UM344-2

CIRCUIT DESCRIPTION

7.2 THEORY OF OPERATION

The transmitter input signal, obtained from a Thermocouple (TC), Millivolt (mV), Resistance Temperature Detector (RTD) , or Resistive (Ohm) source is wired to the transmitter’s signal input terminal block. The standard calibration curves for J, K, E, T, R, S, B and N type Thermocouples and US/DIN curves for 100, 200, and 500 Platinum RTDs are stored in memory. The input is corrected for ambient temperature effects, linearized, and converted to a 4 to 20 mA or HART digital output signal. The specific type of input signal is selected during configuration.

7.2.1 Thermocouple/Millivolt (TC/mV) Input

The range limit for a millivolt input signal from a TC or mV source is -10 mV to +100 mV. Since input circuits for mV and TC inputs operate identically, a TC input will be described.

The input from the thermocouple is applied to a 2-pole filter which reduces RFI and 60 Hz noise. The filter’s output is fed to a high gain amplifier and applied to INPUT 1 of the A/D Converter. An amplified reference junction input from an Integrated Circuit Temperature Sensor is fed to INPUT 2 of the A/D Converter. The Sensor measures the temperature of the TC (+) screw terminal and regulates the current at a rate of 1 microamp per degree Kelvin. The Microcontroller (μC) periodically reads the Sensor signal and compensates the TC input for changes in ambient temperature.

UPSCALE/DOWNSCALE indicates thermocouple burnout (open circuit) by driving the output to a high or low limit depending on the selected configuration parameter. When the transmitter input is configured for a TC, the RTD SEL output from the A/D Converter goes logic low. This turns on the P-channel FET and turns off the CMOS Analog Switch, disabling the 52 μA Current Sink used for RTD/OHMS input.

The V BURNOUT output from the A/D Converter, either a plus or minus voltage depending on the upscale/downscale choice, passes through the FET and is converted to a trickle current. If the TC opens, the trickle current will charge the input filter in the appropriate direction and drive the perceived temperature to the high or low limit.

The input circuitry communicates with the microcontroller (μC) through three pulse transformers (DATA/CONTROL ISOLATION) which provide isolation with minimum current consumption.

7.2.2 RTD/OHMS Input

The range input for an OHM type input is 0 to 1875 ohms (from an RTD or potentiometer source). Input circuits for a potentiometer or RTD input operate identically; here, an RTD input will be described.

The operation of the input circuitry with an RTD/OHM input is essentially the same as with a TC/mV input with the following exception; an RTD/OHM input is read by forcing a 52 microamp current through the sensing device and measuring the resulting voltage drop that is applied to the input filter (52 microamps was chosen to generate standard TC voltages when used with a TC input) .

When the transmitter is configured for an RTD input, the RTD SEL output from the A/D Converter goes logic high turning off the P-channel FET and disabling the burnout trickle current network. The CMOS

September 1995

7-3

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Siemens UM344-2 user manual Theory of Operation, Thermocouple/Millivolt TC/mV Input, 2 RTD/OHMS Input
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UM344-2 specifications

The Siemens UM344-2 is a cutting-edge industrial controller designed to meet the demands of modern automation and control systems. This powerful device is specifically developed for use in various applications, including manufacturing, process control, and machinery integration. The UM344-2 stands out for its versatility and robust performance in harsh industrial environments.

One of the main features of the Siemens UM344-2 is its high processing speed, which allows for real-time data processing and decision-making. The controller utilizes advanced microprocessor technology, ensuring quick response times and efficient operation. Its architecture supports multitasking, enabling it to manage multiple processes simultaneously without compromising performance.

The UM344-2 features a modular design, which provides flexibility in configuration and scalability. Users can easily add or remove modules to tailor the system to specific application requirements. This adaptability is beneficial for businesses that need to scale their operations or adjust to changing production demands. The controller is compatible with a variety of I/O modules, allowing for seamless integration with other equipment and systems.

In terms of connectivity, the Siemens UM344-2 supports various communication protocols, including Profibus, Profinet, and Ethernet. This versatility ensures smooth data exchange between the controller and other devices in the automation ecosystem, facilitating comprehensive monitoring and control. The inclusion of these communication standards enhances interoperability and allows for the integration of legacy systems with newer technology.

The Siemens UM344-2 is equipped with advanced diagnostic tools, which enable users to monitor system performance and detect potential issues in real-time. This feature reduces downtime and maintenance costs by allowing for proactive management of the system. The controller also supports remote access capabilities, enabling technicians to troubleshoot and configure systems from off-site locations.

Safety is a critical consideration in industrial environments, and the UM344-2 is designed with this in mind. It incorporates safety-certified features, ensuring compliance with international safety standards. Users can implement safety functions directly within the controller, streamlining system design and reducing the need for external safety components.

Overall, the Siemens UM344-2 exemplifies the fusion of technology and practical application, making it an essential component for businesses looking to enhance their automation processes. With its high performance, flexibility, and focus on safety, the UM344-2 is a reliable choice for industries aiming for efficiency and innovation in their operations.
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