When programming software (ie. ProWorx) communicate with Schneider PLC on Modbus, often Function Code 126 (Hex 7E) are used. Does anyone know some detail in function code 126 or tell me where I can find more detail in this private modbus function code. I really appreciate any help on this matter.

 

Frankan,

This MODBUS Function Code is used in MODBUS TCP/IP, to have full control over a MODBUS Slave Device, including programming/re-programming if the said MODBUS Slave device is a PLC.

You need to take extreme care when designing a MODBUS Ethernet Network, that this Function Code can only be accessed by certain known and trusted devices in the network.

This is not a "private" Function Code. You only need to read more and dig out more information about MODBUS, since it has slightly changed since the days of the conventional Serial MODBUS ASCII/RTU.

Regards,
Ashraf

 

Annex A of the MODBUS Application Protocol Specification states: "The following function codes and subcodes shall not be part of this published Specification and these function codes and subcodes are specifically reserved."

It then lists a series of codes including 125, 126 and 127. This does indeed appear to be a "private" code, in that it is not defined by the spec.

 

Thanks Ashraf,

I have an application that need to write to 1xxx & 3xxx inputs. I understand I can use function code 126 (7E Hex) to do this. I wonder do you have any more information or where I can find out more on the format in using function code 126.

Regards
Frankan

 

Frankan,

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