ABB CM50 Process Controller Register Address

Check the downloads here, you might find something....

http://www.abb.us/product/seitp330/cdff2a69477a394bc12576e9004c69f8.aspx

cannot find something of help from the manual. the provided manual of the CM50 controller have all the necessary, except for the register address given (7146) which was not operational when programmed as "read-write" register.

thanks and much appreciated for your effort

1) function code
There is no mention of supported function codes/commands in the manual.

It sounds like you can write to register 20000 to change from auto to manual and back, correct?

Presumably you've tried both function code 06 decimal and 16 decimal (10 hex) to write a floating point value to 7146?

2) data format
register 7146 Loop 1 control output is under 5.10.3 Writeable Single Precision Float so presumably you are writing a 32 bit floating point value (not a single register 16 bit integer) to the register.

Are you certain that the byte and word order are the same for writing a floating point value to register 7146 as you can read from 7146?

You can always try one of the generic Modbus Masters, like Modscan32, to try writing in various formats.

What happens if you try writing to register 6638 Loop 1 local setpoint 1?
Does that work OK?

3) One offset in the addressing
ABB uses decimal register addressing starting at zero. Are you sure 7146 is not really 7147 to your SCADA?

It pays to read a known value first (like controller output in manual mode at a fixed value), and determine whether the value makes sense to make sure that the format and address offset are correct. I find that it's more typical for decimal addressing to start at one, but addressing is what it is. You just have to adapt to it.

4) Manual Mode
One would expect that it makes no sense to write to a control output in auto mode, because at the next PID update the output value goes to whatever the PID calculates. I, too, would expect the ability to write to the control output register in manual mode.

But I worked with another brand controller which refused Modbus writes in manual mode. Their argument was that only the local keyboard should have control during manual so that the operator doesn't have to 'fight' the comm link for manual control. "Manual means man means a guy standing there at the keyboard!"

They subsequently changed their manual to indicate that the controller output was a read-only register.

If your HMI/SCADA package can write a floating point value to some other register (like 46638) in auto mode, then the HMI/SCADA Modbus master write function is proven OK.

If the CM50 refuses to accept a write to register 7146 in manual mode then that's an issue you'll have to take up with ABB, because they're the ones who write the firmware.

"Their argument was that only the local keyboard should have control during manual so that the operator doesn't have to 'fight' the comm link for manual control. "Manual means man means a guy standing there at the keyboard!"

sounds good! this is really of great help to my understanding. no more issues.

thank you and very much appreciated

regards