from56
2021-05-06 23:56:49
Modbus does not define how the data types are sent, it only defines the sending of one or more 16-bit words and also bits.
As these registers are interpreted as signed or unsigned integers, or as two consecutive registers are interpreted as a float and even less as 3 registers are interpreted as time, then each implementation of the protocol does as it thinks best.
For this reason, when a float is sent, sometimes the most significant word is sent first and sometimes it is the opposite.The Modbus client should be prepared to invert the words in this case.
And in the case of transmitting a time, it can be even more complicated.
Agree !. Modbus doesn't know the data type. As result, we have defined the datatypes & conversion. Question is looking forward more into the conversion method results correct or wrong.
@Narayanan because the standard does not define data types other than 16-bit words and bits your question is unanswerable (there is no 'correct' method). There are multiple ways of implementing Char, Float and Time; the correct way will be whatever way the manufacturer of the device you are communicating with chose! (if communicating with multiple devices you may well need multiple encoding/decoding methods). I find this calculator useful when trying to work out what encoding has been used.
@Brits, You clarified very clearly,, Thanks !!