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Sunday, May 27, 2018
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%M reading with Premium limited to 16bits
Cannot read more than 16 bits on a PRemium with READ_VAR command

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?

How many bits does the analog to digital converter use?

From your test, apparently 16 bits...


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.