Download Modbus for Field Technicians

Modbus for Field Technicians
11. MULTIPLE CLIENTS OF A MODBUS SLAVE
We are frequently asked how you deal with a situation where you have more
than one client for a slave(s). The Modbus spec does not support this but we
have a solution.
The essence of the solution is to use a multi-port FieldServer. Connect each
client to its own port and the slave(s) to their own ports. Each client will see a
single virtual slave(s) on its network. This not only solves the problem but is
extremely efficient. Of course the FieldServer needs to be correctly
configured.
In a situation like this we exploit the FieldServer technology known as ‘Port
Expansion’.
Figure 1: Normally it is not possible to connect two clients to the same slave.
There
are
two
primary
reasons:
1) If you are using RS232 then there can only be two devices on the cable
segment.
2) If you are using RS485 then the 2nd client will not know to process the poll
from the 1st client. It will cause errors.
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