Diagnostic LEDs
The G128 is equipped with three front panel LEDs. The first is a dual function LED labeled INPUT. This green LED indicates DC power and input signal status. Active DC power is indicated by an illuminated LED. If this LED is off, check line power and wiring connection. If the input signal is more than 110% of the full scale range, the LED will flash at 8 Hz. Below -10%, the flash rate is 4 Hz.

Two red LED’s indicate the relay state for each setpoint. An illuminated red LED indicates the tripped condition.

Output
The G128 is equipped with two SPDT (form C) relays, rated at 120VAC or 28VDC at 5 amperes. Each of these relays is independently controlled by the field configurable set point and deadband.

Operation
The field configurable G128 limit alarm setpoints can be configured for HI or LO, failsafe or non-failsafe operation. Each of the setpoints has a respective HI or LO deadband. In a tripped condition, the setpoint is exceeded and the appropriate red LED will illuminate. The trip will reset only when the process falls below the HI deadband or rises above the low deadband (see Figure 1). For proper deadband operation, the HI setpoint must always be set above the LO setpoint.

In failsafe operation, the relay is energized when the process is below the HI setpoint or above the LO setpoint (opposite for non-failsafe). In the failsafe mode, a power failure results in an alarm state output.

Dynamic Deadband
LSI circuitry in the G128 prevents false trips by repeatedly sampling the input. The input must remain beyond the setpoint for 100 milliseconds, uninterrupted, to qualify as a valid trip condition. Likewise, the input must fall outside the deadband and remain there for 100 milliseconds to return the alarm to an untripped condition. This effectively results in a “dynamic deadband” —based on time— in addition to the normal deadband.

Configuration
Unless otherwise specified, the factory presets the Model G128 as follows:

The DC power input accepts any DC source between 9 and 30V, typically a 12V or 24VDC source is used (see Accessories).

For other I/O ranges, refer to Table 1 and reconfigure switches SW1 and SW2 for the desired input type, range and function.

1. With DC power off, position switch SW1-1, 2, 3 and SW2-1 through 6 for the desired input range (Table 1).
2. Set positions 4 and 5 of function switch “SW1” to ON for a HI trip setpoint or OFF for a LO trip setpoint (Figure 4).
3. Set position 6 of function switch “SW1” to ON for non-failsafe operation or OFF for failsafe operation (e.g. alarm trips upon power failure).
4. Set positions 7 and 8 of function switch "SW1" to upscale or downscale burn-out.

1. After configuring the DIP switches, connect the input to a calibrated TC source and apply power. Refer to the terminal wiring (Figure 5).
2. Setpoint: set deadband at its minimum (20 turns counter clockwise) before adjusting the setpoint. With the desired trip thermocouple millivolt input applied, adjust setpoint until the relay trips. For HI trip calibration, start with the setpoint above the desired trip (20 turns clockwise). For LO trip calibration, start below the desired trip (full counter clockwise).
3. Deadband: Set deadband to its minimum (20 turns counter clockwise). Set the setpoint to desired trip. Adjust thermocouple millivolt input until relay trips. Readjust deadband to 5% (20 turns clockwise). Set voltage/current input to desired deadband position. Slowly adjust deadband until relay untrips

Relay Protection and EMI Supression
When switching inductive loads, maximum relay life and transient EMI suppression is achieved using external protection (see Figures 2 and 3). Place all protection devices directly across the load and minimize all lead lengths. For AC inductive loads, place a properly-rated MOV across the load in parallel with a series RC snubber. Use a 0.01 to 0.1mF pulse film capacitor (foil polypropylene recommended) of sufficient voltage, and a 47W, 1/2W carbon resistor. For DC inductive loads, place a diode across the load (PRV > DC supply, 1N4006 recommended) with (+) to cathode and (-) to anode (the RC snubber is an optional enhancement).