Analog Input Modules

The SLC 100 has an analog input module that allows analog values to be used as alarm inputs. Analog signals vary voltage or current from a minimum value to a maximum value. The analog voltage signal usually is a 0—10 volt signal and the current signal is 4—20 mA. The analog input module for the PLC has four separate channels to read four signals. Each signal has two setpoints that are programmable. This means that the voltage signal can come from a sensor, such as a level sensor in the bins for the carbon brush manufacturing process system. The sensor would send 10 volts when the bin is completely full, 5 volts when the bin is half full, and 0 volts when the bin is empty. The main feature of the analog signal is that every voltage between 0—10 volts will be available for the sensor to send to indicate the level of the raw material in the bin. The analog system generally has resolution of approximately 0.1 volt.

The programmable values for the analog input will act as alarm points. E.g., if it's important to know when the bin is 90% full so the bin would not overfill, the setpoint would be set for that level and a set of contacts in the analog module would transition when the bin reached 90% or above. The second setpoint could be set at 20% to transition a set of contacts to indicate anytime the bin was below 20% full. The alarms would indicate if the bin was too full or too empty. Since the SLC 100 does not have the capability to work with numbers, no other math function such as averaging can be accomplished. In larger PLCs, a full set of math instructions is available to perform math and compare functions. Also output analog features would be available to allow an analog output signal to be sent to motor drives or valves to operate them from 0—100% in smooth increments of 0.1 volt. The SLC does not provide analog output signals.