Fan Mismatch Alarm Logic - PLC5 (? use of OSF, OSR, etc)

Hi, I hope I have what is a basic PLC5 coding question.

I have four fans: two for supply (HVU1, HVU2) and two for exhuast (EF1, EF2).

The start up procedure is such (manual by operator but interlocked in code to work in only this way):

1. Start either one of the exhaust fans (EF1 or EF2)
2. Start either one of the supply fans (HVU1 or HVU2)
3. Start the remaining the supply fan
4. Start the remaining the exhaust fan

The start permissives of the supply fans are such (either/or):

1. HVU-1

• Running Status of HVU-2 is FALSE AND Running Status of exclusively one Exhaust Fan (EF-3 XOR EF-4) is TRUE.
• Running Status of both IVS Exhaust Fans (EF-3 and EF-4) is TRUE.

2. HVU-2

• Running Status of HVU-1 is FALSE AND Running Status of exclusively one Exhaust Fan (EF-3 XOR EF-4) is TRUE.
• Running Status of both IVS Exhaust Fans (EF-3 and EF-4) is TRUE.

Now, my issue is I need to program some fan mismatch alarms which I've already specificed the functional specs for. The only thing I'm having trouble with is understanding what the best way to do program it in PLC5 LL is. Here's an example of a fan mismatch alarm that I'm trying to code:



Note: The reset conditions for the Alarm are the loss of the trigger conditions.

Below is my first crack at implementing the above spec. My concerns with this approach are:

a) in the use of the OSF instruction (I don't like it, but what else can I do to detect a 1->0 transition of the run status of the fan?)

b) the logic order that this snippet of code should be relative to the output command of the EF-3 fan (which this alarm will be made to reset when it is triggered) - should it go before or after?


p.s. I know I haven't set the timer yet


If anyone can help or provide some insight, it would be greatly appreciated.

Thanks,
NM