aerl
COMMS MANUAL
COOLMAX SR
Australian Energy Research Laboratories, Pty.Ltd
AER07.004 – Version G2 v9
13
th
January 2016
5 of 21
3
MODBUS COMMUNICATIONS
3.1
OVERVIEW
Hardware
The Modbus hardware interface used is compatible with the Modbus RTU standard. The supported bit
rates (bits per second) is 9600 bps.
Software
All measurement data is transmitted using IEEE single-precision 32-bit format (IEEE 754) with most
significant byte (MSB) sent first.
Modbus Address
The Modbus address is a single byte field, valid address ranges from 1 to 128.
3.2
MODBUS NETWORK TOPOLOGY
Multiple COOLMAX SR units can exist on the same Modbus and this can be connected to other
networks using a Modbus-Ethernet Bridge adaptor.
The Modbus is structured as a linear network, with short stubs branching from 'T' connectors on the
main bus backbone to each device. The Modbus data lines must be terminated at each end of the
main bus with 120 Ω resistors between the Modbus-A and Modbus-B signals.
A simple method of implementing the 'T' junction is shown in
. The in and out wire of each
Modbus line are twisted together and screwed into the appropriate terminal on the Modbus connector,
as shown in
Figure 4
. The Modbus connector is then connected into the COOLMAX SR Modbus
connector socket as shown in
Figure 5
.
Figure 2 – Modbus Wiring Diagram
