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Wednesday, June 20, 2018
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Schneider Modicon M221 Coil Address
Problem writing to a single coil on a Schneider PLC using Libmodbus library

I'm using the Libmodbus library, with a Swift wrapper on an iOS device. I can read coils and registers no problem, but nothing happening when I try to write a single coil. The response comes back as a success, but nothing seems to be happening.

As far as I can see it's sending 5 as a function number and 0 as the coil number.

Anyone else have any experience of this sort with a Modicon M221 PLC and could help me out here?

Actually, M221 returns error ILLEGAL FUNCTION (code 1). That means PLC doesn't support function 0x05 (Write Single Coil). I found that M221 supports only writing to holding registers (%MW) and reading from all kinds of registers (%M and %MW) in spite of the Schneider's documentation.