are included. For example, some external isolators include LED indicators, special connector types, and/or serious surge protection. When in doubt, the safest design with the DigiOneSP is to either use the Digi supplied, isolated AC/DC power supply or add an external RS-232 isolator. PN: 90000641_B http://www.digi.com/ support/ia Page 23 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 5.3.2 Grounding and RS-422/485 with grounded power Using a shared DC power Ethernet UTP No Ground supply instead of the Wire!! AC/DC supply from Digi "Other" DC Power Supply means the DigiOneSP’s RS-485 Non-Isolated Attached DigiOneSP RS-422/485 circuit is now DC Supply Device DigiTS1 referenced to the power SP Drawing #7 (Warning – has Assumption) supply ground. While you still have the potential for damaging ground loop currents, the RS-422/485 chips have thousands of ohms of impendence (impendence or impedance?), which reduces the likelihood of damage. If the remote RS-422/485 device is also grounded in the same manner, you can now run a pure 2-wire or 4-wire design. [You are still assuming the ground potential difference between the DC Power Supply Ground and the Other Ground will never be greater than ±7-volts.] (maybe it’s me, but this seems to come out of the blue. Is it an engineering thing? If not, then please try to clarify.) This is safer than adding the 3rd (or 5th) ground wire to the RS-422/485. The explicit ground wire would cause substantial ground currents to flow with even a 1 or 2-volt ground potential difference. 5.3.3 Grounding and Ethernet with grounded power The IEEE specification for UTP Ethernet requires 1500v isolation transformers. Thus the DigiOne SP’s Ethernet port is always isolated and not affected by use of isolated or non-isolated power. 5.4 Surge management The DigiOneSP’s serial port does not include transient surge protection, although it does have 15,000v ESD protection (which is not addressed here). If you expect normal-mode spikes and line noise on the serial port, you should add external transient surge protection in the range of ±15v to 18v. This works for RS-232, RS-422, and RS-485. Do not use ±6v surge protection for RS-422/485 since the RS-485 chip within the DigiOneSP is designed to operate from –7v to +12v. If you plan on adding heavy lightning protection to the DigiOneSP, it is safest to use the Digi-supplied isolated AC/DC power supply. This makes the surge path in or out of the DigiOneSP by power cable very high-impedance, and highimpedance is the greatest surge-deterrent available. Successful lightning protection is possible with a DigiOneSP and a central, shared DC power supply, but you must know what you are doing. The heavy surge currents during lightning storms can be very unpredictable; you may even have surges entering other devices pass through the DC power supply and damaging the DigiOneSP to reach its attached lightning protection (please clarify – do the entering surges pass through the DC power supply and DAMAGE the DigiOneSP? Whose lightening protection is attached? What is trying to reach the lightening protection – the surges?). This is why using an isolated power supply is safest – you want the path in or out of the DigiOneSP power cable to be high-impedance. PN: 90000641_B http://www.digi.com/ support/ia Page 24 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges 6 The DigiOne TS2/TS4/TS8/TS16 (Multi-Port products) Overview The DigiOne TS2 does not include internal DC-to-DC isolation. Grounding, isolation, and surge protection issues are nearly identical to the DigiOne SP; read Section 5 for information. The major difference is that there are now between 2 to 16 serial ports all sharing a common ground with the DC power supply. 6.2 Ethernet Ground #1 CPU Shields Ground #2 6.1 AC/Mains Ground #3 The following section assumes you have read Sections 4 and 5 in this document. This section covers the new issues for multi-port TS2, TS4, TS8, and TS16. When we refer to the TS2 below, apply the same information to TS4, TS8, or TS16. DC Power Ground #2 Digi Supplied AC/DC Pack D-Shell Ground #2 D-Shell Ground #2 DigiOne TS2 Issues of Ground Potential Problems with Multiple Devices 6.2.1 The Problem – Killer Ground Loops Alarm Panel with DigiOne TS2 This is essentially the same problem "Filtered Earth" discussed with the DigiOne-IAP used #1 #3 Surge Energy Path as a 2-port device. But because 2-, 4-, 8-, or even 16-port devices are #2 #3 involved, the probability of killer ground loops occurring is much higher. If the connected devices are the same Killer Ground make and model from a single vendor, Loop Example! Computer with "Chassis Earth" killer grounds are less likely. If you are mixing devices from various vendors, however, you should assume killer ground loops are highly probable. 6.2.2 Ultimate Solution – Use Isolated Serial Lines For the ultimate solution you install once and never need to maintain, just make sure all serial lines are isolated. Most small devices powered by an AC power pack (or “wall wart”) have a floating earth by design. The AC transformer in the “wall wart” creates a low-voltage DC supply without direct connection to the AC earth. When people see four devices and four wall-warts all running at, for example, 9-12vdc, they tend to consider using a single power supply for all four. But this does not promise they (the devices and wall warts?) all have the same serial ground. The use of protection diodes in many devices means that even sharing a DC supply will end up with serial grounds varying by up to 1½-volts. This can lead to phantom currents in the serial lines; in fact, it is common for the entire product current (100 or even 800mA) to enter one device from the power supply, then exit that device and PN: 90000641_B http://www.digi.com/ support/ia Page 25 of 26 Using Digi Products in the Real World: Issues of Grounding, Isolation, and Surges return to the power supply through the RS-232 cables and the TS2 to the power supply. For grounded devices or those with shared DC supplies you may need to add external serial isolators. These isolators are available with isolation from 500v to 5000v. Some claim to be “self-powered,” but reliable communication is highly dependent on the attached device. The newer TS2 and TS4 (with MEI or RS232/422/485 ports) allow you to supply extra power to the ports. The TS8 and TS16 should have only externally powered isolators. But you cannot assume the attached device can adequately power its half of the isolator. Using a selfpowered isolator with the wrong devices can create support problems as more errors and occasional communications outages occur. The “professional” answer is to design in powered isolators; they cost a bit more, but you can be sure they will work reliably for many years. Powered isolators strengthen the signal, while “self-powered” isolators weaken it. 6.2.3 Solution – Ground Design at Devices The lowest cost (but error-prone) solution is just careful ground design. You need to make sure all connected devices share a single concept of ground. This can be done with probable success by any of the following methods: • Make sure all devices plug into a single power strip and make sure you inform support staff that this must always be the case. People are too used to the notion that AC power is power and any AC outlet will do. Study every device, locate its main ground, then literally tie all the devices’ grounds together with large gauge ground wire. This (WHAT?) should be connected to the safety or PE earth in the AC power system. Larger 3-prong devices do this already, but many of the small 2-prong devices (or 3-prong devices with 2-conductor power wire) lack an explicit ground connection. Be sure your support staff knows what they need to do and how. 6.2.4 Solution – Ground By-Pass at TS2, TS4, TS8, TS16 Installing external surge protection on all serial lines near the TS2 can help prevent ground loop damage to the TS2. External surge protection does not stop ground loop problems, but it does offer related surge energy a way to avoid passing through the TS2. The surge may still damage the attached devices, but it should spare the TS2. This solution is appropriate only when ground loops are NOT expected. For example, this could be a good compliment design when the same power strip powers all connected devices. PN: 90000641_B http://www.digi.com/ support/ia Page 26 of 26