Despatch Protocol Plus Modbus Communications - Manual (Page 7)

Chromalox Instruments and Controls
A-51643 Rev. 6 10/06/03 1
1 Overview
1.1 Purpose
This document provides all application specific information necessary for developing a Modbus Master
application program for interfacing with the Protocol
TM
Plus slave controller. The assumption is made
that the programmer has at least an intermediate understanding of the Modbus Protocol. Only limited
information is presented here regarding the Modbus Protocol specifications. Below is a list of relevant
Modbus Protocol documents. A quick reference section is provided in this document (see section 6
Modbus Protocol Quick Reference). A programming guideline section provides a programming
example and general communications hints (see section 7 Programming Example and General Hints
for programming help).
Table 1: Modbus References
Modicon Modbus Protocol http://www.modicon.com/techpubs/intr7.html
Schneider Automation Open Modbus/TCP
Specification
http://www.modicon.com/openmbus.html/
Table 2: Protocol
TM
Plus References
Engineering Specification for Despatch Protocol
TM
Plus A-50778
1.2 Conventions and definitions
RTU: Remote Terminal Unit
CRC: Cyclical Redundancy Check
$: Represents a hexadecimal number
XOR: Exclusive OR logic operation
LSB: Least Significant Bit
MSB: Most Significant Bit
XX: Don‘t Care when used in a hex number
--: Field contents are data dependent
NA: Not Applicable
TBD: To Be Determined or To Be Defined