Operating Instructions

Communication via RS485
5.4 Communication using Modbus RTU
Fieldbuses
144 Function Manual, 04/2018, FW V4.7 SP10, A5E34229197B AE
5.4.8
Communication procedure
Procedure for communication in a normal case
Normally, the master sends a telegram to a slave (address range 1 ... 247). The slave sends
a response telegram to the master. This response telegram mirrors the function code; the
slave enters its own address in the telegram and so the slave identifies itself with the master.
The slave only processes orders and telegrams which are directly addressed to it.
Communication error
If the slave detects a communication error on receipt (parity, CRC), it does not send a
response to the master (this can lead to "setpoint timeout").
Logical error
If the slave detects a logical error within a request, it responds to the master with an
"exception response". In this case, the slave sets the highest bit in the function code to 1 in
the response. If, for example, it receives an unsupported function code from the master, the
slave responds with an "exception response" with code 01 (illegal function code).
Table 5- 32 Overview of exception codes
Exception
code
Modbus name
Remark
01 Illegal function code An unknown (unsupported) function code was sent to the
slave.
02 Illegal Data Address An invalid address was requested.
03
Illegal data value
An invalid data value was detected.
04 Server failure Slave has terminated during processing.
Maximum processing time, p2024[0]
The slave-response time is the time in which the Modbus master expects a response to a
request. Set the same slave-response time (p2024 [0] in the inverter) in the master and
slave.