Analog Inputs and Outputs

Analog Inputs and Outputs Analog inputs are generally divided into two categories  

High level. Where the source is a process transmitter, the range resistor in the current loop converts the 4- to 20-mA signal to a 1- to 5-V signal. The conversion equipment can be unipolar (i.e., capable of processing only positive voltages). 

Low level. The most common low-level signals are inputs from thermocouples. These inputs rarely exceed 30 mV and could be zero or even negative. The conversion equipment must be bipolar (i.e., capable of processing positive and negative voltages). 

Ultimately, such signals are converted to digital values via an ana-log-to-digital (A/D) converter. However, the A/D converter is normally preceded by two other components  

Multiplexer. This permits one A/D converter to service multi-le analog inputs. The number of inputs to a multiplexer is usually etween 8 and 256. 

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Analog inputs and outputs in data-based (DDC) controllers are usually standardized and record the signals from sensors or transducers as 0(2)-10 VDC or 0(4)-20 mA. The inputs module can also be standardized for resistances, as Pt 100 DIN, Pt 1000 or Ni 1000 DIN (Pr = platinum, Ni = nickel). 

Processor card with analogical inputs and outputs. This is the most important part of the system. Receives signals from the sensors and formulates bearing response based on control algorithm. 

L Inputs/Outputs (I/O). Up to several thousand discrete (binary) inputs and outputs can be accommodated. Large PLCs have several hundred analog inputs and outputs for data logging and/or continuous PID control. 

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