The MSC-021 is a 1 Duty, 1 Standby Controller that enables equal workload sharing between two devices (e.g. air-conditioning compressors, fans, pumps, etc) through schedule auto duty changeovers while continually monitoring the primary/duty device. In the event the primary device is detected to have tripped, the standby device will automatically commanded to take over. The MSC-021 helps prolongs the lifespan of the device while ensuring 24/7 availability at all times.

In a typical setup, a time-clock (or other start signal) is sent to the MSC-021 controller, which commands one device to run. The controller continually polls for a fault signal (e.g. overload or trip) returning from the device and automatically switches between the duty and standby device in the event of a fault. The MSC-021 also provides workload sharing by rotating the devices automatically between clock/start signals. Even during a power failure, the MSC-021 retains its memory hence ensuring proper workload sharing.

For applications where the process is required to run continuously (24×7), the MSC-021 contains an accurate 12 or 24 hour (user-selectable) timer which swaps the duty and standby devices automatically when the timer expires. This eliminates the requirement of an additional time-clock.

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  • Seamless Switch-Over
  • Last Duty Memory (maintained even during power loss)
  • A+B ON Mode
  • 12/24 Hour Automatic Rotation Timer
  • Emergency Stop

Connection Diagram


SW1.1 – Auto Duty Changeover Timer Setting

  • On = 24h
  • Off = 12h
  • Note: Duty changeover will also be performed when there is an on-off power cycle.

SW1.2 – Output Logic Mode Setting

  • On = Latched
  • Off = Momentary / Pulse (1s)
  • Note: In the Latched mode, the output relay will be energized when ON and de-energized when OFF. In the Momentary mode, output relay will produce a 1s Pulse to toggle the compressor ON or OFF. Whether the pulse is inhibited or not depends on the current running status of the compress (feedback is via Status A and B Inputs).


  • Air-Conditioning System
  • Water Fountains Control System
  • Koi Ponds
  • Plant Processes

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