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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