User`s manual

FX3U Series Programmable Controllers

User’s Manual - MODBUS Serial Communication Edition

7 MODBUS Standard Commands

7.3 Protocol Data Unit Formats by Commands

This section describes MODBUS protocol data unit formats used by the FX-Series MODBUS Communication

ADP.

1) Precautions:

a) When the FX-Series MODBUS Communication ADP receives a broadcast request message:

Although the processing requested by the request message is performed etc., no response message

is sent to the master.

b) When the FX-Series MODBUS Communication ADP receives a request message in the listen only

mode:

Received messages are evaluated, but no action takes place and no response is sent. The only

exception is the "reset communication option" command (diagnosis 0x08 sub command 0x01). If this

command is received the slave recovers from listen only mode without sending a response, all

subsequent commands to this slave will be answered with a response as usual. For further

information refer to Chapter 6.

2) When the processing is completed in error at the slave (FX-Series MODBUS Communication ADP):

When the processing (read/write, diagnostics, etc.) requested by the request message is completed in

error, an exception code is sent to the master.

"Response message formats (when completed with an error)" in Section 7.4 to 7.18.

a) Storage location of exception code and error code. Refer to Chapter 6 for storage location,

confirmation methods, and other detailed contents.