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Preferred Instruments

Preferred Instruments manufactures microprocessor based controllers, instruments, and electric actuators for combustion and process applications. Preferred Instruments also manufactures oil tank gauging and leak detection equipment. A major component of the Instrument group sales is from control systems integration and personal computer based Supervisory Control and Data Acquisition systems. Preferred Utilities' industrial factory is located in Danbury, CT

Plant Wide Controller (PWC)

The Plant Wide Controller is a state-of-the-art equipment sequencing, control and monitoring system.  It combines innovative ease of operation, communication and expansion capabilities with boiler plant control application expertise.


The PWC can be configured for an unlimited number of control applications. These configurations are the most common:

Lead/Lag Controller for Improved Steam or Hot Water System Availability: Automatic Sequencing ensures that the number of boilers in service meets hot water or steam demand.  Tripped equipment is automatically replaced with a standby unit.

Boiler Monitoring: Flue gas temperature, smoke opacity and boiler draft may be monitored and trended.  Warning alarms and burner safety shutdown interlocks may be included.

Unmanned Boiler Plants: Provides for off-site monitoring and control using internal modem or RS485 interface.  Serves as a  single plant monitoring point for Building Automation Systems and personal alphanumeric pagers.

Cooling Tower Optimization: Multiple Tower Cells are sequenced and fan speed controlled with wet bulb optimization.  Substantial fan and chiller electrical savings can be realized.

Improved E-Gen Fuel System Availability: Fuel pump standby sequencing, day tank level control and fuel storage tank level and leak monitoring.

Improved Steam System Availability: Condensate transfer and feed pump standby sequencing, Deaerator and Surge tank level monitoring, alarm and remote communications.

Coordinated Hot Water System Operation: Pumps, isolation valves, distribution pumps and temperature monitoring for reduced thermal stress and energy consumption.

Fresh Air Dampers, Air Compressors and Fans: Sequencing, monitoring, and control are based on the number of boilers online.  A single damper failure will not prevent a boiler from firing.

Custom configurations are available to suit virtually any application.



The Plant Wide Controller (PWC) is a state-of-the-art equipment sequencing, control and monitoring system.  The PWC combines innovative ease of operation, communication and expansion capabilities with boiler plant control application expertise.  Off-the-shelf, standard applications for boiler modulating lead/lag, cooling towers and air compressors can be expanded to include additional monitoring or control additional pumps, variable speed drives and valves.  Multiple communication protocols allow simultaneous communication to alphanumeric pagers, laptops via standard telephone lines and Building Automation System or SCADA Systems using a control network.  The PWC is a complete plant monitoring, control and communication interface.

Easy Installation: The PWC integrates a powerful Programmable Function Controller (PFC), I/O boards, hardwired and LCD HMI, power distribution, 24VDC power supplies, external communications, isolation relays into a single wall mountable controller.  No external control devices are required.

Easy to Operate: Large LCD Display, intuitive operation, setup, alarm / event summary and historical trend displays allow quick process assessment and maintenance monitoring.

Easy to Configure: PWC configuration tools maintain the look and feel of the PCC-III and offer advanced features.  The PWC uses an intuitive “Blockware” configuration language with multiple block outputs and special purpose “Super” blocks that greatly simplify complex logic such as Outdoor Air Reset and boiler sequencing.


Blockware: The PWC uses an intuitive “Blockware” configuration language.  Functions (AIN, PID, LOALM, F(x)… ) are simply copied into a configuration, and then the control signals are “wired” from block to block.  Preferred’s innovative PWC_Draw™ for MS Windows® uses a graphical, “drag and drop” interface.  It allows the user to print or plot Blockware drawings, and then download them to a PWC via a standard RS232 port.  Additionally, Blockware and displays may be edited from the spreadsheet style PWC_Edit™.

Multiple Block Outputs: Using the Analog Input Block’s “BAD” data quality output a user may switch a loop to manual control or initiate an alarm.  The PWC display and any block can access all block outputs.  Other available outputs include cold junction temperature, input is out of normal range,  pulser is missing pulses, input type selector switch position does not match the input type, etc,.

Super Blocks: The PWC provides a collection of special function “Blockware” to enable simplified implementation of complex control strategies.  The function Outdoor Reset Setpoint [ORSP] is used to save energy by changing a setpoint based on the outdoor air temperature.  A typical application is to use the ORSP to generate the Hot Water Setpoint for a Hot Water Heating System.  Another important energy savings block is the Scheduler block.  The Time/Date Scheduler [SCHED] compares the current Time and Date to the schedule defined by the entered parameters, and sets the schedule output to “1” during the “Normal” period, and to “0” during the “Setback” period.  Typically this function is used to conserve energy during low occupancy periods.  It can be used to “setback” hot or chilled water temperature setpoints, activate outdoor lighting, and other time or day of week, or date based control logic.

