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Transcript
PIP6 APPLICATION NOTE RS485/RS422 PIPCOM RS485/RS422 INTERFACE MODULES 1. Introduction The MPL PIP6 can optionally be equipped with RS-485/RS-422 ports, using PIPCOM interface modules. This application note provides an overview of the RS-485 and RS-422 standards and shows the differences between them. Three application examples show the most common ways of interconnection. 2. RS-485/422 Overview The RS-485 (EIA/TIA-485) and RS-422 (EIA/TIA-422-A) are electrical standards, specifying the electrical characteristics of line drivers and receivers, but not the connector, cable or protocol. 2.1 Advantages of differential signaling Both, RS-485 and RS-422 are data transmission systems that use balanced differential signals. Balanced data transmissions requires two conductors per signal (A and B). Both lines are switched and the logical states are referenced by the difference of potential between them, not with respect to ground. Differential data transmission nullifies the effect of coupled noise and ground potential differences. Both of these are seen as ‘common mode’ voltages (seen on both lines, not differential) and are rejected by the receivers. These facts make differential drivers, in contrast to unbalanced (single ended) drivers, less sensitive in noisy environments, allowing operation at higher speed and over longer distances. The high noise immunity can be improved additionally using shielded cabling. Both standards specify a maximum data rate of 10 Mbit/s and a maximum cable length of 1200m (4000ft.) at 100 kbs. 2.2 RS-422 brief description RS-422 specifies a unidirectional, multi-drop interface, with a single driver and up to 10 receivers. The intended transmission medium is a twisted pair cable with a line impedance of 100Ω. The driver resides at one end of the line and a termination resistor with the characteristic line impedance (100Ω) at the other end. Up to 10 receivers can be distributed between the driver and termination resistor. max. 1200m170,76mm. (4000 ft) @100 kbs Termination 100 Ohm Transmitter Receiver 1 2001 by MPL AG Receiver 2 1 ... Receiver 3-9 ... Receiver 10 MEH-10087-001 Rev. A PIP6 APPLICATION NOTE RS485/RS422 1.3 RS-485 brief description RS-485 meets the requirements for a truly multi-point (multiple driver, multiple receiver) communication network, and specifies up to 32 drivers and 32 receivers on a single 2-wire bus. The principle difference between RS-422 and RS-485 is that the RS-485 driver can be put in a high impedance mode, which allows other drivers to transmit over the same pair of wires. The intended transmission medium a twisted pair cable with a line impedance of 120Ω, terminated at both ends in the characteristic impedance (120Ω). Up to 32 driver/receivers pairs can be distributed between the termination resistors. 170,76mm. max. 1200m (4000 ft) @100kbs Termination 120 Ohm Termination 120 Ohm Transceiver 1 Transceiver 2 ... Transceiver 3-31 ... Transceiver 32 1.4 Combining RS-422 and RS-485 components Many characteristics of RS-485 drivers and receivers are the same as RS-422. RS-485 components (drivers and receivers) are backward compatible with RS-422 devices and may be interchanged. In contrast to RS422, RS-485 extends the common mode voltage range for drivers and receivers during high impedance mode. Also, RS-485 drivers have an extended load current capability and are able to withstand data collisions (bus contentions) problems. However, RS-422 drivers should not be used in RS-485 applications. While PIPCOM RS-485/RS-422 modules fulfill the RS-485 specification for drivers and receivers, they can be used in both, RS-485 and RS-422 applications. 1.5 Grounding Arrangement While differential signal does not require a signal ground to communicate, the ground wire serves an important purpose. Over a far distance there can be significant differences in the voltage level of „ground“. RS-485 networks can maintain correct data with a difference of –7 to +12V (common mode voltage range). If the grounds differs more than that amount, data will be lost or the port is damaged itself. The intention of the signal ground is to tie the signal ground of each node to one common potential. The connection between the ground nodes must contain some resistance (100Ω) to limit circulating currents. The PIPCOM modules additionally use an optical isolation, which separate the transceivers ground potential from the system ground. Therefore the isolated ground of a PIPCOM module may be completely floating with respect to system ground and the network ground. The receiver input resistors of the module will cause its isolated common voltage to go to the mean voltage of its receiver inputs. The isolated common ground of each module is accessible at the connector over an internal 100Ω series resistor. 