Ladder counters

[cch1955]

In my application I need to use the ladder logic counters (2) for length measurment taken by sensor wheel running on product. Frequency worst case is 70ms (14hz). I know scan time is dependant on many factors. I am concerned that my frequency may be too high and I will miss counts making length wrong. I am assuming state change for counter input from one scan to the next increments the counter.

Do you have an "guideline" of maximum frequency for the ladder counters based on bare minimum PLC program?

If I had this I could rough out what I may be able to count on with lines of code etc.

Thanks
Chris

[support]

If you only needs 2 counters that must be guaranteed no missing read, try using the High Speed Counter in the  T100MD888+ PLC which can handle up to 10,000 pulses per second.

If you are only using say an E10-npn (provided the programming language is able to handle what you want to do such as computing the frequency) that does not support high speed counter, then the rule is that the maximum scan time of the PLC must be less than the minimum pulse width of the incoming pulses.

For E10, the average scan time per step is about 20 microsecond. So for full program of 216 steps, the scan time is about 216 x 20 E-6 = 4.32 millisecond. So  it can handle incoming frequency of aproximately 100 Hz max.

[cch1955]

I am using T100MD2424. I am using the HSC on two quadrature encoders so must use ladder counters. It sounds like I will be OK.

Thanks
Chris

[support]

Just ensure that you don't have custom function that takes a lot time to excute (e.g. if you have a very large FOR NEXT loop or endless WHILE loop) then you should be OK. Note that the T100MD+ PLC I/O scan time is 2ms. The total scan time = program scan time + I/o scan time, so the pulse width must be much wider than 2ms to guarantee that you will not miss counting any incoming pulses.