Installation, Operation and Maintenance Manual
MAN712 Rev. 9
June 2021
Modbus RTU Interface
Section 8: Modbus RTU Interface
69
8.3
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 17
PLC
Transmitter
ELBS-20
Actuator
Master Device
Slave Devices
8.3.1
Modbus RS485 Transmission Mode
The ELBS-20 Modbus RTU module uses a half-duplex, multidrop, serial communication line
RS485. The module communicates with the masters via RS485 interface.
Transmission speed from 600 bit/sec to 38400 bit/sec 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 colors 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.
General I/O
