M172 Keeps Missing Analogue Input 7

[MacsEngineer]

We have 4 induction loop detectors in our control panels, each with 2 NO outputs; one output closes while the induction loop is detecting an object, the other output gives a single pulse of maybe 100ms when the induction loop is first occupied. Using the 2 outputs, we can determine if the loop is occupied, or if the loop has failed, missing the input is causing false flags for a failed loop.

I have drawn a crude wiring diagram for the detectors below. They are all in block CN5, with the GND terminal connected to each common for the outputs; out of the 8 inputs, only AI7 is having this issue. I have tested continuity of the pulsing contact on the detector itself, and at the PLC, and it is activating every time.

I have also:

• swapped detectors to see if the issue moved to a different input.
• reduced the cycle time for Timed tasks to 50ms
• moved 14 tasks from Timed to Background, leaving 14 tasks in Timed

This issue has happened on 3 different PLCs in 3 different panels but the other 2 just sort of fixed themselves, so I am unsure if it is something I am doing wrong with either the wiring or the program, or if the M172 has a bit of an issue going on.

[ismailozinan]

Hi,

I have tested the analog input on the M172 PLC, and it worked without any issues. However, if in your test the input signal was not applied for longer than the PLC scan time, the PLC might not have detected it. For testing purposes, you may consider adding a relay to the system and trying again.

Please find attached the test I performed without a relay, along with the images showing that the M172 is functioning correctly.

[FedericoM · Accepted solution]

Hello,

I think the problem is due to the fact that the update period of input AI7...AI12 (600ms) is longer than AI1...AI6 (200ms), you should be able to solve remapping input

[MacsEngineer]

Thanks, that will be the issue. Do you know why they are a different update period, as AI1 - AI8 are on the same connector? I thought they would have all been the same

[FedericoM]

This is what the HW allows to do, it is a PLC designed for HVAC applications, performance and cost must be balanced