Modbus protocol function codes


Thread Starter

Vatsal Trivedi

I was reading Modbus protocol specification version 1.1b3. It is mentioned that function codes 1 - 65, 73 - 100, 111 - 127 are public function codes and they are publicly documented except function codes 9, 10, 13, 14, 41, 42, 90, 91, 124, 126, 127 as they are reserved function codes. I want information regarding the function codes which are publicly defined but not reserved such as function codes from 30 - 40. The document does not contain any information regarding it. Kindly mention where can I find the relevant information......

Lynn August Linse

Those codes are reserved - there is no temple of Modbus priests with secret documents. :)

In truth, those functions probably were used by Modicon, Gould, AEG, or any of the other older owners of the Modbus flame. Likely most are esoteric, device specific function, which for all practical purposes are obsolete (like the drum-sequencer memory range of '2x')

Someday, could issue a NEW design for those codes.

For your own functions, there is of course the range like 100 (etc).

Fred Loveless

I have been working with that protocol for many years and I do not believe I have ever seen those particular function codes specified for any device. I suspect that were put in place to allow for expansion of the protocol.