Merlin RS Installation & Operation Manual
REV 06 Page 7 of 12
VFC Relay Wiring
This is a dry contact/volt free connection to a BMS (building management system) or central control panel to
send an alarm signal upon the defined CO2 level. This terminal can be used in conjunction with other
external relays that affect other devices, and controls such as purge fans, audible alarms, strobe/beacons etc.
As a factory set condition, the relay is Normally Closed when the device is energised and in normal operation
(fail safe), ensuring relays are triggered upon a power failure.
Thermistor Wiring
This 2-way terminal provides an output for a
BETATHERM, 10K3A1B
NTC type thermistor fitted to the PCB.
Other thermistor types are available, contact your supplier for more information.
RS-485 Wiring
Ensure the equipment is 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.
Ensure A+/B- signal polarity is maintained throughout the network.
Connect GND wire to 0V common grounding terminal.
Daisy chain wires running to additional RS sensors
–
127 sensors max per cable run. First and last device in
bus requires a termination resistor. Use EOL switch to terminate RS sensor. Recommended 3-core, 2 twisted
pair + ground, shielded cable with 120 Ω characteristic impedance.
Shield terminal [SH] is used to maintain shield line only. Shield is cut and taped on last device.
Consider voltage drops due to cable resistance for long distances/large networks. Cable specifications
ultimately determine the distance the sensors can be mounted away from a control panel or power supply.
Strip cables as short as possible.
See
RS BACnet or Modbus Functionality
Document for more information and parameterization.