Two issues regarding a problem with a 1747-L20C. It had been pulled from scrap with no firmware IC. I had an L20D and so decided to try and see if that firmware would work. When trying to copy a program configured for the L20C, RSLogix 500 says "Downloading Program - (TEST) for 1747-L20C/F ... to (DEFAULT) 1747-L20A...
This would indicate that the firmware made the processor think it was a 1747-L20A, yet it works in the L20D.

Regarding programming, I'm in a catch-22 situation. Attempting to program the 1747-L20C using a 1747-PIC and RSLogix 500, RSLinx sees the processor as a SLC500 with no problems. Memory was reset to default on the L20C, so when trying Who Active Go Online, I get a window saying "Corrupted Processor". If I try downloading to the L20C, I get the message about it being an L20A, asking if I want to procede with the download. I answer YES, and then get "Download Failed! - Communication Errors!" So, as you can see, it won't go online while only having the default program and won't accept a download while being corrupt, therefore there is no way to go online and clear faults or anything. The RSLogix 500 program definitely supports the 1747 processors as they are all listed in the selection under Controller Properties - so I don't think it is an issue requiring upgrade of software.

I did ask the AB distrib if they can supply a new firmware IC for my L20C, but have not heard back from them yet.

So, I am confused, my software is confused and my processor is confused. Please, if anyone out there is NOT confused, maybe I can get this to work - or maybe not. Any help is greatly appreciated

James C. Williams

For those of you following along at home, the 1747-L20 is the oldest and smallest of the SLC-500 "fixed I/O" controllers. These controllers were most popular in the late 1980's.

1747-L20A 115VAC Inputs, Relay Outputs
1747-L20C 24V DC Sinking Inputs, Relay Outputs
1747-L20D 24V DC Sinking Inputs, Triac Outputs

The firmware in these controllers is a socketed ROM chip, so some users attempted to repair damaged controller by taking ROM chips from another controller. The ROM chips were never meant to be removed or sold separately and I think your only chance for getting one is to send the controller to Rockwell for repair. I've had a few customers buy these controllers on the aftermarket only to find they had been sold a controller from which the ROM had been removed.

My guess is that there are firmware issues deep in the host hardware that prevent the ROM chips from one type off controller from being used in another. Maybe the -L20C controller with the transplanted -L20D ROM chip reports itself as a -L20A only because -L20A is the first part number in sequence.

Re-stated, the controller may be incompatible with the transplanted chip and the mis-identification of the controller type by RSLinx is just a symptom of that incompatibility.

Thanks Ken. I found that my communication problem was largely due to a conflict of a change in motherboard/RAM and my 1747-PIC. I switched to using the PCMK driver for DH485 and had no problem logging onto the L20D and programmed it fine. Maybe my PIC was pic-ky about only using 80MB of RAM running W2K.

Jury is still out on the firmware. If you look at the cases of both L20D and L20C they both say FRN 6. I will be spending some more time on this and will come back with what I find for future reference.

~JCW