On Friday, December 18, 2015 at 11:58:31 AM UTC-5, Ralph Mowery wrote:

Some of the computer controlled processes use what is called a PLC that has
lots of modules about the size of the cable modems that plug into a rack.
They will have a few small switches or jumpers inside them and the new one
will need to be set the same as the old one. Need to keep up with which is
the old part and which is the new one so the switches can be set correctly.

Worked at a plant that used Allen Bradley PLCs.

The program resided in memory but there was a backup EPROM in the cabinet.

You could also type it in by hand but it was thousands of lines of ladder logic, any line of which could point to any other, it would take weeks.

So we lost the program in a power surge or something. No problem, pop the EPROM in and download the program.

Meanwhile the production manager is screaming we're going to miss the ship date and lose the customer. No sense of humor, those guys.

Here's the catch: on some PLCs download the program means install it FROM the EPROM, but on these A-Bs download meant install it TO the EPROM. So we downloaded the empty program into the EPROM, erasing it.

No problem, we have a spare safely locked in the maintenance office. Notice I said A spare, as in, one. You already know what we did, popped that one in the PLC and erased that one too.