LCD Display Commissioning: Plant Wide Controller configurations are designed to allow commissioning to be accomplished from the controller mounted displays.  Project specific tuning displays may be created to present and group key “Blockware” parameters for field tuning.  Additionally, any block parameter may be edited from the front panel display using the “Parameter Edit” mode.  Laptop computers are only required when it is necessary to change wiring between blocks or add additional blocks.

PWC_Edit™: The “point and click” simplicity of the PWC_Edit software makes “Blockware” configuration simple and intuitive.  The program uses a straightforward spreadsheet format with a convenient fill-in-the-blanks approach.  Each Block has an unlimited length “comments” field for clear documentation.  The “Blockware” data and comments can be printed to any MS Windows® compatible printer.  PWC_Edit offers fill-in-the-blanks style display generation.  Display text can be presented as either regular or bold.  Dynamic-text, softbuttons, status, numeric values, time values and alarms may be added to any display.  The Chart Edit display allows configuration of trace and chart selections using a menu style system.  The generated configurations are then easily downloaded using a standard RS232 DB9F cable.

PWC_Draw™: The powerful object-oriented CAD interface in PWC_Draw makes the program the ideal choice for rapid “Blockware” programming in a visual environment.  The program is built on a Visio® platform with extensive Visual Basic automation.  Standard functions are included in menus of pre-drawn figures for each PWC Blockware Function Type.  Functions are simply dragged onto the drawing page and connected with “Smart Connector” lines to interconnect the Blocks.  Block inputs are automatically generated by placing the Block connections.  Double clicking on any block allows the user to edit data within the Block.  Drawings can be saved as AutoCAD® drawings and can be printed on any MS Windows® compatible printer or plotter.  “Blockware” data can also be printed in the PWC_Edit tabular format.

Historical Trend Display

Each Chart can display up to 4 traces, called ‘Pens’.  The bottom of the screen shows the symbol and name of each Pen.   Charts can be a mixture of analog and discrete data.  A specific chart is displayed by selecting a Menu line that is linked to the chart.  The PWC can save up to 32 analog values plus up to 32 discrete values every 1, 5, 15, or 60 seconds in the 32 Mb non-volatile memory.  The 32 Mb Historical Memory can store up to 1-6 months of data (number of points monitored, sample interval, and data compression ratio affect duration).

Pen Selection: Each “Pen” trace has a unique name, chart scale, and engineering units.  However, only one Pen Scale can be displayed at a time. The Up and Down Cursor Arrows may be used to display the desired Pen Scale.

Chart Cursor Readout: When a chart is first displayed, the Chart Cursor is located at the right hand edge of the screen.  Using the Numeric keypad Arrows the operator may move the Chart Cursor.  The number on the top line of the screen is the value of the currently selected Pen trace where it touches the Chart Cursor. Use the Cursor Up and Down Arrows to display the values for the other Pens.

Start Time Panning: Use the Cursor Left and Right Arrows to shift the Start Time backward or forward in time.  The time is shifted 7/8 of the span to provide chart display overlap.

Changing Chart Span: Using the PAGE UP and PAGE DOWN keys, the operator may change the Chart Span between 8 minutes, 40 minutes, 2 hours, 8 hours, or 24 hours.

‘Span’, Start Time and Date Selection: When a Chart is selected, the first screen that appears is the “Setup Display.”  This display allows the operator to easily select “Span” (width), Start Time and Date. This screen defaults to the current time and date with a 40 minute wide chart.


Telephone Modem: The internally mounted Telephone Modem permits the PWC to “dial out” to an alphanumeric pager and allows a user to “dial in” to the PWC to view all displays and make tuning adjustments.  Selected alarms cause the modem to dial a pager service center telephone number.  A 20 digit phone number can include outside line codes, access codes, and pauses.  The modem uses the TAP protocol to send a text message to an alphanumeric pager. The message can include a 30-character facility name plus a 20-character alarm message.  Upon receipt of the page, the user can “dial in” to the modem to acknowledge the message. If the system does not receive a dial back acknowledgement or front panel Alarm Silence button “press” within 5 minutes (adjustable), the system will dial a second backup pager and re-send the alarm message. The system will log the time and date of all pages and acknowledgments in the Alarm/Event List.  Using PWC_Remote™ software, which is included with the Telephone Modem (option p/n 190604), a remote user is able to “dial in” to the modem to view any screen and remotely “press” any keypad button just as if they were standing in front of the controller. PWC_Remote™ software running on the user’s personal computer can “dial in” to any PWC site, and does NOT require site specific programs or custom configurations.

