M&M
R
EFRIGERATION
I
NC
. C-1
APPENDIX C
Communication Protocols
Introduction
The hardware interface and software protocol are defined for per-
forming serial communications with the M&M Screw Compressor
Computer Control System (CCS). Two open protocols are defined
in this document, Modbus RTU and Allen-Bradley (AB) DF1. In
each case, the implementation of each protocol is limited to the
sub-set of the complete set of possible message types. Communi-
cations with one or more compressors is possible using a multi-
drop RS-422 interface. M&M Refrigeration also supports its own
internal communications protocol which is available upon request.
NOTE: Modbus is a registered trademark of Modicon, Inc.
Real time monitoring of the following data is provided:
• Analog input parameters
• Discrete I/O parameters
• Mode and State information
• Computed parameters
• Alarm Information
Real time control of the following parameters is provided:
• Compressor Control including start/stop and mode
changes
• Compressor Capacity Control Setpoints
• Economizer Control (optional)
Applicable Documents
Familiarity with the following documents is highly recommended:
• M&M Refrigeration CCS Screw Compressor Overview
• Modicon Modbus Protocol Reference Guide (PI-MBUS-
300)
• Allen-Bradley DF1 Protocol and Command Set (1770-
6.5.16)
• Allen-Bradley Enhanced and Ethernet PLC-5 Programma-
ble Controllers (1785-6.5.12)
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