Modbus MFU Option Card Installation Manual
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Publication PUB091-004-00_0918
4.5.3
Single Highway with Internal Repeater
With this option a single highway passes through the Modbus Module and an internal repeater
regenerates the data signals. The input to output connection is also isolated and this then allows the
next section of cable to reach the maximum specified distance and the number of connected devices
on the section to be 32. Biasing and termination resistors will be required at all repeater connection
points.
Each repeater has a small propagation delay (micro seconds), this should be taken in to consideration
in systems design when using multiple repeaters
– for example when creating a ring network.
Data line 1A is positive with respect to data line 1B when the MFU is transmitting
a ‘1’.
The internal repeater supports data speeds from 300 to 115,200 baud.
The repeater operates in both directions such that communications that appear on channel 1 will be
repeated onto channel 2, and communications that appear on channel 2 will appear on channel 1. If
the message is addressed to a device, the message will both be repeated and processed by the
Modbus Module. The reply will appear, only, on the channel side where the message came in, i.e. a
message received on channel 1 will be repeated over to channel 2 and once the message is
processed the reply will appear only at channel 1. This is to avoid issues in a ring network that may
be caused by the same message being received at both ends of the ring.
Modbus Ring, using Rotork K-Range and HIMax controller
Channel 2
A out
B out
Channel 2
Channel 2
Channel 2
Channel 1
Channel 1
Channel 2
Channel 1
Channel 1
Channel 1
Each actuator is fitted with a
Modbus Repeater card
Here is a typical Modbus ring configuration. The PLC normally communicates with all the actuators on
the A out channel. If there is a break or short-circuit in the cable between any of the actuators, the B
out channel will communicate with the actuators beyond the damaged cable.