Control Network: The PWC includes an RS485 Modbus port to communication with Building Automation System (BAS), Building Management System (BMS) or Supervisory Control and Data Acquisition (SCADA) systems.



  • Programmable Function Controller (PFC)
  • Hardwired Panel
  • Easy To Configure


Case Size: 16½” H x 14½” W x 6¾” D

Enclosure Type:  Wall Mounted

Case: 7 Slot, (CPU + 6 I/O Slots)

Weight: 55 lbs


Operating Temp: 32° to 122° F (0° to 50° C)

Storage Temp: -20° to 150° F (-28° to 65° C)

Humidity Limits: 15 to 95% (non-condensing)

Enclosure: NEMA 1


Accuracy: 0.025% Analog I/O

Resolution: 16 bit input/12 bit output

Microprocessor: 32 bit, 128k EEPROM

Execution Cycle: Five per second

Time/Date Clock: (battery backed)

Operator Control Panel

LCD Graphic Display:  2.9″ H x 5.1″ W

Keyboard: Membrane, tactile feedback

Historial Data (Optional)

Displays: 8 or 40 minute; 2, 8 or 24 hour charts

Memory: Non-Volatile, 32 Mb.  48 points every 15 sec for 30 days, or greater with datacompression


Standard Lead/Lag: Menu style, “Fill-In-The-Blanks” setup.

Control Language: Function block style, 60 functions, 600 Blocks

Security: 2 password levels

Custom Blockware Configuration Software: PWC_Edit™ spread sheet based or PWC_Draw™ graphical, editor (Windows PC Required).


Control Network

Protocol: Modbus (ASCII or RTU mode)

Speed: 1200 to 38,400 baud

Type: RS485, optically isolated

Telephone Modem: Internal Card 33,600 baud, RJ-11 Jack, Data and Pagers

Printer Port: Alarms/Logs, DB25F connector

Programming Port

Speed: 9600 to 38,400 baud

Type: RS232, DB9F connector


Input Power: 120 VAC (+/- 15%), 12A total, 0.7A internal Built in surge suppressors

Internal Power Supply:24 VDC @ 300 mADC for external use

Input/Output Specifications

CPU Board

Analog Inputs: Quantity, 2; Type, 4-20 mADC or -20°F to +300°F Thermistor

Relay Output: Quantity, 5; Type, SPDT, 8A, ½ HP, 120VAC

Hand-Off-Auto Relay Output (HOA-ROUT) Board

Relay Output: Quantity, 5; Type, SPST, 8A, ½ HP, 120VAC

Toggle Switches: Quantity, 5 Type, Hand-Off-Auto (hard wired) SPDT, 8A, ½ HP, 120VAC

LED Indicators: Quantity, 10; Type, “Call for Operation” and “Output Status”

Auto/Manual Analog Output (A/M-AOUT) Board

Analog Output: Quantity, 5 Type, 4-20 mADC or 0-135 ohm (any combination)

Toggle Switches: Quantity, 5; Type, Auto-Manual

Control Dial: Quantity, 5; Type, 0-100% (Manual Potentiometer)

Bargraphs: Quantity, 5; Type, 0-100%, 10 segment

Discrete Input (DIN) Board

Digital Inputs: Quantity, 15; Type, 120 VAC, optically isolated

LED Indicators: Quantity, 15; Type, Status Indication

Analog Input (AIN) Board

Analog Input: Quantity, 8; Type, Universal, Switch Selectable as: 4-20 mADC, 2 wire Thermistor, -20°F to 300°F, Thermocouple Type J, 0-1200° F, 0-5 VDC, or Potentiometers Pulse, 0.01 – 4000 Hz, 0-15 VDC

LED Indicators: Quantity, 8; Type: Status Indication

Relay Output (ROUT) Board

Relay Output: Quantity, 8; Type, (2) SPDT, (6) SPST-NO, 8A, ½ HP, 120 VAC

LED Indicators: Quantity, 8; Type, Status Indication


General: Supply a microprocessor-based control system with field expandable plug-in Input/Output modules.  Control logic shall be either Ladder Logic or Function Block based.  Any/all loop controllers, programmable logic controllers, and/or historical trend recorders within the Control System shall be interconnected via serial links to minimize wiring of internal control signals from device to device. The control system logic and calibration data shall be stored in a non-volatile memory that does not require battery backup.  A field replaceable battery back-up shall be included to maintain the system time/date clock. The control system shall operate on 120 VAC and include a surge suppressor.  The control system shall include a 24 VDC power supply with 300 mADC available for external use that is UL508A rated for 120° F.

