Communication between 4 AB 505

I just started doing some programming for a company that has usually used 1 AB 505 and 1 Panelview per machine. However, there is a project that they did that has 4 AB 505s and 4 Panelviews connected with ethernet. The programmer before me set up a write MSG Block to each plc. This just seems like a crude way to do this and it is very hard to keep the addresses straight, but it does work. The company is adding equipment and will need a lot of extra programming, with data send to all 4 PLCs. I have 2 free spaces in each rack. The max distance between the PLCs is 100'.Any help on the proper way to do this would be appreciated.

Thanks,
Jim

Welcome to the Forum, Jim !

The MSG instruction is the ordinary way to exchange data between SLC-500 family logic controllers. The ControlLogix family of controllers has a cyclic data exchange called "Produced/Consumed Tags" but that feature is not supported in the older SLC-500 architecture.

The programming of MSG instructions is easy to do poorly; the SLC-500 doesn't have an independent message processor so the main CPU has to handle all the messaging tasks. It's highly recommended to limit the number of simultaneously active messages to four or less, and to use delay timers between message triggers.

You will find some disagreement among longtime users about the "best way" to manage addressing. My preference is to use unique data table registers in each controller for the source and destination data, so that if I'm looking at "Controller B" I'm going to find data in my "N11:x" file that was sent from Controller A's "N10:x" file, rather than having the data file numbers match.

Thanks Ken,
I was afraid that was going to be the case. I have gottem the company to switch to CompactLogix for all new machines, but no help on this one. The last programmer made data files match. I like your idea to help somewhat in the addressing. so will at least change that. There are no delay timers either so will add that as well. Thanks again for the help.
Jim