Specifications

Multi-Zone Gas Monitors
P/N: 3015-5074 Rev 13 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 MODBUS Protocol Reference Guide for details 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 with the assumption that the reader is familiar with the
various setup parameters and operational modes for the Multi-Zone.
B.3.2. Protocol Details
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.
Three MODBUS functions are supported. They are function 03 (read holding register), function 06 (write single
register), and function 16 (Preset Multiple registers – for block mode only). 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.
In addition to the standard register groups found in the 2000 and 3000 address ranges, a block mode access is
provided for OEMs who need efficient access to the complete range of system variables using the least amount of
queries. The block modedata 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 the preset multiple register function. If the transaction is successful, the MZ
monitor sends the appropriate MODBUS response. It is the responsibility of the master device, when making
modifications, to insure that all parameters transferred are within the working limits of the MZ.