The Snooper PD865 is an 1/8 DIN Modbus® meter that can be programmed as a Modbus® RTU master, slave, or snooper. In snooper mode, the PD865 listens to the Modbus traffic and displays a process variable being transmitted on the bus. As a master, the Snooper can read up to 8 Modbus variables; it will display one and make the other variables available for other Snoopers to display in snooper mode. As a slave, a Modbus variable may be written to the PD865 by a Modbus master. The PD865 is available with two (2) relays, 4-20 mA analog output, and multi-pump alternation control.


  • Snooper Mode: Listening for the PV you want
  • Proxy Polling Mode: Poll up to 8 PVs
  • RS-485 Modbus® RTU input
  • Six full digits 999,999
  • Master, Slave, or Snooper mode
  • Sunlight readable LED display
  • Scale in engineering units
  • 16-point linearization
  • Square root & programmable exponent
  • Exact display of input data
  • 4-20 mA analog output option (Modbus to 4-20 mA)
  • Two Form A relays standard
  • Two Form C relays option
  • Up to four pump alternation control
  • Universal power supply


Input/Output: Modbus RTU RS-485
Display: 0.56" (14.2 mm) red LED, 6 digits; displays from -199999 to 999999, automatic leading zero blanking
Display Intensity: Eight user selectable intensity levels
Display Decimal Point: Up to five decimal places: d.ddddd, dd.dddd, ddd.ddd, dddd.dd, ddddd.d, or dddddd
Float Decimal Point: Select the number of decimals to use for the floating point data expected from the slave or master device
Front Panel: NEMA 4X, IP65; panel gasket provided
Programming Methods: Four front panel buttons or via Modbus registers (Slave mode only)
Scaling: Input may be scaled from -199,999 to 999,999
Math Functions: Linear, square root, or programmable exponent from 0.50000 to 2.99999
Multi-Point Linearization: 2 to 16 points
Noise Filter: Programmable 2 to 199 (0 will disable filter)
Bypass: 0.2 to 99.9% of full-scale
Cutoff: 0 to 999999; 0 disables cutoff
Display Update Rate: Master: 10/sec to 1 every 25.5 seconds;
Slave and Snooper: Dependent on master device (PLC, DCS, RTU, etc)
Overrange: Display flashes 999999
Underrange: Display flashes -199999
Max/Min Display: Stored until reset by user or meter is turned off; user can reset by front panel buttons or via Modbus registers.
Password: Restricts modification of programmed settings.
Non-Volatile Memory: Settings stored for a minimum of 10 years.
Power Options: 85-265 VAC, 50/60 Hz; 90-265 VDC, 20 W max or 12-36 VDC; 12-24 VAC, 6 W max.
Required Fuse: UL Recognized, 5 A max, slow-blow; up to 6 meters may share one fuse.
Normal Mode Rejection: 64 dB at 50/60 Hz
Transmitter Supply: Isolated 24 VDC ±10% @ 200 mA max
Isolation: 4 kV input/output-to-power line, 500 V input-to-output or output-to-24 VDC supply
Operating Temperature: -40 to 65°C
Storage Temperature: -40 to 85°C
Relative Humidity: 0 to 90% non-condensing
Connections: Removable screw terminals accept 12 to 22 AWG
Enclosure: 1/8 DIN, high impact plastic, UL 94V-0, color: gray
Weight: 9.7 oz (275 g) (including options)
UL File Number: E160849; 508 Industrial Control Equipment
Warranty: 3 years parts & labor

Operating Modes

Master: Processes data read from a Modbus RTU slave device (only one process variable at a time can be displayed on the master).
Master Proxy Polling: Polls up to 8 process variables up to 8 slave devices. The Master processes and displays one PV and allows other PD865s in Snooper mode to read any of the variables being polled.
Snooper: Listens to the Modbus traffic and picks up a specific process variable being polled by a Master device from a specific slave device and processes the data being read.
Slave: Processes data sent from a Modbus RTU master device.
Note that the relays and the 4-20 mA output are functional in all modes.

Master and Snooper Modes Settings

Function Code: Modbus function code 03 or 04.
Number of PVs: Number of process variables to be polled (PV1-PV8)
Slave ID: Address (1-247) of up to 8 slave devices
Register Number: 1 to 65536. Specifies which register(s) to read in the slave device. Register number is preceded by register type (3xxxxx or 4xxxxx). Five or six digit register number allowed.
Data Type: Select the data format that the slave device uses; select between Short (2 byte) and Long (4 byte) Integers or Floating point (4 byte), Signed or Unsigned (integer only), and byte order (big-endian vs little-endian).
Slave Response Timeout: 0 to 25.4 seconds.This is the time allowed for slave to respond to a command. 0 disables timeout (Master Mode Only)
Poll Time: 0.1 to 25.5 seconds between read commands