Enclosure: A wall mounted, factory-assembled steel enclosure shall be provided. All operator interface control switches, indicators and displays shall be physically separated from any field terminations.  During normal operation it shall not be possible for an operator to come in contact with 120 VAC wiring.  Manual Backup control switches and indicators must be protected from unauthorized operation by a key lockable door with a viewing window.

Operating Displays: The control System shall have a flat panel LCD Display for operator control, alarm listing, control tuning and troubleshooting functions.  Provide tactile feedback, numeric keypad for data entry.  Provide dedicated pushbuttons for Alarm Silence and to view a Plant Overview displays.   The display shall be 5″ x 2.9″, 8 line x 40 character or larger.   The Control System shall include a password protected menu system for controller tuning functions.

Historical Trend Display: The Control System shall provide historical trend displays by using a paperless chart recorder or other video graphic hardware.  This recorder shall include a 100 x 150 pixel resolution, up to 4 traces per chart, 8 minute to 24 hour chart “width” and a non-volatile memory for up to 32 data points for at least 45 days of history.  Arrow keys shall be provided to scroll backward and forward thru time.  For efficiency monitoring, tuning, and troubleshooting, a technician shall be able to re-configure trace and chart selections using a menu style system.

Alarm and Event Management: Alarms, events and operator actions shall be logged with Time/Date stamp and English language description.  The control system shall include a 200 point memory minimum.  Provide an Alarm Display page for viewing the most recent 8 alarms/events with scrolling capability to view the complete 200 point alarm/event memory.  New alarms shall trigger the common alarm output relay. Events shall be recorded, but shall not trigger an alarm. A dedicated Alarm Silence button shall silence the alarm output.

Control Panel Mounted Indicators: Provide individual long life LED status indicators for all controlled equipment.  All indicators shall be labeled with a permanent marking.

Input / Output Signal Types: The Control System shall include the following input/output signal types: Analog inputs shall be universal type and must be field selectable between 4-20 mADC, Thermistor, Thermocouple, Potentiometer and pulser.  Analog outputs shall be 4-20 mADC and 0-135 ohm.  Discrete inputs shall be 120 VAC, optically isolated type.  Relay outputs shall be SPDT and SPST, 8A, ½ HP, 120 VAC.

Reliability: Field wiring shorts or ground loops within one pump, valve or fan shall not affect automatic or manual operation of other devices.  Provide electrically isolated relay contact and isolated 4-20 mADC/0-135 ohm modulating control outputs.  Each Transmitter and Sensor shall have individual power supply short circuit protection.   “Hard Manual” backup stations shall be provided to ensure continued central operator control in the event of CPU memory corruption or failure.  Include hard wired “Hand-Off-Auto” control switches inserted directly into every boiler, pump, damper, fan, etc… Start/Stop  circuit.  Each 4-20 mADC or 0-135 ohm modulating control output must include a hard wired Manual Backup Station with Auto/Manual Switch, output control knob or pushbuttons, and output level indicator (bargraph, analog meter or digital display). The Manual Station hardware must function when the CPU is not functioning.

Remote Monitoring and Paging System: Selected alarms shall cause a modem to dial a pager service center telephone number. Provide a 20-digit phone number that can include outside line codes, access codes, and pauses. The modem shall use the TAP protocol to send a text message to an alphanumeric pager. The message shall include a 30-character facility name plus a 20-character alarm message.  Upon receipt of the page, the person shall “dial in” to the modem to acknowledge the message. If the system does not receive a dial back acknowledgement or front panel Alarm Silence button “press” within 5 minutes (adjustable), the system shall dial a second backup pager and re-send the alarm message. The system shall log the time and date of all pages and acknowledgments in the Alarm/Event List.  A remote user shall be able to dial in to the modem and be able to view any screen and remotely “press” any keypad button just as if they  were standing in front of the control system.  Provide software to allow a remote user’s personal computer to “dial in” to any Control System site, without custom configured for each site.

Control Network: In addition to the Remote Monitoring and Paging System features, the Control System must include a RS485 Modbus communication interface to a Supervisory Control And Data Acquisition (SCADA) System, Building Automation System (BAS), or Building Management System (BMS).

Quality Assurance: The control enclosure shall be manufactured and labeled in accordance with UL508A (CSA C22.2 #14 for use in Canada).  Simply supplying UL recognized individual components is not sufficient.  The assembled control enclosure, as a whole, must be inspected for proper wiring methods, fusing, etc., and must be labeled as conforming to UL508A.  Inspection and labeling shall be supervised by UL or other OSHA approved Nationally Recognized Test Lab (NRTL).  Lack of an NRTL certified UL508A wiring methods inspection and labeling will be grounds for control enclosure rejection.