How to Interpret DCS and PLC Symbols on a P&ID

Learn about how to correctly interpret PLC and DCS symbols on a P&ID.
Listen to this article

In this article, we’re going to talk about how to correctly interpret PLC and DCS P&ID symbols on a typical instrumentation drawing.

Before we get started, you might want to review 2 of our other articles called How to Read a P&ID and What is an HMI?.

Alright then…let’s get started.

A P&ID is a road map

Anybody such as Instrumentation & Control engineers, technicians, and maintenance personnel working on industrial plant control systems depend heavily on P&ID’s.

A P&ID is like a road map telling us what the instruments and devices are, where they are located, and how they interact with the rest of the plant.

P&ID is a Road Map

P&ID symbols

Original P&ID symbols

Original symbols and terminology for P&ID’s were created long before the introduction of PLC’s and DCS’s in plant control.

P&ID Original Symbols

Old industrial processes and control rooms

Before computers came along, process control loops were stand-alone. In other words, each process had its own individual controller.

Control rooms were massive and had sprawling control panels with lots of controllers and other devices such as strip chart recorders and switches.

Old Control Systems

New industrial processes and control rooms

Today’s industrial processes are controlled by computerized systems using virtual controllers found in PLC’s and DCS’s.

New control rooms are filled with HMI’s and keyboards connected to a DCS or a PLC system. All of this computerization required that new P&ID symbols had to be developed.

New Control Systems

P&ID symbols for PLC and DCS

P&ID symbols for DCS

So, let’s look at the P&ID symbols for PLC and DCS. If you recall, stand-alone instruments are indicated on a P&ID by a circle with a tag number.

The horizontal bar across the middle of the circle indicates the physical instrument resides in a primary location accessible to an operator on the main control panel.

Instruments P&ID Symbols

If we take that same symbol and draw a box around it, it now means that it is no longer a physical instrument. It is now part of a shared display and shared control in a DCS.

No Physical Instrument
P&ID symbols for DCS on HMI

Ok…Let’s look at a P&ID with DCS symbols and see how it links field instruments with the DCS and how it’s displayed on the Operator HMI.

LT501 and FT501 are 2 field instruments both sending electrical signals to DCS controllers.

P&ID Field Instruments
P&ID symbols for DCS on Flow Control Loop

Let’s take a closer look at the Flow Control Loop. Flow Transmitter FT501 sends an electrical signal to the DCS Flow Indicating Controller FIC 501, which in turn sends an electrical signal to operated Flow Control Valve FCV501.

Operating A Control Valve

Here’s what the Operator sees on the HMI screen…We’ve put a big red circle around FIC501 and FCV501.

As we discovered on the P&ID, the controller FIC501 is part of the DCS. FIC501 controller functions can be adjusted at the HMI by the Operator.

All FIC501 inputs and outputs can be observed in real-time on the HMI as well.

P&ID Diagram
Real_Time Observation on HMI

Ok…Let’s drill down a bit deeper and look at how the P&ID relates to the Flow Loop Diagram. We’ve placed green circles around FT501 and FIC501.

Referring back to the P&ID, you will see that FT501 is a Flow Transmitter in the field, and FIC501 is a shared control in the DCS.

This is a good time to point out the blue circle around the Software Signal lines as they show how the DCS connects with the HMI.

P&ID and Loop Diagram

P&ID symbols for PLC

PLC symbols don’t show up on P&ID’s nearly as often as DCS symbols. But, when they do show up, they are drawn as a diamond inside a box.

All you have to know is that the symbol represents a software instruction to perform some function in the plant.

P&ID PLC Symbols

For example, let’s revisit our P&ID. The PLC symbol indicates that the SDV503 valve is operated by the Emergency Shutdown software instruction written in the PLC program.

Emergency Shutdown Software Instruction


Let’s review what we’ve discussed today:

– DCS and PLC symbols became a necessity with the computerization of process automation.

– If you see a square box drawn around the circle on a P&ID, that means a DCS is part of the control process.

– If you see a square with a diamond in it on a P&ID, that means a PLC is part of the control process.

Want to Learn More?

If you would like to get additional training on a similar subject please let us know in the comment section.

Check back with us soon for more automation control topics.

Got a friend, client, or colleague who could use some of this information? Please share this article.

The RealPars Team

Join the Top 1% of Automation Engineers

Start your 7-day free trial

Learn from Industry Experts

Start your learning journey today!
With a 7-day trial, then 25/month
Join for Free