Choosing a replacement CPU, AB 5/04, 1747-L541

I'm trying to remember the lessons for replacing a SLC CPU, but I so rarely have to do it, I'm afraid of screwing it up. I'm hoping to give my office some guidance in choosing a replacement for this customer. They said the only exact match online seemed too risky. Can someone give me some guidance for choosing? I'd like to print this out afterward and keep it in my notes.

The available CPUs are evidently newer revisions... I'm not sure what else is important to look at. It would be nice if I could avoid flashing the firmware, but I suppose whatever it takes.

With that 5/04, I don't think you will have a problem. The FRN 7 is very old, and the newer firmwares only added features to the best of my recollection.

We recently were awarded a job adding radio comms to a system with a 5/04. My radio hardware requires Modbus RTU capability and their old FRN doesn't support it. For about the same price as a firmware upgrade kit, I bought one used from Mark and I trust his used equipment.

The SLC 5/04 he sold us has more memory than the old one, but that is also not a problem.

Here's another one:
https://www.plccable.com/allen-bradley-1747-l541-series-c-cpu-rev-7-frn-8-processor-scl-500-5-04/

When you swap the CPU to one that is different, there are a couple things I typically do. Since the new unit will also have the same comm ports (DH+, serial) nothing has to be checked there. Since it will likely have a newer FRN, I still like to select the controller (offline) in RSLogix 500 with the right FRN range. I think in most cases you don't have to modify the controller properties, and the software will detect the difference and warn you when you attempt to download. I like to do this offline manually and Save As with a new file name.
When you change the controller selection, the software will prompt you to clear the IO and resize data tables. I almost always uncheck the option to resize data tables. If you leave that box checked, the software will truncate data tables to the last element in use and that can screw things up if there is indirect addressing or just wipe out your spares that I always like to keep.

Aside from that, you should not have to do anything else. Since it is a DH+ CPU, you will probably want to dump the program into it through the serial port, or isolate it from the DH+ network until you have the node number set so there is not a duplicate node number when you power on the new CPU.

It's only using the serial port for an Panelview 600. The PV couldn't communicate with the CPU. At first, I thought it was an issue with the PV. Just to check, I tried connecting my laptop to the SLC through the serial port. RSLinx couldn't find it. Just to make sure I had the connection settings right, I connected with the exact twin machine right next to this one. ...It connected. I noticed the twin machine had the RS232 LED lit, whereas this machine didn't. So at this point I think it's a bad comm port on the CPU.

...Good thing I uploaded the program when I was working on the machine last year!

Thanks for the info!

Hi Brain,

Got your email and I will reply to it... but one thing you may want to try a reset on the CPU, if you just had a power glitch than maybe just needs to be reset

Thanks for the kind words Paul!

Update: I sent Mark (geniusintraining) an email pointing him to this post and expressing my hope that I could take care of the customer without a glitch. I also included the file from the old CPU just in case it would be helpful. Later in the morning, he asked that I call him when I had the opportunity. We chatted on the phone about the old CPU issue, then he mentioned that he'd be able to load the program on the new CPU later that night, before sending it out 2nd day air the next day. I don't have a way to use the DH+ port (Btw... 1784-U2DHP on my Christmas wish list if anyone cares. ) so having Mark load the program made sense to keep everything simple, and he did it for free. I received the CPU and installed it earlier today. It took less than 15 minutes to install and test, and the customer was all smiles.

I mention this because I believe one of the best features of this board is establishing contacts for things we need. This was not an expensive transaction (this time), but the confidence of being able to take care of the customer without issues was a great thing. Mark prepared the CPU so everything would go smoothly. But I also knew if I had any issues, I had someone who would be a phone call away. I'll be doing this again.

Thanks, Okie and Mark.

Yep... There are a lot of true Gem's on this forum!

Hi Brian,

Glad it worked out and let me know if you need anything else, happy to help and yes happy customers are a good thing

BTW try resetting the old PLC and see if it can be saved as a backup

Thanks again for the lead Paul!