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Option SSR=m” factory configures the WattNode® Modbus meter to use the spare X terminal on the “MODBUS” connector (upper left corner) for switching external loads up to 500 mA at 60 volts (peak AC or DC). The C terminal provides the ground reference.

Option SSR may be used for any of the following purposes, and others:

  • Signal or control an external device.
  • Send a load shed signal.
  • Control an external relay.

Notes: A separate Modbus master (data logger, energy management system, PC, etc.) is required to control the output state. The SSR output shares the C terminal ground reference with the Modbus interface. When connecting the C terminal to external control circuits, be careful not to create a ground loop. For application assistance call CCS Tech Support.

SSR Configurations

When specifying “Option SSR=m“, the m value is used to configure the IoPinMode (1623) register. You can also change the IoPinMode in the field. The m value or IoPinMode may be one of the following values:


  • 0 – The SSR output X pin is disabled.

Output Modes

A solid state relay acts much like a traditional electromechanical relay. It has a conducting or closed state and a non-conducting or open state. There are three output modes:

  • 3 – The SSR output (X pin) is active. If power is lost and restored, the output is restored to the state (open or closed) it was in before power was cycled. IoPinState controls the output logic state.
  • 4 – Same as mode 3 except the SSR will always start closed (conducting) after power is cycled.
  • 5 – Same as mode 3 except the SSR will always start open (non-conducting) after power is cycled.


The following registers are associated with Option SSR:

IoPinState (1361)

Reading IoPinState reads back the output state (the last value written to IoPinState.

Writing IoPinState does the following:

  • 0 – Set the SSR to the closed or conducting state.
  • 1 – Set the SSR to the open or non-conducting state.

IoPinMode (1623)

This disables (mode 0) or configures the output (modes 3 to 5).

OptSSR (1735)

This reads 1 if the WattNode was ordered with the “Option SSR” feature, or 0 if not.


The SSR is rated for ±60 Vdc or 40 Vac at 500 mA continuous with 1 amp peak surge current. Voltages above ±60 V peak on the X pin or currents greater 500 mA for sustained periods may damage the WattNode meter.

ESD / EMI Precautions

The normal Modbus RS-485 lines are rated to handle 15 kV of ESD (electrostatic discharge). Furthermore, the WattNode Modbus meter without Option SSR has passed all FCC Class B and CE Mark EMC (electromagnetic compatibility) tests. However, the Option SSR version has not been separately tested.

Output Specifications

  • Load Voltage: ±60 V (peak DC), or 40 Vac
  • Load Current: 500 mA
  • Peak Current: 1.0 A
  • On Resistance (typical): 0.85Ω
  • On Resistance (maximum): 2.50Ω
  • Turn On Time (typical): 1.8 milliseconds
  • Turn On Time (maximum): 5.0 milliseconds
  • Turn Off Time (typical): 0.5 milliseconds
  • Turn Off Time (maximum): 2.0 milliseconds
  • Off State Leakage Current (maximum): 1μA

See Also