PLC versus PC with I/O cards


Thread Starter

Andy Hill

A simple control system that will switch on and off some valves (10) and report some data all from a Touch screen LCD display.
System will not have to think just convert analog into pressure etc.
Valves will be manual from the screen not as part of algorythms.

The System will have the following i/o:

44 Digital Inputs
22 Digital Outputs
15 Analog Inputs

1 LCD(TFT) display with touch screen for User interface. ( Hazardous area Classification for Europe Zone 1)

All digital inputs are feedback from (Open & Close) of the solenoids connect to the digital ouputs.


Analogs report some pressure etc.

The question is it cost effective and justified to put this onto a PLC with a seperate PC running an HMI or just add i/o card to the PC.

Second question if it is done by the Cards in the PC method can it still be connectedc to a SCADA package like IFIX or WonderWare.

Shiv Hiremath

One workable solution is to have
1 PC Card on PC for say Profibus
Run Intellution FIX or equivalent.
Depends on your budget if you want to go this way
It works for sure.

Jake Brodsky

Is it cost effective? The question really ought to be "Do you trust your PC well enough to use it as a PLC", or to put it another way: "Can you afford the lower overall reliability common with most Personal Computers and the associated software?"

If you won't lose any sleep over using a package such as InControl on a Windows NT PC then by all means do it. However, you need to understand the failure modes and whether they are something worth risking.

It's not a matter of I/O or capabilities. It's a matter of reliability and fail-safes. Good Luck!

Steve Myres, PE

Sounds to me like this would be a good job for a hardware HMI with some built-in I/O. I know Siemens makes some like that in the Coros series, but I don't know about the explosion proof / intrinsic safety capabilities.
I would give a look to bekchoff system. They have very good soft-PLC, which you can connect by a bus system like profibus to their. I/O Modules. For your application it looks to be well priced to 'cause the I/O system is highly customizable.

For HMI you could consider writing your own HMI-Application in a language like C++ or use one of the proformatted HMI-packages like sietec.

Kind Regards,

Dennis Patterson

With that much I/O. you might want to get a siemens Profibus scanner card and some ET200s I/O modules. just use a cheap scada like siemens protool CS/RT, as something like WinCC may be to costly. In development time, this would be cost effective, and for the customer, it is easily servicable.

Jesper M. Pedersen

Dear Andy,

I put the emphasis on reliability, speed of development, availability (will spareparts be available after 5-10 years), maintainability and price.
For that, I recommend that you pick one of the smaller PLC's from one of the major manufacturers:
Siemens S7-200 + OP170A or
AB MicroLogix1200 + PV600Touch or
Omron or others.
You will get a well working system up and running in no time for a minimum price.
And importantly: Both the OP170A and the Panelview are approved for installation in hazardous areas, which I believe was a priority for you.

If you still need to connect to a PC for datalogging/SCADA then pick a S7-200 with 2 serial ports or pick an AB SLC5/03 with 1 serial port and 1 DH485 port.

Best regards,

GE has a neat little HMI/PC control software called Cimplicity Machine Edition (from the Taylor Industrial people up in Canada). I have been using it for the last couple of months and really appreciate the quality and thorough engineering of this product. You could use a PC/touchscreen to run the graphics and logic, and use ethernet/profibus/devicenet to talk to any remote I/O.

GE also has some cheap I/O called VersaMax that would do the trick. This is actually the kind of application that came to mind when I first started working with this product. It compares with Intouch and Fix and RSView as an MMI, but not at the same high cost...and you use the PC (or touchscreen PC) for both the PLC and HMI.

As stated previously, as long as this is an application that is not critical, remember you are STILL running on a Windows machine. This means you have to deal with viruses, operating systems, registry issues, creative operators, and other evils.