Modbus TCP

[Toby Mills]

Has anyone got Modbus TCP to work with a relay board.
I'm finding Loxone is sending 16bit integers instead of a binary bit for the relay and I can't see where you change the type in Modbus TCP in  Loxone config.

Toby

[Jonathan Dixon]

Try using the "write coil" rather than write integer command 

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[Toby Mills]

Thanks, yes loxone config only 4 options for Modbus TCP and I'm using write single coil..


But in the logs this is going out as an unsigned integer.


I just have it connected up to a switch for testing so I'm wondering if switches output integers or if I'm missing something stupid.
Have also tried an analog actuator and different addresses.
I can control the relays when I send raw hex from using a TCP sender. But if I even try and send that hex using the Modbus Test command, it sends it as an integer.


Toby

[Jonathan Dixon]

>But in the logs this is going out as an unsigned integer.

Yes this is correct - at the TCP layer everything is integers, and modbus TCP tends to encode things into 16 bit values.

See https://www.waveshare.com/wiki/Modbus_Protocol_Specification#0x05_Write_Single_Coil - Coil logic "on" is encoded as 0xFF00 which is 65280 decimal which is what you're seeing.

The relay board should be able to decode this, but depending on the relay board there can be an additional layer of shenanigans at play. For example, Waveshare ethernet relays default to modbus RTU over TCP, you need to use some proprietary software to reconfigure them to modbus TCP mode. See https://www.waveshare.com/wiki/Modbus_POE_ETH_Relay#Modbus_TCP_Protocol_Settings

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