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AB expertise needed...

AB expertise needed...

AB expertise needed...

I am putting together a machine which requires the use of a lot of 1771-VHSCs (Very high speed counter). Can I use a 1771 chassis with these counters in them as an expansion unit for use with another controller than the PLC 5?

These counters are necessary because I need the 2 outputs per counter based on the inputs of a flow meter.


Can I use a PLC 5 setup for the counters and slave that to another controller?

AB's website didn't have much as far as intermixing the platforms. Thanks for all your help!!!

RE: AB expertise needed...

   Wether you need to use VHSC's or the internal counters in the PLC depends on the frequence or pulse rate. As long as the pulse rate does not exceed about 2 Hz you can use a standard input module and the internal counters of the PLC 5. Above that you will need to use the VHSC.

  You can use any number of VHSC's a system. You can even use them in remote racks. Please be aware that AB is not recommending the PLC 5 line for new installations. If this is a new system, I suggest using the ControlLogix platform.

RE: AB expertise needed...

The pulse rate is approx 10 kHz, so I know I need a VHSC.

But can you completely fill a 1771 chassis with VHSC's or do you need some other module to connect to a ControlLogix system?

RE: AB expertise needed...

First off the 1771 VHSC will only work in a PLC 5 chassis. And yes in a 16 slot chassis you can place 15 modules (need one slot for the RIO adapter.)

For the ControlLogix system you will need to use the 1756-HSC card. Again you can place any number in a rack. Please refer to the web page :

RE: AB expertise needed...

So the RIO will allow me to use the 1771 VHSC on a ControlLogix system; obviously not in the same chassis but they can be integrated...

I would prefer the 1771 because it has controller ability on board, and I don't have to wait for scan times.

RE: AB expertise needed...

I would suggest calling your Allen Bradley rep to help you select the correct method.  You may also want to consider using the high speed counter cards for a slc500.  The slc cards should be a cheaper solution.

If you need more help let me know.

RE: AB expertise needed...

The 10Hz pulse rate should be okay (barely) with a standard module in the local rack.  This, of course, depends upon the duty cycle of the input and the normal program scan.

Think of it this way.... a 10 Hz signal is 100msec.  Of this, 50% (assumption) is ON and 50% if OFF (50 msec each).  A typical program scan of 20 msec will properly detect this.  (Nyquist criterion - scan at approx 2.56x the highest frequency --- as I recall).  The only "hic-up" which I can think of is abnormally long program scans, such as during extended on-line programming/testing/etc.  It is absolutely critical that the standard IBD cards be in the local rack.

Don't waste your money on the VHSC cards and programming.  Good luck and have fun!

RE: AB expertise needed...

Thanks for the advice, but my meters are pulsing at 10,000 Hz (10 kHz). So I think I'm stuck with the VHSC family

RE: AB expertise needed...

Right you are.  I should have read a little closer!

RE: AB expertise needed...

Just a general idea regardless AB PLC or others, for high speed pulse inputs (10Khz) I would look for counter card (module) that reads the pulse input, includes set point and also the output. so the module compares the pulse inputs (count) with the set point that already stored in and activate or deactivate the outputs that on same board. PLC CPU update the set points.
if you would relay on CPU to read the counter, then set the output according to the set point there would be long delay due to CPU scan and execution time.

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