 The input module of a PLC converts field voltages such as a 120V limit switch to 5V DC logic levels. Output modules perform the same function but in reverse order where 5V DC is converted to field voltages to control devices such as 120V relay coils. Starting with the AC discrete input module, the circuit is comprised of two sections. The first section is referred to as the power section and the second is called the logic section. Using an optical isolator, electrical isolation is provided between the power section and the logic section. When the 120V push button is actuated in the circuit, voltage is applied through a pair of resistors and a capacitor that filters false signals and electrical interference. Continuing through the bridge rectifier, a low-level DC output is applied to the LED of the optical isolator as voltage passes through the Zener diode. When light is received at the phototransistor, the push button on state is communicated to the processor and the status indicator is illuminated. For the AC output module, the circuit is comprised of a power section and logic section coupled by an isolation circuit similar to the input module but in reverse order. When the output status is commanded from the PLC program, voltage is transmitted to the LED of the optical isolator. As light is received at the phototransistor, a triggered pulse at the gate of the triac allows current to pass to the output terminal and changes the state of the output device.