18
Detecting Gas Saving Lives
English
7. RS485 Modbus configuration
7.1 General
This section explains how to use
IRmax
detectors in a Modbus configuration. An RS485
enabled version is available for such applications. Up to 32
IRmax
detectors can be linked
in star or bus configurations as described below.
Connections to the RS485 version of the
IRmax
must be made via a spigot gland to an
auxiliary junction box (see Section 5 on page 16), rather than by cabling directly into
the detector.
Please note that the
IR
Display
cannot be used with RS485 enabled
IRmax
.
The RS485 enabled version of the
IRmax
has five field terminals . The terminals are:
1
Positive supply
12 V to 30 V above 0 V
2
0 V
Return and RS485 reference
3
4-20 mA signal
Source or Sink signal (optional)
4
RS485 B
RS485 differential signal
5
RS485 A
RS485 differential signal
The signals on the RS485 terminals conform to the EIA/TIA-485 standard, which means that
the common mode range is -7V to +12V with respect to the 0V terminal.
Note that not all RS485 manufacturers agree on the polarity of the A and B signals. If the
wiring does not work one way, users should switch the RS485 A&B wires. There is no risk
in making the wrong connection.
The communication settings are 9600 bps, two stop bits and no parity.
When compiling an interface for a control system, it is important to consider the amount of time it
takes for the system to collect information from each detector in turn. The fastest speed at which
multiple detectors can be polled is 14 detectors per second; practical conditions may reduce this to
7 per second. Users must ensure that the arrangement allows alarm signals to be registered within
acceptable time limits.
It is also important to ensure that the system can supply enough power to keep all the
detectors working. To calculate the amount of power required in a linear bus connection,
see Section 7.3, Cabling requirements, on page 20.
For full Modbus instructions, contact
Crowcon
(see addresses on back cover).
