User manual BENSHAW RTD MODULE

[. . . ] This is accomplished by using three wires to connect the RTD to the module. The Remote RTD module provides 10mA of current from H (Hot) and C (Compensation) terminals. The two currents add at the RTD and 20mA is returned to the R (Return) terminal. The following diagram illustrates this; C R 10mA Compensation 20mA Return Remote RTD Module H 10mA Hot RTD 4 1. [. . . ] Avoid using star or stub configurations. The RS-485 shield should be terminated at only one point in the network, preferably at the master device. Do not connect the RS-485 shield to the RTD shield terminals. 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. Each RTD is connected to the three terminals with the common number. 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. · 19200 bits per second communications rate. · 8 data bits, 1 stop bit, no parity. · 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 response will be in the form; · Byte 1: RTD module address. · Byte 2: Function code with most significant bit set to 1 indicating an exception. · Byte 3: Exception code 03h indicating an illegal value exception. For example, the Remote RTD Module will respond with the following if it receives an invalid preset single register command; Address 10h Function Code 86h Exception Code 03h CRC code 52 64h 4. 6 Description Indicating LEDs There are three indicating LEDs. [. . . ]