DF-1 communication via PLC-5 Channel 0 Serial Port.

It is possible to exchange status and variables between one MC³ or Genetix GCM controllers and an Allen-Bradley PLC-5 PLC using the standard serial port (Channel 0) on the PLC. It replaces both analog and digital I/O. Full Floating Point accuracy is maintained for variables. Up to 10 variables and 80 I/O points can be configured.

See Genetix GCM Configuration and Troubleshooting for DF-1, or MC3 Configuration and Troubleshooting for DF-1 for details.

The Common Interface Table (CIT) in the Genetix GCM and the MC³, is transferred to and from the A-B “Common Interface File” (CIF). PCCC functions 1 (PLC2 Unprotected Read, 485CIF Read), and 8 (PLC2 Unprotected Write, 485CIF Write) and 6 (PLC-2 Diagnostic Status) are supported.

For performance reasons, data exchange takes place in block mode. The read segment of the CIT is transferred to the PLC in with one MSG read instruction, and a part of the write segment is transferred in one MSG write instruction. The positioning and content of the data elements in the tables must be tracked all the way from the internal controller's register database to the data structures in the PLC. This is done in several steps:

·         Between the GCM or MC³ register database and the CIT. Some of this mapping is fixed, and some is configurable within the GCM or MC³.

·         Between the CIT and the target data tables in the PLC. This is defined in the MSG instructions.

·         Between the target data tables in the PLC and the final, formatted data tables in the PLC. This is done with PLC programming methods, using COP instructions.

Control/Status bits and Integer numbers are organized in 16 bit words. Parameters are organized as IEEE 32 bit floating point numbers.

The serial cable we used was an 18 GA one pair shielded, where we used the shield drain line for connecting digital ground. Max cable length is 50 ft. The PLC end has a 25 pin DSub (Male) connector. The Genetix GCM end has a 9 socket mini Phoenix connector. The MC3 end has a 9 pin DSub (male). Connect according to the following table:



GCM Term

MC3 Pin





Data to GCM/MC³




Data to PLC




Ground. Use drain wire


This solution is fully supported by Merrick, meaning that we have equipment on hand to furnish tested configurations.

We have a test set-up a Merrick, that we use for support. An MC³ 20.20.B or a Genetix GCM controller is connected to a PLC5/11 series C, Revision E PLC. The message logic can be found in Ladder file 2. Data  is red into N10:1 and written from N11:1. Parameters are then copied in and out of F8. The main 'gotcha' is the way elements in the message rung are represented:

·         Size in elements: Decimal words.

·         Data table address: Octal words.

Here are the files from the test setup:

·         RSLogix5 (V 6.2) RSP file.

·         Here is the ladder listing, in PDF format.

·         The set up screens for the MSG instructions: Read Write.

·         Channel 0 properties set up screen.

The DF-1 Specification is useful for troubleshooting.

This Babbage site is useful to resolve IEEE floating point problems.