PLC 5 Program Reset

MarkNightingale

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Good Morning All,

I have a customer with a problem with a PLC5 installation.

There is basically a loop which is writing values to 32 filling valves.

The logic is therefore identical for all 32 valves.

However, one valve is playing up and not opening an closing like it should.

The valve is opened using 4-20mA. I can force the valve open and closed to the exact position i require, by forcing a value into the 4-20mA.

The problem it seems is the value coming from the 'loop' is always wrong. But it works for all other 32 'loops'.

The one that isn't working is Valve number 4, so it's not even like its and issue with the loop cutting off early.

Before i have exported RSLogix 5000 files to a .L5K file and then re-opened, and that has cured some anamolays i have had.

Is there some sort of feature like this with PLC5?

I am baffled as to whats happening.

Cheers

Mark
 
The analog cards on PLC5 has scaling integrated in the card.
Maybe the scaling configuration for the channel in question has been messed up.

There is a dialog if you open up the configuration of the card in RSLogix.
As far as I remember the scaling is always 0-4095 on the hardware side after scaling and the engineering value must be an integer. There are some caveats, you cannot scale beginning at less than zero, -20 .. +20 for example, and you cannot scale in inverse from, 100..0 for example.

Disclaimer, this is from memory from approx 25 years back !
 
Yes, PLC-5 has an ASCII export/import file format that can clean up corrupt Logix files (which may produce corrupt compiled code in the PLC). I don't remember the file format extension -- ?? .PLC5 perhaps. Do a SaveAs and see what your options are -- it should be pretty clear.

Beyond that, there could always be the chance that there's logic code somewhere that is attacking Valve 4 directly, after the loop has done the normal stuff. Search for N10:4 or whatever drives that loop directly, working backwards from the output.

It's also possible that if the loop is "cutting off early", there is a conditional TND or JMP or other instruction inside the loop that is getting triggered by something in the valve 4 indirect / indexed addressing inside the loop that isn't true for any of the other valves.

But without seeing the code, there's no way for us to help.
 
In the prehistoric era earlier PLC 5 BTR/BTW config registers used on analog modules were normal N registers and not BT registers. I have seen the N register points innocently re-used elsewhere in the program causing all sort of strangeness.
I had this in my old notes

Analog Modules
A/I BTW 39 words (Rev C) 37 Words (Rev A/B)
A/I BTR 22 words (Rev C) 20 Words (Rev A/B)

A/O BTW 13 words
A/O BTR 5 Words (Not actually needed at all, just reads status)

NOTE: These are DATA lengths. ALL CONTROL Lengths are 5 words!
Use BT DataType whenever possible to avoid conflicts
 
If the value from the loop is wrong, then it is not the Block transfer or scaling at the card. What determines the loop output? Plus OP, said forcing a value to the output works correctly.
 
Thanks for all the answers so far.

The customer is well versed in PLC5, and I trust him impeccably.

everything was working, and no changes were made, but all of a sudden filler valve 4 started to have this problem.

The output comes from the loop on N104.4, and it is this that is not changing.

This value is then copied directly to O:035 which is the analogue output.

When we force values to the output we get the correct value and the valve opens appropriately.

I have attached a screen shot, and you can see the forces we have put it. Bear in mind this is offline.

As the customer has said nothing has changed, and there have been no downloads to the PLC, then I thought about something getting screwed within the loop.

I will try to convert to ASCII and back again and get the customer to re-download.

Thanks for all your help so far.

Cheers

Mark

Output.PNG
 
Addressing an AO directly at O:035 on a PLC 5 doesn't sound right.

MarkNightingale said:

This value is then copied directly to O:035 which is the analogue output.

When we force values to the output we get the correct value and the valve opens appropriately.
Mark
Click to expand...

Analogue modules require setup BTW/BTR instructions. Usually the addresses would end up as a N# register.
 
Hi Chelton,

It has been working this way for years.

All of the other valves work this way.

I know it's not a problem with the addressing because it works when we force the value.

Cheers

Mark
 
So here's my 2 cents. If you are 100% sure, and I mean absolutely positive, that nothing has changed in the PLC then this leaves you with these possibilities:
- Defective output on the card.
- Damaged wiring to the output device.
- Faulty output device.
 
muusic_man said:
So here's my 2 cents. If you are 100% sure, and I mean absolutely positive, that nothing has changed in the PLC then this leaves you with these possibilities:
- Defective output on the card.
- Damaged wiring to the output device.
- Faulty output device.
Click to expand...

Op said forcing the output works so correctly, so not the issue.

I am thinking the bits that drive the various output signals are not being made correctly.
 
I think somewhere a point was missed. OP said that N104:4 wasn't changing. If it doesn't change, the output won't change either without forcing a new value into O:035. What controls N104:4? Perhaps seeing that logic could shed some light on the situation. What input device decides the position of the valve? Is that device working correctly?
 
Is the analog output a 1771-OFE, or another device that takes a bit pattern and converts to analog going out to the valve? Writing to an output word implies the latter, possibly something on remote I/O or discrete-to-analog.
 

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