While implementing a protocol between two (or more) oxygen analyzer instruments of different makes, if it has to be MODBUS, what are the recommended function codes, data formats etc. to claim that the complete system is MODBUS compatible?

 

All recommended codes have described in the MODBUS specification on www.modbus.org. You can any function codes, if your device has different node address and you use master-slave protocol.

For testing purposes You can use Advanced Serial Port Monitor with MODBUS plugin from AGG Software (http://www.aggsoft.com). It's shareware.

 

Sounds like a good Modbus.org RFC (if we can ever get that going!)

Minimum functions would be:
- Function 3: for reading 16-bit ints and packed 1-bits
- Function 16: for writing the same

Minimum Data Types:
- 16 x 1-bit values packed into registers (bit #1 is most-significant bit or 0x8000)
- 16-bit registers (can be any signed or unsigned 16-bit)
- 32-bit ints and floats in 2 consequtive registers, LOW word first

I won't design such a limited product myself - writing 1-bit values will be tricky. But the above should be considered the absolute BARE minimum for a "Modbus product".

best regards
- LynnL, www.digi.com

 

I suggest that you start at the web site http://www.modbus.org