Pulse Counting - Flow Conversion

I'm a real novice so please be kind. I have an Automation Direct DL-450 PLC. I plan on taking the pulse outputs from a flowmeter into the DC Input Module, counting them and displaying the flowrate (gpm)on a touchscreen. My question is can I use the built-in counter (RLL) or do I need to add a HSC module instead of a simple DC Input?

Expected pulse rate is in the range of 5-50 pulses/sec.
I would like to update the flow display every 1 or 2 seconds - any problem with updates this fast?

I think you are going to want the high speed counter. At 50 Hz your interval on pulses is 20 msec, which may approach your scan time. Also, ths specs on a standard 24 VDC input are 10 msec off to on and 12 msec on to off, again getting very close to the period of your pulses.

I haven't used this but - is the F4-8MPI Magnetic Pulse Input Module appropriate? The description mentions flowmeters.

The ol'man at CPU-Central might end up spending a lot of time waiting for those signals. He might even catch a few winks!

It depends on a couple of factors...

-how large is your program (time-wise)
-how long is the pulse (time-wise)

The shorter your scan time, the more often the ol'man can check for pulses.

The longer the pulse-duration the more likely he is to catch it.

In general, the ol'man needs to be able to "look" at twice the rate that the pulses might occur, and the pulses need to be long enough to be caught.

Bear in mind, the ol'man should watch for the pulse "occurring"... not simply "existing". That is, if the ol'man looks and sees the pulse signal on during this scan and then, on the next scan, looks and sees the pulse signal again... did two pulses happen? Or was it just one long pulse?

You need to watch for the pulse signal GOING ON. As in... it was OFF, now it's ON.

And, of course, like Lefty says, if your input responds slower than the rate of the input signal, then you might...

- never see the input go ON,
- never see the input go OFF (once it goes ON),
- or you might receive what appear to be sporadic pulses.

Timing is everything!

Place yourself in a small world where you can slow time down to as low a rate as you need (rate of time???) so you can "see" whats going on and examine the extreme cases.

A High Speed Counter makes a moot (<--take note, RSD) point out of the question.

I actually have done as you suggest, to use a standard PLC digital input and use it for flowrate calculation, based on impulses from a flowmeter. But the max pulse rate was approx 10 imp/sec.
The PLC was AB SLC5/04 and later S7-400.

Any pulse rate faster than that (or less if the PLC has a longer scan time) will require that you use a dedicated counter.
The good news is that dedicated counters often have automatic flowrate conversion of the input signal built in.

Some small brick PLCs have one or two dedicated counter inputs, so you could use one of these, instead of purchasing an expensive counter module.