Allen Bradley 1771-CFM or PLC fast input module?

[ckchew666]

No no, sorry for not explain it clearly (diff comm language bet our side with cust). The PlC will count the pulses input, and it'll trigger some outputs based on the pulse. For example, when it reaches 20k pulses, it trigger the first out....etc...

 

[ckchew666]

From that, I take that you want the CPU to evaluate the signals programmatically.

At 2000 Hz, you would have to scan the signal every 0.2 millisecond - or faster ! (*). That is impossible in the PLC5 program, even if it was dedicated to this application alone.
Even Selectable Timed Interrupts cannot be faster than 10 milliseconds. And if you programmed Input Interrupts, I dont think it would be possible, but if it is possible it would make the CPU do nothing else but react to the interrupts.

Dedicated counter cards in the PLC, or standalone counter to analog converter, or changing to something else than 2kHz pulse signals is the solution to your problem.

*: 0.2 milliseconds scan assumes that the pulses have evenly distributed pause and pulse periods. If the pause and pulse is not evenly distributed, the scantime would have to be proportionally shorter !

If I understand u correctly, this means a 1MHz Input freq 1771-VHSC will do the counting of pulses itself rather than by the PLC CPU. This could be another reason it cost a bomb?

 

[JesperMP]

If I understand u correctly, this means a 1MHz Input freq 1771-VHSC will do the counting of pulses itself rather than by the PLC CPU.

That is correct.
The PLC sends setpoint and a "start counting" command to the counter mdoule. The module then responds with a count value and "counted up" message when done. If you need to act immediately when the setpoint is reached, then there is a direct output on the module you can use for that.
Some counter modules have an integral conversion of the pulse input to a rate value, maybe that is useful for you. Dont know if a VHSC module has such a conversion, but there is a good chance that it has.

By the way, what is it that these pulses indicate exactly ? Level, flow, speed, or .. ?