SoftPLC Specs


Thread Starter

Joe Montagna

I need to evaluate some softPLC packages - what are the most important specs when choosing a softPLC?

Joe Montagna
Depending on your softPLC application, the first thing to determine is how soft or hard the real time aspect of the project needs to be. Soft real time usually runs as a guest of the operating system (many packages run under NT, some under Linux). Hard real time runs NT or Linux as a guest which allows the system to specify and guarantee minimum latency times far less than soft real time.

To help decide between hard versus soft try the following:

If the computer occasionally takes over 50ms to respond to an event will something get damaged, if the answer is yes, you need hard real time (maybe even go back to a real PLC).

If you are running NT, the next question is what happens if NT blue screens - what does your application need to do. If it can die right along with NT, then soft real time is probably just fine. If it needs to keep right on chugging until someone can push a button to stop it, hard real time is better.

What about networking? Does the computer need to have integral network communication with other devices without going through some fancy interface program? If yes, soft real time tends to do this better.

Finally, what else is the computer going to be doing? Does the computer need to be running other applications that could impact a soft real time system?

Beyond the soft/hard part, the actual packages should be evaluated. Keep in mind how many I/O points you will be using, the size of the application code, and what must it interface to: I/O interfaces, Human-Machine-Interface(ie. graphics), and other computer applications. Lastly, if it is a big project, don't forget training, many of the big name packages assume that training will be purchased and their supplied documentation re-enforces that mind set.
PLC were not designed to be number crunchers. In other words, they rate low on the maths capability if any at all. The microprocessor does not tell how it will digest maths.
Unless you specify what that soft is intended to perform and what you are been offered, help is no help.
Keep in mind form of separation between the worker and the supervisor.
Generaly, extra software are just add-on. There is a or more weak link, because good integration means redesign.
Simply be nasty in your evaluation.
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