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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.
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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.
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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.
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Using Digi Products in the Real World:
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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
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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:
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•
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
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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.
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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.
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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.
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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
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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
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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
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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-
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Using Digi Products in the Real World:
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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.
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Using Digi Products in the Real World:
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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.
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Using Digi Products in the Real World:
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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.
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Using Digi Products in the Real World:
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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.
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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
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Using Digi Products in the Real World:
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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-
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Using Digi Products in the Real World:
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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.
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Using Digi Products in the Real World:
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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.
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Using Digi Products in the Real World:
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•
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.
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Using Digi Products in the Real World:
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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.
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Using Digi Products in the Real World:
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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.
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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
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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.
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