Allen Bradley PLC3 DF1 communications


Thread Starter

Jon Stewart

We have an IBM computer communicating with a PLC3. The PLC3 is a slave to the IBM. The IBM reads and writes registers to a single file within the PLC over an RS232 DF1 communications link.

If we were to change the PLC to either a PLC5 or ControlLogix but keep the communications configuration the same including the PLC file number, would any changes need to be made to the IBM?

In other words are there any changes in communications protocol between the three PLCs?
Not all DF1 calls work on all DF1 controllers (in fact I don't think there are ANY calls that work on all controllers). Refer to page 7-2 and 7-3 of the DF1 spec 1770-6.5.16 for a compatibility matrix with various controllers and controller families. The only calls I see that work in a PLC3 but not in a PLC5 are File Read 0f/04, File Write 0f/03, Physical Read 0f/09, Physical Write 0f/08, Restart Request 0f/0a, Shutdown 0f/07, and Upload 0f/06 (and even the first two work in a 5/250). So it will
depend on whether they used any of these seven calls. You could put a serial spy PC in between the IBM and the PLC-3 and get a capture of the
calls and responses to see what calls they used.

Jerry Miille

There are definitely differences between the communications capability between the three PLCs. But, you may be able to replace the PLC-3 with another model depending on the CMD and FMC codes used with your current software. You should probably use some sort of eavesdrop program to "listen in" on your current communications to determine what CMD and FNC codes are being utilized. Then look here (see pages 7.2 and 7.3) to see if the PLC 5 or ControlLogix recognize these codes.

Jerry Miille