OT PLC's

[Chuck Wintle]

Is there anyone on this forum with experience with PLC's? In particular it is the SLC500 made by Allen Bradley.

[Peter Stahl]

We have one at work that controls our equipment. I'm just a panel board operator so I don't know how much help I can be.

[Kyle Kraft]

I've installed, wired, programmed and and and and and them in the past. What would you like to know? BTW I'm not the authority on them in any way.

[Chuck Wintle]

I've installed, wired, programmed and and and and and them in the past. What would you like to know? BTW I'm not the authority on them in any way.
We have an Allen Bradley SLC 500 that was replaced with another one from the manufacturer who claims it is an exact replacement. yet when all the connections were made some parts of the machine do not work. I was told that the SLC may be waiting for certain inputs before enabling other outputs. What can i tell from the input lights to the PLC?

[Justin Leiwig]

We have an Allen Bradley SLC 500 that was replaced with another one from the manufacturer who claims it is an exact replacement. yet when all the connections were made some parts of the machine do not work. I was told that the SLC may be waiting for certain inputs before enabling other outputs. What can i tell from the input lights to the PLC?

Did they program it for you? If so then they need to fix it. Without seeing the hookups and the ladder logic there is no way in my opinion someone could tell on a forum what is wrong.

[Michael Weber]

What Justin said. No way to know otherwise.

[Kyle Kraft]

The lights on the input card simply confirm a signal from your input device (limit switch, prox switch, etc.). Troubleshooting a program based on input lights is in my opinion futile at best. You must have the A-B software running while on line with the processor, a printed copy of the program, and preferably the electrical drawings in front of you to start debugging a program.

If an outside vendor did your upgrade, I'd put it back in their lap to correct the problem.

[Myk Rian]

Did the old PLC get thrown out? Hope not.

[Matt Meiser]

I'm not a PLC guy but I do deal with them, have programmed them, and hear a lot about them. First I agree with whatl's been said about loading the program. If you just went to a Rockwell distributor and bought a new processor then it isn't going to work. Worse if you bought used there could be an old program in there. Most likely there would be a fault in this case I believe because the rack config won't match.

With respect to who's responsible to fix it, that depends on warranty for the piece of equipment the PLC is controlling, any support contracts, etc. You will probably need to hire a system integrator or the original manufacturer of the equuipment to fix it at an hourly rate if you don't have the expeience, programming software, and cables (which are both fairly expensive.) I believe Rockwell has a directory of certified system integrators on their web site. Some distributors can provide services as well.

[Chuck Wintle]

Matt,
The original PLC on this machine at work developed a CPU fault and just was not working at all. So the manufacturer of the machine offered to sell us a used PLC that was "supposed to be programmed identically" to the old one. After all was said an done it did not work. And to make matters worse it was sold without a warranty and tech support meaning I am now on my own to get it running. Luckily one person was nice enough to send me the logic ladder. We have a laptop with the necessary software so all I need is the cable to connect to the PLC. Hopefully this week I can make it work. As it stands now the PLC does work, i.e. the CPU led is on and i did put back tghe wires exactly as they came off the old one. So with any luck as they say.......:D:D

[Mike Bensema]

I spent 10 years commissioning equipment and was heavily involved in programming and troubleshooting AB PLCs. If the manufacturer is anything like a company I worked for, the PLC programs were modified heavily to get them to work during commissioning and the changes often did not get filed back at the office. If you have not done so already, I would recommend backing up every PLC program you have so you don't run into problems with other machines. I would also recommend that if you have any service techs coming in to work on the equipment that they give you a copy of the program if any changes were made before you sign their report.

The first thing to check is make sure no emergency stops are actuated! I learned that the hard way early in my career, spent several hours troubleshooting a machine to find out someone tripped an e-stop.

When a program was not working right, the first thing I would do is verify all inputs are working. If you know a certain input should be on and it is not, then that would be the place to start to see if it is the sensor or the PLC card not working. Since you had a CPU fault, it is possible there was damage to other cards as well which could have shorted out some of the inputs and outputs. If all your inputs are working correctly, then I would check to see if the correct output lights are on. If the outputs are not correct, then I suspect the program loaded is not what was in your old PLC or you have faulty outputs. Since you have the ladder logic, you can check in the program to see if the correct outputs are energized and then check to see if you have power at the output. If no power, then it is a faulty output.

[Sonny Edmonds]

We have an Allen Bradley SLC 500 that was replaced with another one from the manufacturer who claims it is an exact replacement. yet when all the connections were made some parts of the machine do not work. I was told that the SLC may be waiting for certain inputs before enabling other outputs. What can i tell from the input lights to the PLC?

Obviously, they lie. :rolleyes:

I've had some to do with PLC's and much bigger control schemes in my Career, but I forgot most of what I used to know.
Other than the laughter as foreign ingineers (German) would run back and forth to the computer controls pulling bugs from sockets, reprogramming them, and taking them back like sushi servers with a styrofoam tray in their hands.
They finally got the stuff to work, but one side of the station burned to the ground 4 years later.
Not a lot of PLC's in my life anymore. Just another of those things I'm remembering to forget as I close in on retirement, I guess. I leave that to the younger guys who have to work on it. :D
Ah, I can smell that pasture grass from here. :)

[Matt Meiser]

I suspect Mike is right. I never trust when an OEM or customer tells me that things are the same. :D