YOKOGAWA UPM100, User Manual

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5-7
IM 77C01H01-10EN
MODBUS Communication
5
5.1.4 Responses from Slaves
The UPM100 receives a command message from the higher-level device. If the received
command message is found to be normal and directed at the station number of the UPM100
itself, the UPM100 concludes the content of the message to be normal. Thus, the UPM100
enters the phase of executing message processing, deciphers the content of the command
message, and processes with the message.
The UPM100 does not execute message processing, however, if the received command
message is found to be abnormal. In that case, the UPM100 either ignores the received
message or creates a response message telling the received message is erroneous.
After receiving a normal command message and executing a given process, the UPM100
creates and sends a response message to which error check data appropriate for the
command function code of the higher-level device is added.
Responses to Normal Messages
For a loop back function or a function for writing to a single register, the UPM100 returns the
received command message as a response message.
For a function for writing to multiple registers, the UPM100 returns part of the received
command message as the response message.
For a readout function, the UPM100 adds the read data to the ends of the station number
and function code of the received command message, and returns the message as the
response message.
5.1 Overview