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General => Technical support => Topic started by: Christian Ziedins on October 29, 2003, 09:29:17 PM

Title: force a single bit through Modbus
Post by: Christian Ziedins on October 29, 2003, 09:29:17 PM

I'm currently testting the T100MD with the Matrikon OPC server for Modbus.
I can't find a way to force one single output bit without forcing the whole row (the 16 bits)
The manual (page 5-3) seems to suggest that modbus function code 05 (force single coil) is supported.
A possible workaround would be: read the whole row --> do an OR calculation --> send the result back......if no better solution I will stick to this one but I don't like the idea of overwriting the 16bits each time..
Any suggestions ?

Cheers,
    Christian

Title: Re: force a single bit through Modbus
Post by: support on October 30, 2003, 07:00:32 AM

The M-series PLCs do support MODBUS function 05 which is to force set/clear a single output bit. However, your OPC server must support function 05 in order to do so. The bit address in this case can be found in the following link:

http://www.tri-plc.com/appnotes/modbus-map.pdf

This is an updated document of Chapter 5 of the user manual which now include the document on the Bit addressing. To turn ON output 1, execute a Function 05 at address 257.