10 • MRLDS-450 Refrigerant Gas Detection I&O Manual
026-1316 Rev 0
Using appropriate cable glands, connect the wires for relay 1, relay 2, and relay 3 to
the terminals as indicated in
Table 3-2
- Relay Wiring Configuration
Relay
Function
1
Low Alarm
2
High Alarm
3
Fault Alarm
When configured according to the factory default settings, the relays are
de-energized during normal operation (not fail-safe). Fail-safe mode can be
configured. When configured for Failsafe operation, relays are energized during
normal operation. Failsafe operation ensures relays are triggered in cases of power
failure at the instrument. In Failsafe operation, normally open and normally closed
terminals are reversed as indicated in
Table 3-3
- Relay Wiring Terminal Configuration
Terminal
Normal Operation
Failsafe Operation
NC
Normally Closed
Normally Open
COM
Common
Common
NO
Normally Open
Normally Closed
3.4.4. Modbus RTU RS-485 Interface
For the Modbus RS-485 network use a 16 to 24 AWG (0.5 to 1mm
2
) 3-core,
2 twisted pair + ground, shielded cable with 120Ω characteristic impedance.
IMPORTANT: Recommended: Belden 3105A (or equivalent).
The Modbus address, baud rate, stop bit, parity and slave termination is configured
through the setup menu. No jumpers or hardware switch settings are required.
Ensure that the communication parameters within the network, including the BMS,
are configured identically.
To ensure optimal performance of the Modbus network ensure the following
guidelines are implemented:
• Ensure instruments are configured in a single bus topology, connecting multiple buses in
parallel or branching multiple units from the main bus, may introduce impedance mismatches,
reflections and/or signal distortions.
• Avoid long stubs when connecting instruments to the bus, stubs should be less than one (1)
meter in length.
