Writing Bits in Modbus

Hi folks

All my various motor control bits and setpoints are mapped to holding registers in a Schneider PLC. I can read/write the setpoints fine. But I can only read the bits. Attempting to write to them causes an error.

Here’s a typical address

ns=1;s=[Acid Dosing PLC]HR201.8

And the error generated

Could anyone help me diagnose the error? Is it something I’ve messed up in Ignition or is it possible the Schneider doesn’t support bit writing in holding registers.

It might not support it - check the console in the gateway for error messages when you attempt to write.

Looks like you’re right. Do I have any other options?

Derived tags or scripting functions that do the appropriate masking.

The derived tag approach is discussed here: Troubles making a Derived Boolean Tag work as a mask for a Short Tag

Thanks for that Kevin. I’ll take a look.