normal input & high speed input

It is usually the time each type of input takes to switch on when presented with a 'true' or '1' signal.

"Normal" inputs may have an input switching time of 1mS or more. In many cases additional delays may be desired to eliminate 'contact bounce' so provided your process can accomodate it, delays of many milliseconds may be common.

If we have an input which takes 1 millisecond to switch on, and say, 1mS to switch off, then the fastest signals you can detect in sequence will be 1 every 2mS, or 500Hz. For a high-speed counter you may have to deal with signals arriving at 20KHz, 30KHz, 40kHz or higher. Obviously the circuitry in that input must be able to switch on and off in very much shorter times - down to a few microseconds.

How they do that goes much too deep in semiconductor design for me!

regards

Ken

A normal input has a capacitor as part of it's noise immunity circuit. The cap gives the input a slow response.
The cap is removed on a high speed input. A good example for a high speed input application is an encoder feedback.
Edit:Typing while Ken posting.
There is other components and software involved in the buffering of a Normal input, but the cap is the major part.

Another major difference is that the high speed inputs operate outside the normal scan cycle of the program. So even if the program takes 30ms per scan it can still cope with inputs into the kHz range on the high speed count input.

Bryan

Have we completed your homework for today?

Excellent point about scan time.

Warning - OT -Soapbox.
I will take issue with the student jab.
I trust the instructor to be smart enough to recognize a student that doesn't do the work. Full blown answers here won't be enough to compensate. I wish people would quit trying to be hall monitors, and just answer the question. Others will search this forum for years and benefit.
My answers are free to anybody, same as Google. Students welcome. I believe my Creator gave me knowledge. I got it for free, I give it for free. WWJD?

BryanG said:

Have we completed your homework for today?

Click to expand...

http://www.plctalk.net/qanda/showthread.php?p=237141#post237141

Not sure if he's a student.



Another note, with some PLC's you cannot access the input directly, instead its linked to a counter and you see the count value and can reset it.