AB microLOGIX1100 Programmable Limit Switch

I’m experiencing some unusual behaviour when using the Programmable Limit Switch Function in an AB microLOGIX1100 PLC.

For test purposes, I’m using the example in the “MicroLogix 1100 Programmable Controller Instruction Set Reference” manual.



PLS10:0.HIP=250 PLS10:0.LOP=0 PLS10:0.OHD=0000000000000001

PLS10:1.HIP=500 PLS10:0.LOP=0 PLS10:0.OHD=0000000000000010

PLS10:2.HIP=750 PLS10:0.LOP=0 PLS10:0.OHD=0000000000000100

PLS10:3.HIP=1000 PLS10:0.LOP=0 PLS10:0.OHD=0000000000001000



What I’m expecting to happen is:

From 0 to 250 - no outputs are on

From 250 to 500 – output 0 is on

From 500 to 750 – output 0 is off and output 1 is on

From 750 to 1000 – output 1 is off and output 2 is on

At a count of 1000 the cycle resets and repeats.



What is happening however is:

The HSC accumulator counts from –

0 to 250 and then

0 to 500 and then

0 to 750 and then

0 to 1000 and then repeats

0 to 250 and so on



Unfortunately, I have to use the HSC and PLS functions as, the PLC scan rate is too slow. My HSC input is switching at 200Hz and I need to turn an output on for two counts.



What am I missing??

Why use the PLS function? What happens when you use a compare function to turn on whatever outputs you want. Something like a limit function with high and low set 250 and 500= output 0 turned on. I'm not at all familiar with the PLS function you mention.

It seems as though the HSC accumulated value is resetting to zero each time it reaches the end of a PLS step. In the reference manual section for the HSC Mode, it says for MOde 00 (which the example specifies):

"Up Counter - The accumulator is immediately cleared (0) when it reaches the high preset. A low preset cannot be defined in this mode."

I found a "quick start" document for the PLS function in MicroLogix 1100/1200/1500 and it is exactly the same as the example in the Reference Manual.

So maybe invoking the PLS file changes the Mode 00 behavior.. or maybe there's something else we're overlooking.

Unfortunity I can't do it this way. The issue is with the scan time. I need an output to switch in under 3mseconds. The scan time at the moment is at 5mseconds, and I still have more code that needs to be written. By using the HSC function and a PLS file, the counting and switching happens in the hardware, not the software, meaning the the scan time is no longer a factor

Reading the info only from the MLX12-1500 information, try setting the LOP of the next step equal to the HIP of the previous step.

[Edit] OK Ignore the above. Found the MLX1100 manual. It's confusing.

[Edit 2] Try using a different HSC mode. Or just consider your steps as "number of pusles for this section"