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cwalter-at / freemodbus

BSD licensed MODBUS RTU/ASCII and TCP slave
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queston about eMBFuncReadInputRegister #16

Open glct12fsl opened 4 years ago

glct12fsl commented 4 years ago

at line 71 of mbfuncinput.c there is an increment of usRegAddress variable before calling at line 96 the function eMBRegInputCB(). i think, the function eMBRegInputCB() will read the next register address of the desired.

George

yannickasselin commented 4 years ago

I noticed the same thing. Every read/write functions have this increment of usRegAddress. So when testing modbus TCP using Codesys I always end up reading/writing the wrong registers (Offset by 1 more than desired). I know some modbus device registers start at 1 others starts at 0. This may be the reason why but I don't quite understand. Can anybody give an explanation?

karlp commented 4 years ago

freemodbus does this +1 to just hard code all values to be "register numbers" ie, 1 based. If you want to work with "register addresses" ie, 0 based, you must subtract one in the beginning of your user callbacks. Should this be configurable? Maybe. Would it perhaps have been simpler to not do the +1? Maybe, but that's how it is.