The type of input modules used by a PLC depends on the type of input device. For example, some respond to digital inputs, which are eitheronoroffwhile others respond to analog signals. In this case, analog signals represent machine or process conditions as a range of voltage or current values. The plc digital input output module converts signals into logic signals that the CPU can use. The CPU evaluates the status of inputs, outputs, and other variables as it executes a stored program. The CPU then sends signals to update the status of outputs.
Output modules convert control signals from the CPU into digital or analog values that can be used to control various output devices. The programming device is used to enter or change the cpu modules program or to monitor or change stored values. Once entered, the program and associated variables are stored in the CPU. In addition to these basic elements, a PLC system may also incorporate an operator interface device to simplify monitoring of the machine or process.
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