Dual Channel Mode - rotork Modbus Technical Manual

Modbus MFU Option Card Installation Manual
The Single highway with inbuilt Isolating Repeater version of the Modbus Module introduces
propagation delays as mentioned above. Care must be taken in any system design to allow for these
delays.

6.4 Dual Channel Mode

The dual channel version of the Modbus Module has two ports and allows two data highways to be
used for communication from the host system to the module. These highways operate independently
with a limit of 32 devices per segment and the ports on the module communicate with their associated
highway. There is some interaction between the ports since a single processor serves them and the
following points must be noted:

The module only has one setting for the unit's communication speed and format. Both the
comms ports share one set of common values for these parameters. The channels may
have the same address or may be different.

When communication is active, every device on the highway reads all the data request or
command messages. Each unit then responds only to those messages aimed at its
address.

Port 1 has priority over Port 2. Since every message is read, the host system must allow
an idle time on Port 1 to allow Port 2 time to read its messages completely.

A minimum time gap equivalent to 11 bytes of data should be left between data requests
on both highways. This will ensure that the processor has sufficient time to respond to
messages directed at Port 2. Without this idle period messages directed at Port 2 may
not be acknowledged and a response may not be generated.
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