Too fast for CLICK PLC?

[WaxhawFive]

I have a simple timing application that the customer wants to solve with an Automation Direct CLICK PLC. I need to "measure" the time difference of two different discrete inputs. The expected time gap will be from 0 to 60ms. I know that resolution will be a function of scan time, since there is no high-speed timer module, so I'll be essentially counting scans between the two inputs. If I can get 10 counts in 60 ms, that will be enough resolution. It will be a very small program, so do you feel that I can rely on 6ms scan times or faster from this PLC? Or should I punt and suggest some other hardware?

 

[controlsdude]

If you look at the input module specs they give you a max 20msec (typical 5msec) for the input module point to respond to an input changing state. Just on the input spec you cant meet the customers needs.

For you to sample correctly the scan time needs to be 1/2 of the input time. So you scan time needs to be 3msec to pick up the input correctly without missing it.

you need a high speed counter input.

 

[controlsdude]

sorry mispoke

your looking for a change in 60msec? Just as long as the input changes at 2x in that time you ok. then the scan time needs to be 15msec to be safe. Or at the longest 25-30msec. Scan time is for continuous scan is based on the processor and how many instructions you execute. Each instruction eats up so much scan time to execute.

 

[WaxhawFive]

Your first answer was correct, I need to at least divide the 60ms time into 10 6ms pieces, so that seems too fast for this unit. I'll have to look for an inexpensive high speed solution. Any ideas?

 

[controlsdude]

peruse that website for the plc that gives you a high speed input counter. Most shoe box (brick type) plcs like this will have a cheap plc that does this. Then you dont have to worry about scan time.