Section 6: Modbus Wirings; Rs485 Transmission Mode - Emerson Bettis XTE3000 Installation, Operation And Maintenance Manual

Installation, Operation and Maintenance Manual
VCIOM-15081-EN Rev. 7

Section 6: Modbus Wirings

The maximum number of master and slave devices in a BUS segment is 32 without repeaters.
The maximum cable length depends on the speed of transmission. Higher is the speed shorter should
be the cable length.
The figure below shows a Modbus RTU configuration with 1 master device and different slave devices.
Figure 4
6.1

RS485 Transmission Mode

The XTE3000 Modbus RTU module uses a half-duplex, multidrop, serial communication line RS485.
The module communicates with the masters via RS485 interface and the transmission media consists
in a shielded twisted pair cable. Transmission speed from 600 bit/s to 38400 bit/s is available.
One unique transmission speed is allowed for all devices on the BUS when the system works.
All devices are connected in a BUS structure. Up to 32 stations (master and slaves) can be connected
in one segment. Repeaters should be used to extend the number of devices on one BUS.
Addresses range is from 1 to 247. Address 0 is reserved for broadcast messages.
The BUS must be terminated by a resistor of 120 Ω at the beginning and at the end of each segment.
Only two terminations in one BUS segment must be provided. To ensure error-free operation and to
increase driving capability, pull-up and pull-down resistors should be provided on the termination
network. The maximum cable length depends on the transmission speed.
The data lines must not be reversed. To avoid polarity inversion, it is suggested to use different
colours for each data line. Use of shielded cable is mandatory for having high system immunity
against electromagnetic disturbs. The data lines should be kept separate from all other cables.
It should be laid in separate, conductive and earthed cable trunking. It must be ensured that there is
not voltage difference between individual nodes of BUS.
Modbus Wirings
Transmitter
General I/O
Slave Devices
Section 6: Modbus Wirings
Master Device
PLC
Actuator
Actuator
October 2022
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