5/04 Fault

Tark

Member
Join Date
Apr 2004
Posts
503
I have a 5/04 which is in Fault and I am unable to clear the fault. I am also unable to go online with the unit. I have tried the following to clear the fault –

1. Placed the keyswitch in Program then Run.
2. Cycled power.
3. Removed CPU from the rack.

The PLC consists of a 5/04, AC Input Card, and a Analog Input Card. The RS-232 is used to communicate with a Redlion HMI. The system has been in service for at least 5 years.

I have been trying to chase down this problem for a few months. I get a call from the customer and they tell me that the CPU is in Fault. I instruct them to cycle the keyswitch and they tell me it’s still in Fault. When I get there (generally the next morning) the CPU is no longer in Fault. Either my version of RSLogix (V6.0) or the CPU does not have a Last Fault, so I have been unable to determine the problem.

This morning I went to the sight and the CPU wasn’t in Fault. I went online with the processor. I noticed that the units that send the 4-20mA signal to the PLC were not functioning correctly. We turned off the power to the cabinet and the customer swapped out the 4-20mA units. When we turned the power back on the CPU was in Fault. I tried going online with the CPU but couldn’t. RSLinx will show the CPU when it first goes searching for processors, but on the second round of the search it shows the processor unavailable.

My first thought is that it has something to do with the 4-20mA units (load cell displays) but since I can’t even go online with the CPU I’m not sure what’s up with it.
 
The first question is always what has been changed, program
new hardware?
If nothing then I would suspect power supply first.
 
Mickey said:
The first question is always what has been changed, program
new hardware?
If nothing then I would suspect power supply first.

Nothing has changed. The only thing that has happened is that the loadcell displays have gone out from time to time, mostly due to storms. This is why I thought it might have something to do with the 4-20mA signals at first, but now since I can't even go online with the unit I'm not sure.

I guess a question would be, if the 4-20mA signals were wrong would that cause the CPU to be locked in Fault? Or if the power supply was bad would that lock the CPU in Fault, preventing even comms?
 
4 - 20 signals can cause math overflows if they are messed up. Have you been able to get the fault ID at all? Make sure that the analog signal is limited so it cant go out of bounds.
 
Could you have an intermittant ground short on your 4-20 ma circuit? I would start by disconnecting all external devices that you can and see if you can reestablish comm's. Then reconnect one by one if you were sucessful hopefully one will cause your comm's to drop out again
 
Tark, I have a SLC5/04 that I keep on my desk for experimenting with. Everyonce once in a while it does the same thing - faults out and I cannot go online. I think the battery is getting weak - esp since it spends most of the time turned off, ubt it doesn't give a battery alarm yet. I disconnect the battery and short the Vbb pin to the Gnd pin to reset the PLC and wipe memory. I've been able to get back online with it everytime that way. I don't know if that is of any help to you or not.


We had three SLCs at our New Jersey plant that would fault out and have to be relaoded everytime there was a power failure. They asked me and I asked AB. AB suggested we check the grounding of the chassis and the panel. When I conveyed that back to the NJ maintenance manager he dismissed the suggestion. Later I flew out there and took a look for myself. There were no grounds whatsoever, the entire panel was ungrounded. We grounded it and viola, problem solved. So you might want to take a look at that as well.
 
Last edited:
I did disconnect all the 4-20mA lines while it was faulted and was not able to get it to come out of fault. If the A/I was receiving 0mA would that cause a fault? I am using the scale instuction in the program.

Alaric - I'm not going to try that at this time since I don't have a copy of the program on my laptop. I'm a bit worried about losing the program all together. For some stupid reason when I first connected with the processor I didn't save the program to my harddisk.
 
The next time you connect to it with the actuall program loaded and not default after a vbb/gnd. Check the watchdog in the processor properties panel. If it goes over the max setting you will get a fault at start-up. 02h usually...
 
Is the falt light flashing red or solid red?

Do you have an HMI available? If so, program in a screen to view the fault code. Even if you can't go online perhaps the HMI can still talk to the PLC and display the fault info.

OG
 
I don't believe seeing 0 ma on the A/I would cause the fault as while the program is faulted none of the inputs/outputs are being examined. Its too bad you couldn't default the processor then reload the program at least that would confirm the processor. If you happen to have another processor you could temporarily place that onto the rack and try to connect to it.
It will show as a faulted processor initially as it will not recognize the rack assignments and probably not have a program on it. but it can help you verify your I/O cards and rack as a problem on those two items could also cause similar problems
 
Here's an update. If power is cycled from the PLC the PLC will go into Fault. There is no communication with the RS-232 port. Don't know about the DH+. Cycling the keyswitch from Program to Run does not remove the Fault. At some point (hours later?) the PLC drops out of Fault and starts running.

Best guess to the problem? Power supply?
 
I'd get online asap with it running and make a backup of the program. Ive never had a fault prevent me from getting online with the PLC, so it sounds like something more dreadful is going on that just a simple math overflow, invalid pid setting, or such. So, get that backup. Then you can try replacing cpu, grounding the current cpu, etc.

matt
 
I would like to know what leds are flashing in what sequence when power is first applied, that may be able to help point you to what is going on. Also is there a line power filter prior to the plc power supply?
 
I got a backup of the program this morning. When the PLC was in Fault, there were no falshing lights. The Fault light was steady on. There isn't a powerline filter before the unit, although I'm thinking this isn't the problem since the unit goes into Fault at powerup and drops out of Fault at some point later in time. As best as I can tell the unit only goes into Fault on powerup.
 
...the unit goes into Fault at powerup and drops out of Fault at some point later in time. As best as I can tell the unit only goes into Fault on powerup.
If the SLC 5/04 drops out of Fault mode without user intervention, then this is a good definition of a malfunctioning CPU. Replace it before it gets worse.
 

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