Modbus & Serial Output Transmitter for Resistance Input in Ohms

Exceptional accuracy at high update rates.DIN Rail mounted, digitally programmable.
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Modbus Transmitter for Resistance in Ohms
Key Features
  • Five precalibrated resistance input ranges from 20.000 Ω to 200.00 kΩ
  • 1 mΩ resolution on 20 Ω scale
  • Exceptional accuracy and repeatability
  • 2, 3 or 4-wire connection with lead resistance compensation
  • RS232 or RS485 serial communications, half or full duplex, isolated
  • Modbus RTU, Modbus ASCII or Laurel ASCII protocol
Standard Features
  • Output update rate to 60/sec
  • Isolated 4-20 mA, 0-20 mA or 0-10 Vdc analog output (jumper selectable)
  • DIN rail mount housing only 22.5 mm wide
  • Detachable screw-clamp connectors
  • Dual solid state relays for alarm or control, isolated
  • Universal AC power, 85-264 Vac
  • Easy setup with PC-based Instrument Setup Software
  • Priced at only $235 (basic resistance transmitter)
Options
  • Low voltage power, 10-48 Vdc or 12-32 Vac
Description

The Laureate™ Modbus & serial output, resistance input transmitter is factory calibrated for five jumper selectable resistance ranges from 20Ω to 200 kΩ. Accuracy is an exceptional ±0.01% of full scale ± 2 counts. Resolution is one part in 20,000. In the 20-ohm range, resolution is 1 milliohm, making the transmitter suitable for contact resistance and conductance measurements.

Transmitter connections can be via 2, 3 or 4 wires. With 4-wire hookup, 2 wires are used for excitation and two separate wires are used to sense the voltage across the resistance to be measured, thereby eliminating any lead resistance effects. With 3-wire hookup, the transmitter senses the combined voltage drop across the RTD plus two excitation leads. It also senses the voltage drop across one excitation lead, and then subtracts twice this voltage from the combined total. This technique effectively subtracts the lead resistance if the excitation leads are the same.

All resistance ranges are digitally calibrated at the factory, with calibration factors stored in EEPROM on the signal conditioner board. This allows ranges and signal conditioner boards to be changed in the field without recalibrating the transmitter. If desired, the transmitter can easily be calibrated using external standards plus scale and offset in software.

Resistance input signal conditioner board
High read rates at up to 60 or 50 conversions per second while integrating the signal over a full power cycle are provided by Concurrent Slope (US Pat 5,262,780) analog-to-digital conversion. High read rates are ideal for peak or valley capture, real-time computer interface, and control. Peak and valley values are standard. These may be displayed via a front panel pushbutton command or a control signal at the rear connector, or be transmitted as serial data.

LTM Series serial output transmitters send data via an isolated serial port, which is user configurable for RS232 or RS485 and full or half duplex operation. Three protocols are user selectable: Modbus RTU, Modbus ASCII, or Laurel ASCII. Modbus operation is fully compliant with Modbus Over Serial Line Specification V1.0 (2002). The Laurel ASCII protocol allows up to 31 Laureate devices to be addressed on the same RS485 data line. It is simpler than the Modbus protocol and is recommended when all devices are Laureates. It also allows use of Laurel Datalogging Software.

Standard features of Laureate temperature transmitters include:

  • Serial communications, isolated. User selectable RS232 or RS485, half or full duplex, Modbus or Laurel ASCII protocol.
  • Analog transmitter output, 16-bit, user scalable and isolated. User selectable 4-20 mA, 0-20 mA or 0-10V levels.
  • Dual solid state relays for control or alarm, isolated. Rated 120 mA at 130 Vac or 170 Vdc.
  • Universal AC power supply for 85-264 Vac. Low voltage power of 10-48 Vdc or 12-32 Vac is optional.

Easy Transmitter programming is via Laurel's Instrument Setup Software, which runs on a PC under MS Windows. This software can be downloaded from this website at no charge. The required transmitter-to-PC interface cable is available from Laurel (P/N CBL04).

Pinouts
Laureate Modbus & Serial Output Resistance Input Transmitter
Pinout for resistance transmitter
Resistance Measurement with Excitation & Lead Compensation
RTD hookup can be via 2, 3 or 4 wires to the J5 connector. The meter applies an excitation current of 196 ¼A (Pt 100 and Ni 120) or 5 mA (Cu 10).

In 4-wire hookup, different pairs of leads are used to apply the excitation current and sense the voltage drop across the RTD, so that the IR drop across the excitation leads is not a factor.
In 3-wire hookup, the meter senses the combined voltage drop across the RTD plus two excitation leads. It also senses the voltage drop across one excitation lead, and then subtracts twice this voltage from the combined total. This technique effectively subtracts all lead resistance and compensates for ambient temperature changes if the two excitation leads are identical.
In 2-wire hookup, the meter senses the combined voltage drop across the RTD and both lead wires. The voltage drop across the lead wires can be measured by shorting out the RTD during meter setup, and this voltage is then automatically subtracted from the combined total. However, changing resistance of the lead wires due to ambient temperature changes will not be compensated.