Bacharach MultiZone, Installation & Operation Manual, Page: 61 / 84

Multi-Zone Gas Monitors
P/N: 3015-5074 Rev 11 55
APPENDIX B. RS-485 COMMUNICATIONS PROTOCOL
B.1. Overview
The following instructions are intended as a guide for integrating the MZ network into a Building
Management System. If you are unfamiliar with complex systems of this type, it is recommended that you
contact Bacharach for technical assistance.
B.2. MODBUS RTU Protocol
The MZ monitor communicates with master devices (such as the Remote Display or a Building
Management System) over the RS-485 serial interface. Because the monitor is configured with a two wire
RS-485 serial bus, data transmission occurs in “half-duplex” mode. Therefore, only one device may be in
transmit mode at any given time.
This document specifies the MODBUS communications protocol as implemented on the applicable
instruments. Programmers should refer to the MODB US Protocol Reference Guide for det ails and more
comprehensive instructions. The guide is available at
www.modbus.org . Specific communications
software is available at: http://www.MyBacharach.com/downloads.htm
B.3. MZ MODBUS RTU Operation
B.3.1. Overview
The MZ and RD are equipped to communicate with other equipment using the MODBUS RTU protocol.
Using this communications channel a MODBUS master device may communicate with up to 15 MZ
monitors on a communications network, exchanging measurement information, alarm data, fault data,
history (logs and trends) and setup information. Additionally, the MODBUS master can control the operating
state of an MZ monitor, placing the MZ in any of its different operating modes. The network may be
configured so that the MZ monitors are connected directly to the MODBUS master device, or the MODBUS
master device may communicate with the MZs through the RD.
NOTE: This document was written wit h the assumption that the reader is familiar with the
various setup parameters and operational modes for the Multi-Zone.
B.3.2. Protocol Detail s
A 2 wire RS-485 bus is used for transmission, therefore communications occurs in a Half-Duplex mode.
The MZ is a slave device and will respond to queries in the MODBUS RTU format from a master device.
Two MODBUS functions are supported. They are function 03 (read holding register) and function 16
(Preset Multiple registers). Please refer to the MODBUS Protocol Reference Guide (available at
www.modicon.com/techpubs/techPubNew/PI_MBUS_300.pdf) for protocol detail and use instructions.
Using the two MODBUS functions, a master device may read, modify and write data and status information
to any monitor on the network. Data is organized into structures (internal to the MZ monitor) which can be
accessed by the MODBUS registers defined in this document. A corresponding set of data structures
should be maintained by the master device. These master device data structures become the destination
for responses to read queries and sources for preset register commands. When a read holding register
query is made by the master device the MZ monitor responds by sending the contents of the structure
referenced by the specified register. After the master validates the MZ response using the CRC bytes, it
must then move the data into its matching data structure before individual items may be accessed or
modified. Therefore, the master data structure should correspond to the MZ data structure byte for byte.
Note that some data structures have been divided into multiple registers due to MODBUS RTU message
length constrains. To change a setting in the MZ monitor, the master device first reads the register structure
that contains the data item to be modified, makes the desired change, then sends the structure back using