Modbus Registers

[Lare]

I found an old manual from 1981 for the Modicon Micro 84 and found I misspoke.

The addressing is decimal, not octal. The numeric data in 3xxx and 4xxx memory is interpreted as BCD. That particular model also includes 2xxx memory addressing for sequencers.
None of the above has any bearing on the Modbus protocol. The Modbus spec doesn't care about how the data transmitted is to be interpreted. And there aren't any Modbus command codes for 2xxx memory.

On old Modicon Bits inside words where also swapped. So if you readed word address bit %MWx.15 on old PLC it pointed on PLC to address %MW.x0 or %MW.x7 not sure anymore if it was 0 or 7.

Still if you readed whole word register on word command then value was same on both new (recent PLCs) and old modicons.

This was probably from endian difference and byte swap.