MicroLogix Missing Pulse Counts

[DRSapach]

I was having a problem with a MicroLogix 1000 processor (DC model 1763-L16BBB) missing pulse counts from a hall effect (NPN open collector) flow sensor. Flow generating up to about 30 pulses per second would register but above that no pulses would be counted. I have the sensor connected to input 0/0 of the processor which supports high speed counting. After much troubleshooting which included connecting an digital storage scope to the input and verifying that the sensor was operating correctly (i.e., outputting a proper square(ish) wave form), I was able to find the problem with the help of Rockwell Automation Tech Support and want to pass it on here...

The issue was an obscure project setting for the PLC program. This setting was set to a generic processor type which configured the inputs to be 'regular speed'. Once I changed the processor type to match what I had, things magically worked. Here are the details on what to change in RSLogix:

In OFFLINE mode:
- From the project tree, open the Controller folder and double-click on IO Configuration.
- Double-click on the first item in the part list which should be the processor (MicroLogix 1100).
- On the first tab change the Base/Type from Any Base to the processor type you have (BBB Base in my case).
- Click OK to save the change.
- Save and download the project to the PLC.

Also, if you need to count very fast pulses, you will likely need to use the High Speed Counters. These are also enabled on the IO Configuration for the processor.

Hope this helps...

 

[duckman]

Thanks for passing on your hard earned experience, l will be filing it away.
Could a OSR pre the counter picked it up at 33m/s or what speed are they good for?
Regards

 

[strantor]

Thank you!

 

[DRSapach]

Duckman,
I was not using an OSR; I had configured the input as latching (rising) which is a specific feature supported on the first 4 inputs of the processor (same inputs that support high speed counting). Funny how this setting worked even though the high speed counting did not.

When not using the HSC feature, the performance would depend on scan time but in my case my program is so small and the maximum frequency output for my max flow is about 200Hz which is no problem for just tying the input into a regular counter. The HSC is suppose to be good up to 20K Hz (or 40K Hz for newer processors).

 

[Brijm]

Also note, that under the advanced config for the I/O, on the next tab (Embedded I/O Config) There are input filters also. I'm not sure what the default is for the ML1100, but I have had this byte me before in the past