2 Bi-Directional Input Signal Detection

For AC input signals to a PLC module, the input voltage alternates between positive and negative relative to ground — making this bi-directional. However, isolated digital input devices like the ISO1228 are typically designed to detect unidirectional (DC) signals. To correctly detect the AC signal while protecting the ISO1228, the input circuit can be designed as shown in Figure 2-1.

In this configuration, a series diode (highlighted in red) is used to block the negative half of the AC signal. This allows only the positive half-cycle to reach the ISO1228 input, effectively converting the AC signal into a form the device can process. The diode essentially performs a half-wave rectification.

Since the diode experiences inrush or surge currents—especially during switching or when the AC signal has fast transients—this must be selected with appropriate ratings. The diode can be capable of withstanding:

• High surge current
• Frequent switching
• Must have designed for voltage and current ratings for the application

This design makes sure of safe and reliable detection of AC signals using a digital input device intended for DC operation, without damaging the device or misreading the signal.