How does a PC or Touchscreen control...


Thread Starter

Clarence Y


this is my first post and I'm not really a technical person, but I'd like to know something that I've always been curious about. How does a PC or Touchscreen control a PLC or the mechanical parts of a system?

For example, if I have a solenoid butterfly valve set to open or close and the display on my screen is "Open" and "Close"...what exactly happens when I click or touch on "Open?" Does the PC send a direct signal to the valve? Or does it send a signal to the PLC (if there is PLC controlling the system)? What kind of signal is it? Electric or electronic? How different is this set up as opposed to if I just use a direct push button to control the valve?

I apologize for the naive post. I just wanted to learn more.
There are too many different ways a system can be designed to try to describe them all here, so I'll just try to walk you through a typical system.

Since you mention the "PLC" or programmable logic controller, it means you have one (in some systems, control can be directly in the PC that has the MMI).

So in this typical system with a PLC, the MMI is connected to the PLC via some type of digital communications (Ethernet, RS232 serial, RS485 serial, USB, etc., and there are others as well, mostly specific to one PLC manufacturer), but not to any of the actual field devices, solenoids, sensors and so on.

So when you press the "Open" or "Close" button, that affects one of the bits in the next cycle of comms with the PLC. The PLC now knows the operator wants to change the state of the butterfly valve, and can set it's output to the valve accordingly.

You ask what's different between this setup and using a button. There are a couple levels of different answers. Mainly, the PLC can inject other logic and interlocks into the control of the valve, or it may need to know the state of the valve to USE in other logic or interlocks. For example, say this valve is not to be opened except when Pump 23 is on, or Tank Level 17 is above its low limit, or both. The PLC can enforce those interlocks and override the operator's input when appropriate. That could be done with relays too, but that's essentially what a PLC is, is just shorthand for a thousand relays.

Also, the PLC may have other logic that needs to operate the valve at certain times (the valve will open whenever the tank level reaches a certain value and close at a certain level, but can also be manually operated from the MMI).

One final case that happens occasionally is that there is no difference between the PLC and a button. In that scenario, the PLC is in the system to perform logic as described for the rest of the system, and has no direct interest in the state of this particular item and simply follows the operator input, but since the PLC and MMI are already there for the rest of the system, it's simpler and cleaner to wire it as a PLC output and manually control through the MMI. I do the controls for some spray dryers where the inspection light in the drying chamber works like that.