E.1 Introduction - Honeywell CM4 Technical Handbook

CM4 Four-Point Continuous Monitor

E.1 Introduction

E.1.1 Overview
The CM4 serial communications protocol was designed and developed
by Honeywell Analytics, and is proprietary information. This manual will
describe the setup and operation of the communication protocol.
Your CM4 four-point continuous monitor is equipped with an optional
serial remote device communication port. With this communication
port, you can monitor the CM4 system's operation with equipment
from a remote location. It will also allow you to gather gas concentra-
tion data for analysis or reports.
The CM4 monitor accepts commands and issues responses to any
valid command it receives. The CM4 monitor is always considered the
"slave" device, and the remote equipment is the "master" device. The
remote equipment can be a personal computer (PC), a programmable
logic controller (PLC), or other device capable of RS-232, RS-422, or
RS-485 serial communications. This configuration requires a "master"
device. Therefore, one CM4 monitor will not communicate directly with
another CM4 monitor. However, one master can be used to communi-
cate with more than one CM4 monitor on a two-wire RS-485 bus.
The CM4 monitor supports multiple baud rates. These user selectable
rates are 1200, 2400, 4800, 9600, and 19,200. Additional port settings
are 8-bit, 1 stop bit, and no parity.
The slave is identified by an address programmed into the CM4 moni-
tor (selectable 1-255). If more than one CM4 monitor is used, each
must have a unique address.
The equipment (master) is always at address 0. Each CM4 monitor
will respond to a signal directed to it from the equipment. To prevent
a collision of messages, the master must avoid transmitting any
information after a packet until the slave responds. Typically this occurs
within 1000 milliseconds.
CM4 Technical Manual
E.1.2 Communication Port
Access to the CM4 protocol is through the COM port. This port is
a DB-9 female connector. This port is designed for bidirectional
communications between the CM4 instrument and your equipment.
Signals present at the port conform to RS-232, RS-422, or RS-485
specifications. Again, this is based on which interface option was
installed in your CM4 instrument. The pin-out specifications for each
option are as follows:
RS-232 (Part Number 874270)
Pin Number
2
3
5
RS-422 (Part Number 874326)
Pin Number
2
3
4
5
RS-485 (Part Number 874556)
Pin Number
2
3
7
E-2
Signal
Transmit
Receive
Ground
Signal
Receive +
Receive -
Transmit -
Transmit +
Signal
B (Transmit/Receive +)
A (Transmit/Receive -)
Signal Ground