Rating: 2 Form A (SPST) standard; 2 Form C (SPDT) optional; rated 3 A @ 30 VDC or 3 A @ 250 VAC, resistive loads; 1/14 HP @ 125/250 VAC inductive loads
High or Low Alarm: User may program any alarm for high or low
Deadband: 0-100% FS, user selectable
Relay Operation:
1. Automatic (non-latching)
2. Latching
3. Pump alternation control (up to 4 relays)
Relay Reset: Selectable via front panel or Modbus registers
1. Automatic reset only (non-latching)
2. Automatic plus manual reset at any time (non-latching)
3. Manual reset only, at any time (latching)
4. Manual reset only after alarm condition has cleared (latching)
Automatic Reset: Relays reset when input passes the reset point
Manual Reset: Front panel button, Modbus registers (Slave only)
Time Delay: 0 to 199 seconds, on and off delays, programmable and independent for each relay
Fail-Safe Operation: Programmable, independent for each relay
Communications Break: No change, Relay on, or Relay off. Controls the condition the relay goes to when a slave device does not reply (Master and Snooper modes).
Auto Initialization: When power is applied to the meter, relays will reflect the state of the input to the meter

Serial Communications

Compatibility: EIA-485 (RS-485)
Protocol: Modbus RTU
Slave ID or Address: 1 to 247. Specifies the ID or address of the slave device (Master and Snooper modes) or the address of the PD865 (Slave mode).
Baud Rate: 300 to 19,200 bps
Data: 8 bits (1 start bit, 1 stop bit; 1 or 2 stop bits with no parity)
Parity: None, even, or odd
Byte-to-Byte Timeout: 0.01 to 2.54 seconds
Turn Around Delay: Less than 2 ms (fixed)

Isolated 4-20 mA Transmitter Output

Scaling Range: 1.000 to 23.000 mA; reverse scaling allowed.
Calibration: Factory calibrated 4.000 to 20.000 mA
Accuracy: ±0.1% FS ±0.004 mA
Recalibration: Recommended at least every 12 months.
Temperature Drift: 50 PPM/°C
Transmitter Supply: Isolated 24 VDC ±10% @ 200 mA max
Isolation: 500 V input-to-output or output-to-24 VDC supply; 4 kV output-to-power line
External Loop Power Supply: 35 VDC maximum
Output Loop Resistance:

 Loop Resistance
Power SupplyMinimumMaximum
24 VDC10 Ω700 Ω
35 VDC (external)100 Ω 1200 Ω 

Data Source: Display value, maximum display value, minimum display value, or Modbus register
Overrange: Programmable mA output for overrange condition
Underrange: Programmable mA output for underrange condition
Communications Break: Programmable mA output when a slave device does not reply within the Slave Response Timeout
Maximum Output: Programmable absolute maximum mA output
Minimum Output: Programmable absolute minimum mA output


Master and Snooper Modes

In this application, the PD865 at the top is acting as the Master and requests eight process variables (level & temperature) by polling four multivariable tank level sensors. Seven of these variables are polled by the master on behalf of the seven PD865s acting in Snooper mode (a process called proxy polling). The seven PD865s display specific data captured during the polling process without detection by, or interfering with, either the master or slaves on the network.

Multivariable Level Transmitters

The top Snooper is displaying product level and it is set up as a master device to the level transmitter's slave device. The next two Snoopers are set up in Snooper mode and they are displaying interface and product temperature. Each Snooper's two relays are connected to the PD158 annunciator to indicate alarm conditions.

In Slave & Snooper Modes with PLC Master

Many digital process meters get their 4-20 mA input from a PLC. This requires a costly analog output card in the PLC and also degrades the accuracy of the signal as it is converted from digital to analog and then back to digital. Using a Snooper in these applications means the PLC communicates digitally with the digital display.

Remote Display for Process Meter

The Snooper can be connected to the Modbus serial output from devices like digital panel meters, temperature controllers, flow indicators, etc to provide indication of the process at another location. The advantage of going digital, is both indicators always read the same value. If an analog output is used, the two displays often read different values.

Alternate up to Four Pumps

The Snooper, in conjunction with a Modbus output level transmitter, can be used to control and alternate up to four pumps. In the application above, the Snooper's two Form C relays are being used to control and alternate the Main and Backup pumps, and the Snooper's two Form A relays are being used for High and Low Alarm.

Multi-Point Linearization

The Snooper's multi-point linearization feature can be used to display the volume in a round horizontal tank or any other non-linear vessel. In addition, it can generate a 4-20 mA output that is linear to volume.


connection diagram

Mounting Dimensions

Modbus Serial Input Meter PD865 Snooper: Quick Installation and Diagram

1. Panel cutout required: 1.772" x 3.622" (45 x 92 mm)
2. Panel thickness: 0.040" - 0.250" (1.0 mm - 6.4 mm)
3. Mounting brackets lock in place for easy mounting

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