2001 by MPL AG 2 MEH-10087-001 Rev. A PIP6 APPLICATION NOTE RS485/RS422 3. Application Examples 3.1 RS-422 Point to Point Connection RS-422 is often used to simply extend the distance between two nodes over the capabilities of RS-232. The example shows a 4-wire connection of a PIP6 to an RS-422 device, using 2 PIPCOM modules. The modules have to be configured for the use with one single serial port (full duplex mode, ‘RS485 FD’). One module operates as transmitter, the other one as receiver. Please refer to the users manual of the PIP6 for the correct setup procedure. The RS-422 specification requires a single 100Ω termination at the transmission line end. Since the PIPCOM modules use RS-485 drivers, the line pair connected at the PIP6 transmitter module may also be terminated with 120Ω at each end. Therefore the internal PIP6 termination resistor may be used. The PIP6 receiver module line pair may also be terminated with the internal 120Ω resistor (which may also be a suitable value) instead of an external 100 Ω resistor. Note that this line pair must not be terminated on both ends (RS-422 driver). PIP6 RS-422 Device A3 120Ω UART3 RxD Isolation TxD 120Ω B3 RxD Jumper removed SHLD3 PIPCOM 100Ω 100Ω A4 UART4 Isolation 120Ω TxD B4 SHLD4 PIPCOM 2001 by MPL AG 100Ω 3 MEH-10087-001 Rev. A PIP6 APPLICATION NOTE RS485/RS422 1.2 RS-485 4-wire Master Slave System 4-wire networks have one master port with the transmitter connected to each of the slave receivers on one twisted pair and the receiver connected to each of the slave transmitters on a second twisted pair. As many as 32 driver/receiver pairs can be share a multidrop network. The slave devices are addressed by the master, allowing each node to be communicated to independently. The example shows a 4-wire master – slave system with a PIP6 operating as master. 2 PIPCOM modules are required and have to be configured for the use with one single serial port (full duplex mode, ‘RS485 FD’). One module operates as transmitter, the other one as receiver. Please refer to the users manual of the PIP6, for the correct setup procedure. The RS-485 specification requires a 120Ω termination resistor at each end of the twisted pair. If the PIP6 is connected at one end of the cable, the internal termination resistors may be used. PIP6 A3 UART3 RxD 120Ω Isolation TxD 120Ω B3 Jumper placed SHLD3 PIPCOM 100Ω 100Ω 100Ω 100Ω A4 UART4 Isolation 120Ω 120Ω B4 Jumper placed SHLD4 PIPCOM 100Ω Node 1 2001 by MPL AG 4 Node 2 Node 31 MEH-10087-001 Rev. A PIP6 APPLICATION NOTE RS485/RS422 1.3 RS-485 2-wire Master-Slave Systems In a 2-wire network transmitter and receiver of each device are connected to a single twisted pair cable. As many as 32 driver/receiver pairs can be shared in a multidrop network. The slave devices are addressed by the master, allowing each node to be communicated to independently. The example shows a 2 channel, 2-wire master - slave system with a PIP6 operating as master. 1 PIPCOM module is required for each channel. The modules have to be configured for the use with independent serial ports. (half duplex mode, ‘RS485 HD’). Please refer to the users manual of the PIP6, for the correct setup procedure. The RS-485 specification requires a 120Ω termination resistor at each end of the twisted pair. If the PIP6 is connected at one end of the cable, the internal termination resistors may be used. Node 1 Node 2 Node 31 PIP6 100Ω RTS A3 Isolation RxD 100Ω 120Ω TxD UART3 100Ω 120Ω B3 Jumper placed SHLD3 PIPCOM 100Ω RTS A4 UART4 RxD 120Ω Isolation TxD 120Ω B4 Jumper placed SHLD4 PIPCOM 100Ω 100Ω Node 1 100Ω Node 2 100Ω Node 31 1.1.1 Half Duplex Driver Control In a two wire configuration the driver must be set in high impedance state, when not in use, to allow the other nodes to use the shared pair of wires. The RTS handshake line of the serial port is used to enable or disable the driver. This requires that the host software asserts the RTS line before beginning a transmission to enable the driver and releases the RTS line after the completion of the transmission. If a slave device attempts to reply before the master has put the line in high impedance mode, a collision will occur and data will be lost. Since a two wire configuration connects the driver to receiver in a loopback fashion, the receiver monitors the transmitted data (local echo). 2001 by MPL AG 5 MEH-10087-001 Rev. A PIP6 APPLICATION NOTE RS485/RS422 COPYRIGHT AND REVISION HISTORY Copyright 2001 by MPL AG Elektronikunternehmen. All rights reserved. Reproduction of this document in part or whole, by any means is prohibited, without written permission from MPL AG Elektronikunternehmen. DISCLAIMER The information contained herein is believed to be accurate as of the date of this publication, however, MPL AG will not be liable for any damages, including indirect or consequential, arising out of the application or use of any product, circuit or software described herein. MPL AG reserves the right to make changes to any product herein to improve reliability, function or design. TRADEMARKS Brand or product names are trademarks and registered trademarks of their respective holders. 2001 by MPL AG 6 MEH-10087-001 Rev. A