MICRO800 CCW Modbus messaging

[LogixLad]

I'm still a beginner when it comes to PLC's programming in general but I've had a fair bit of experience with SLC 500's and micrologix, but I've been playing about recently with the micro 800 and I was doing some modbus messaging to an ABB Acs550, it took many reads and rereads of the manuals to get it going and I can read holding registers and write to coils just fine. However the only way I figured out how to write to coils was mostly just by guessing..

So CCW uses words to read and write modbus messages, which was confusing coming from RSLogix 500 where it uses bits. The part I don't understand is why I have to write 1,2,4,6,8, etc to set coils 40001,40002, 40003, 40004, 40005? This may seen very basic to some people but after some research I still can't figure it out.

[drbitboy]

Welcome to the forum!

Which Modbus Function Code are you using to write to the output coils: 05 (0x05; Write Single Coil); or 15 (0xF; Write Multiple Coils)?
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[LogixLad]

I've been using function code 15

[drbitboy]

At this link, you can see and example how the Modbus protocol encapsulates the values of each group of eight coils into one byte comprising eight bits, and each bit of a byte corresponds to one coil to be written. The mapping may seem a bit () odd, as the 16-bit words in the Modbus protocol are supposed to be MSByte-first, but it does work.

If you do not understand how bits compose bytes, then let us know.
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