DSQC658 3HAC025779-001 Security system module

Isolated A/D Converter Each input channel has an A/D converter (delta-sigma type). Isolation is done by DC/DC converters (one per channel) and optocouplers on the digital serial line of the A/D converter. Each channel accepts voltage and resistance inputs. Resistance measurements are made by digitizing the voltage drop created across the input resistance source. A precision constant current source supplies the current used to measure the input.Once isolated, digitized, and buffered, the input signal is sent to the microcontroller by a digital multiplexer. All the inputs, including the references and cold junction input, are multiplexed as shown in Figure 2-1.

Cold Junction Reference

 The ASI module measures the cold junction RTDs on the termination unit of the analog input module. This results in an accurate reading of the ambient temperature at the field wire termination area. This value can be used by the ASI module to compensate for voltages generated from the bimetal connections made by terminating the field wires (thermocouple wires) onto the terminal blocks of the termination unit. The block address of the cold junction reference used by thermocouple inputs on the ASI module is contained in FC 215, specification S3. Each analog input module configured for a thermocouple input requires a cold junction reference. Each analog input module can only have one reference which can be used by up to 16 thermocouple inputs. The exception to this is when one of the inputs is used as a remote cold junction reference. Refer to Section 4 for more information.

Microcontroller and Memory 

The onboard microcontroller coordinates ASI functions. 

The main functions are: 

• Channel and cold junction reference switching. 

• Programming A/D converters.

 • Reading the A/D converters and applying all necessary correction factors. 

• Interfacing to switch and LEDs (used for diagnostic mode). 

• Background integrity checking. 

• Reading and writing expander bus data through dual port RAM (DPRAM)