%M reading with Premium limited to 16bits


Thread Starter


I send a READ_VAR command in Modbus on a Premium PLC.
I read 16bits it's working.
I tried 17 bits, 32 bits, etc. and never could have it work.

Does anyone have already seen this problem?

Actually I read digital informations on a modbus device, not from an A/D converter.

%M is an internal bit information.
By definition, the MODBUS specification declares that a holding register or input register is 16 bits.

MODBUS does not define the format of the data in a register, it defines only the register size, 16 bits.

A 32 bit floating point value requires two 16 bit registers. A 17 bit data word would require two 16 bit registers, too.