ISA Tag for a PLC 1

[jcfoley]

This a follow post to my "P&ID Symbology" last week. I have worked through ISA S5.1 and created the tags for my instruments with one major exception. I do not see what tag should be used for a PLC. I have used YC for an annunciator as it is really just a state controller. A plc is quite different however. Any suggestions on how to handle this?

Chris Foley
Midland, TX

 

[jsummerfield]

You can tag systems, cabinets and components with a U prefix and a C suffix. Process loops have hardware tags for the measurement instruments and final control elements. Typically the system I/O uses the tag of the associated field instrument. The P&ID normally includes a tag for the operator interface based upon the old hardware instruments.

Example:
A flow loop may use a measurement element such as an orifice plate, V-cone, etc. tagged FE and a transmitter tagged FT. The wiring to the analog input is labeled with the FT tag. The PID loop available on the Wonderware or such HMI is tagged FIC for flow indicating controller. This is shown with a diamond inside a square for the PLC and a horizontal line that indicates that it is on an operator display. The analog output to a control valve transducer is labeled FY or perhaps the valve tag FV.

John

 

[jcfoley]

John,
Thanks for the response. Just to clarify, the one example that I am using is a simple thermocouple. I would label the Thermocouple as TE, the PLC input (with the shared diamond symbol) as TIC with the solid line. This would imply that the OIT and PLC are a single system. Then for the Pressure Transmitters, I would used the nonshared circle & PIC with the shared diamond symbol. Does this sound correct?

Thanks,
Chris

 

[jsummerfield]

I would show the operator interface using the shared diamond inside square symbol in each case if there is an operator accessible function. One could also use the DCS symbol. The PLC and DCS technologies are quite similar these years. We often use the square with circle inside for regulatory/sequential or normal control and the diamond in a squre for a segregated process safety shutdown system. On the P&ID we typically do not differentiate the PLC system from the operator interface - unless there are PLC points with no operator visible/accessible information.

John