FMC Technologies MPU B Series - Hardware Interface Details; Timing Considerations for RS-485; Modbus on TCP;IP

MPU Dispatch Check Procedure
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2.1.3. Hardware interface details
The actual hardware interface used can be one of the following:
RS-232 (Full Duplex)
2-wire RS-485 (Half Duplex)
4-wire RS-485, i.e. RS-422 (Full Duplex)
2.1.4. Timing considerations for RS-485
For the Half Duplex RS-485 (2-wire) interface, the following
timing considerations must be taken into account by the software
on the ModBus master:
There must be at least a 20 mS "silent interval" from receiving a
ModBus reply from the MPU, before the next ModBus request
message issued by the master to the MPU. The software on the
ModBus master will typically need to follow this pattern of
communication:
Send a request message A to the MPU
Read the reply message from the MPU
Wait for at least 20 mS
Send the next request message B to the MPU
...etc...
This will ensure that the MPU has sufficient amount of time to
change the driver direction before the master sends the next
request.
2.2. ModBus On TCP/IP
The MPU Series B ModBus on TCP/IP implementation conforms
to the definition in section 1.2 References [2].
The following requirements apply to the ModBus Application
Protocol (MBAP) header used for TCP/IP:
Fields Length Client
Transaction Identifier 2 bytes Must be 0
Protocol Identifier 2 bytes Must be 0
Length 2 bytes Minimum 6
Unit Identifier 1 byte Must be 1