The Woodward Digital Reference Unit is a device designed to provide a precise 0 to 8 volt reference signal to an attached control system. It serves as a more precise and dependable alternative to Motor Operated Potentiometers (MOPs), offering easier adjustment of setpoints and adjustable ramp times, which are typically not available with MOPs. The unit is suitable for use with speed or load controls, as well as Process and Import/Export controls.

Function Description:

The Digital Reference Unit generates a linear ramp to produce its 0 to 8 volt reference signal. It features two separate, switch-selectable ramps that control this output signal. The unit allows for the selection of high, low, and intermediate setpoints on the ramp, and these same setpoints are utilized by both ramps. One of the ramps is always engaged, ensuring that any movement of the output occurs at the speed of the currently engaged ramp.

Important Technical Specifications:

Output Signal: 0 to 8 Vdc reference signal.
Power Supply Options:
- Low voltage: 10 to 45 Vdc.
- High voltage: 90 to 150 Vdc or 88 to 132 Vac at 45 to 440 Hz.
Power Consumption: Maximum 3.0 W.
Operating Temperature: -40 to +185 °F (-40 to +85 °C).
Vibration Resistance: Withstands 4 Gs of vibration between 5 and 500 Hz.
Shock Resistance: Withstands 60 Gs of shock.
Ramp Rates: Independently adjustable from a minimum of 20 seconds to a maximum of 120 seconds. Miniature rocker switches inside the unit's cover allow for changing ramp rate ranges:
- 1st (default) Ramp:
  - Switches 3 & 4 open: 5 seconds to 25 seconds (full CCW to full CW adjustment).
  - Switch 3 open, 4 closed: 20 seconds to 120 seconds.
  - Switch 3 closed, 4 open: 100 seconds to 10 minutes.
- 2nd Ramp Selected:
  - Switches 1 & 2 open: 5 seconds to 25 seconds (full CCW to full CW adjustment).
  - Switch 1 open, 2 closed: 20 seconds to 120 seconds.
  - Switch 1 closed, 2 open: 100 seconds to 10 minutes.
- Ramp time is linear and independent of the limit values, representing the time to ramp from the low limit to the high limit.
Relay Option (for models 8272-683 and 8272-685):
- Three relays for indication of low limit, setpoint, or high limit, or for automatic switching functions.
- Relay contacts: 5 A resistive at 30 Vdc, or 2 A resistive at 115 Vac.
Part Numbers:
- 8272-682: 10-45 Vdc, No Relay Indication.
- 8272-683: 10-45 Vdc, Yes Relay Indication.
- 8272-684: 90-150 Vdc or 88-132 Vac 45-440 Hz, No Relay Indication.
- 8272-685: 90-150 Vdc or 88-132 Vac 45-440 Hz, Yes Relay Indication.

Usage Features:

Output Limits Setting: The Low Limit potentiometer sets the minimum output, and the Range potentiometer sets the maximum output. It is crucial to set the Low Limit first, then reset the Range (High Limit) after any change to the Low Limit.
Setpoint Adjustment: The set-point potentiometer allows for adjustment of the intermediate setpoint.
Input Priority Logic: The unit's electronics always respond to the lowest signal selected if multiple selection switches are closed simultaneously. Set Point has priority over Raise, and Lower has priority over Set Point. If Raise is permanently closed, the unit will automatically go to the High Limit when neither Lower nor Set Point switches are closed.
External Reference: The unit can provide ramp-speed setting for EPG and 2500 controls. This feature is enabled by connecting an external reference from these controls to terminal 7 on the reference unit and jumpering terminals 8 and 9.
Installation: The unit can be installed in any position that provides adequate ventilation and servicing space. It is not designed for mounting on the engine or exposure to rain/pressure wash.
Wiring: Wiring should follow the plant-wiring diagram and application charts. Shielded wiring is required between the reference unit and the control system, with all shields terminating at the control system only to maintain effectiveness.

Maintenance Features:

Electrostatic Discharge (ESD) Awareness: The unit contains static-sensitive parts. Precautions must be observed during handling, including discharging body static, avoiding synthetic materials, keeping plastic/vinyl/Styrofoam away from the work area, and handling printed circuit boards (PCBs) by edges only, avoiding contact with components or conductors. New PCBs should remain in antistatic bags until installation, and old PCBs should be immediately placed in them.
Battery Charging Caution: When the control system uses an alternator or battery-charging device, it must be turned off before disconnecting the battery to prevent damage.
Service Options: Woodward offers several service options:
- Replacement/Exchange (24-hour service): A premium program for immediate service, providing a like-new replacement unit. A core charge is applied, credited upon return of the field unit within 60 days. A Return Shipment Authorization Label is provided to ensure proper processing.
- Flat Rate Repair: Repair service for standard products with a known, fixed cost. Includes Woodward service warranty on replaced parts and labor.
- Flat Rate Remanufacture: Similar to Flat Rate Repair but returns the unit in "like-new" condition with a full standard Woodward product warranty. Applicable to mechanical products only.
Returning Equipment for Repair: Contact Woodward in advance to obtain a Return Authorization Number. Attach a tag with installation details, contact information, part/serial numbers, problem description, and desired repair type.
Packing a Control for Return: Use protective caps, antistatic bags for electronic modules, non-damaging packing materials, at least 100 mm (4 inches) of tightly packed, industry-approved packing material, a double-walled carton, and strong tape.
Replacement Parts: When ordering, provide the part number(s) from the enclosure nameplate and the unit serial number.
Technical Assistance: Contact Woodward's Customer Service Department for troubleshooting or support. Provide general information (name, location, phone, fax), prime mover information (model, manufacturer, cylinders, fuel type, rating, application), and control/governor information (Woodward part number, revision letter, description, serial number). For electronic/programmable controls, have adjustment or menu settings available.

Woodward Digital Reference Units - Figure 4-2. Wiring Diagrams

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