Programmable temperature transmitters with Modbus
September 2005
Temperature Measurement
Programmable transmitters have many advantages such as digital design, catering for any temperature range, ability to re-program sensor inputs and a range to suit any application. However, these transmitters normally have only a 4 to 20 mA output signal. Unitemp is now offering a range of transmitters with a Modbus output signal, thus, ensuring easy integration into PLCs and scada systems.
Conventional temperature transmitters are based on an analog design and can only handle one temperature range and one sensor (eg, a Pt100 with a temperature range of 0 to 100°C). They are also subject to drift and the potentiometer slides out of calibration due to vibration.
The newer programmable temperature transmitters are based on a digital design and thus drift has been greatly reduced. They can handle all temperature ranges, any sensor and can be re-programmed via a PC to suit every new application (eg a Pt100 with a temperature range of 0 to 100°C or a thermocouple with a temperature range of 100 to 200°C). However, these transmitters only have 4 to 20 mA output signals and cannot directly communicate with PLCs or scada systems.
UniTemp offers the new Novus DigiRail family with Modbus communications. Included in this range are units with a relay output, a digital counter and a programmable module for temperature and analog signals. In these modules the conventional 4 to 20 mA output is replaced by Modbus communications. This allows easy integration directly into PLCs or scada systems. Cabling is done on an RS485 basis allowing distances up to a few hundred metres.
The Novus range of programmable temperature transmitters bridges the gap between programmable and analog versions and is priced to compete with analog transmitters at a fraction of what programmable transmitters used to cost.
The PC programmable TxBlock and TxRail family is already, and consists of head and DIN rail mount transmitters. One single model can be configured to accept all thermocouple types and Pt100 RTDs. These transmitters are available with a 0-10 V or a 4-20 mA output and can be loop powered or powered by an external power supply of between 18 V and 30 V. Therefore, only one model is needed for various temperature applications greatly simplifying spares holding for users.
Both the headmount and DINrail versions are also available as isolated models. These transmitters are isolated between the sensor input and the transmitted current/voltage, which prevents interaction between multiple sensors linked to a common system (eg, PLC). Thus, damage is greatly reduced when a fault occurs, as one circuit does not interfere with another.
For more information contact Willem Krige, UniTemp, 021 762 8995, [email protected], www.unitemp.co.za
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