User manual BENSHAW RTD MODULE

[. . . ] INTRODUCTION The connection wire from the Remote RTD Module to the RTD will typically have all three conductors the same size and length. From the above, the following equations can be seen; VRH = VR lead + VRTD + VH lead = 20mA ´ Rlead +10mA ´ RRTD +10mA ´ Rlead VCR = VC lead + VR lead = 10mA ´ Rlead + 20mA ´ Rlead Subtracting these two equations and subsituting Rlead for the different lead resistance provides the following; VRH - VCR = (30mA ´ Rlead + 10mA ´ RRTD) - (30mA ´ Rlead) VRH - VCR = 10mA ´ RRTD This calculation gives the voltage across the RTD independant of the connection wire resistance. By performing this calculation, the Remote RTD Module compensates for the conductor resistance and gets the true value for the RTD resistance. The Remote RTD Module then converts the measured voltage across the RTD to the temperature that the RTD is measuring. [. . . ] The connections are as follows; · A (­) - RS485 negative communications connection. The RS-485 communications cable should conform to the following specification; · Conductors - 2 twisted pair (one pair signal plus one for signal common). The RS-485 shield should be terminated at only one point in the network, preferably at the master device. A 120W resistor may optionally be installed between the A(-) and B(+) terminals if the Remote RTD Module is the end device on the RS-485 network. The terminating resistor may be necessary if the RS-485 cable is extremely long. Do not install a terminating resistor if the Remote RTD Module is not the end device on the network. There should never be more than two terminating resistors on the network. RS-485 Cable Specification RS-485 Topology RS-485 Shielding RS-485 Termination 8 3. INSTALLATION OF THE REMOTE RTD MODULE 3. 4 RTD Connections RTD Wire Connections Each Remote RTD Module has connections for up to 8 RTDs. The terminals for the RTD wires are as follows; · R - RTD return wire. For example, RTD number 5 connects to the terminals numbered 5R, 5C, and 5H. Benshaw recommends using 3-wire twisted (triad) shielded cable to connect the RTDs to the RTD module. It is important to keep the three wires of the same length and type for accurate lead resistance compensation. The RTD cable shield should be terminated at either of the shield terminals. For a simpler installation, a cable containing multiple triads with an overall shield can be used to connect a group of RTDs to the module with a single cable. RTD Cable Specification RTD Cable Shielding 3. 5 General Wiring Diagram The following is the wiring diagram detailing the connections of the RTD module. Only the wiring for RTD #5 is shown as the typical connection, but all the other RTDs connect similarly. Status Tx Rx BENSHAW ADVANCED CONTROLS & DRIVES Remote RTD Module 3C 7C 3R 7R 4H 8H 4C 8C 4R 8R Com 24 VDC + A (-) B (+) RS-485 1H 5H 1C 5C 1R 5R 2H 6H 2C 6C 2R 6R Shld Shld 3H 7H 24 VDC Power Supply VV+ RS-485 Master Device Shld B(+) A(-) Com H C R RTD #5 (Typical) 9 4. COMMUNICATING WITH THE REMOTE RTD MODULE 4. 1 General Modbus Communications The Remote RTD Module uses an industry standard RS-485 port, communicating as a Modbus RTU slave. The Remote RTD Module has the following communications features; · RS-485 port. · Modbus RTU slave, addresses 16 to 23. 4. 2 General Modbus Input Registers The Benshaw Remote RTD Module has a number of Modbus input registers. These registers are used to read the temperatures measured by the eight RTDs. The following table lists the Modbus input registers; Register 30001 30002 30003 30004 30005 30006 30007 30008 30009 Description RTD 1 RTD 2 RTD 3 RTD 4 RTD 5 RTD 6 RTD 7 RTD 8 Shorted RTD Values 0 to 200 - Temperature in degrees Celsius 0 to 200 - Temperature in degrees Celsius 0 to 200 - Temperature in degrees Celsius 0 to 200 - Temperature in degrees Celsius 0 to 200 - Temperature in degrees Celsius 0 to 200 - Temperature in degrees Celsius 0 to 200 - Temperature in degrees Celsius 0 to 200 - Temperature in degrees Celsius A bit is set if an RTD has a shorted lead. [. . . ] · Byte 2: Function code with the most significant bit set to 1 to indicate an exception. · Byte 3: Exception code 02h indicating an illegal data address exception. For example, if an attempt is made to read a holding register that does not have a valid address, the Remote RTD Module will respond with the following; Address 10h Function Code 83h Exception Code 02h CRC code 90 F4h Illegal Data Value If the Remote RTD Module receives a preset single register command that contains an illegal value, it will respond with and illegal data value exception response. The response will be in the form; · Byte 1: RTD module address. [. . . ]