AB remote I/O


Thread Starter


Hello there,
We have an application where we would like to place a high speed counter card in a remote I/O rack. My concern is communication speed in relaying the counter values from the counter card back to the plc.

The field devices are flow meters. I don't know exactly what the pulse train frequency is. Maybe around 5 khz. Can someone explain the steps involved in retreiving the counter values from the counter card and resetting the counter register once the plc has retreived the current value. What is the most accurate way of recording flow totals to a register in the PLC?

Thank you in advance.

Roland Lilja

Hi there,
I realy dont know anything about the AB-PLC but i know in general terms som about counter cards. In the first place the object of using a counter card is to captuer pulses that are shorter than the "Scantime" for the PLC-program (Else you could used a digital input insted) OR you use a countercard along with a incremental sensor with two fase shifted pulstrains to count upp/down for positioning reasons.

Now to your concern about comunicationspeed. The probably slowest thing in the PLC is usualy the "Scantime" witch means that the speed of the running program is the most critical factor for the accurasy of the messured value. For example: If you use a signal with the frequency 5 Khz then the time between two pulses is 0,2 mS and if the PLC-program runs every 20 mS the counter value is changed by aprox. 100 each scan.

This means that if you whant to count pulses and for instans stop att a certain value by switching of a output, that value will NOT be accurat to less then 100 pulses.

As you mentioned the objective is maybe only to record the total flow at som distant point. For the same reson as abow there is no good to reset the countercard from the PLC-program. (The minimum reset-time is 1 program scan witch would caus you to maybe "loos" 100 pulses.) Another way to make an accurate calculation is to make the PLC-program calculate the difference between the countervalue "this" scan and the "last" scan and then use this differance to accumulate the value in a register. One important thing to remember is when the counter reaches the maximu value (Usually 65535) it will start over at 0. And in fact MUST do so for the abow mentioned sollution. But again the PLC-program MUST also deal with this.

Hope this will help you.
Roland Lilja

Alan Rimmington

At least some of the Remote IO devices will not work with a HSCE. Depends on your application, but if all you are doing is totalising a flow then this could be done in another PLC then transferred (DH485/DF1) to the PLC you need it in. You may be able to get away with using a micrologix if its high speed inputs are fast enough, if not a 5/03, 4-slot rack, power supply and HSCE will do the trick.

[email protected]
Gather and post some information on your system and it will help the Control.Com regulars size up your application.

You mention "remote I/O", which implies Allen-Bradley Universal Remote I/O with a PLC-5, but it could just as easily be a PLC-2, SLC-500, or ControlLogix controller you're thinking of.

You'll want to find out about the electrical characteristics of your flowmeter output. These can be as varied as 50 millivolt magnetic pickups to 24 VDC square-wave outputs.

Spectrum Controls makes some 1746-type (for the SLC-500 chassis) modules that are aimed directly at the flowmeter business. Although you will still have to execute Block Transfer Read instructions to the remote chassis, these modules have a very accurate "Rate Average" that measures the input frequency average over 1 second. It
might be appropriate for your application.
Are Spectrum control an Encompass partner? Where can we see some info on what they offer?