SLVAG20 June 2025 ISO1228
Digital Input (DI) modules are used in Programmable Logic Controllers (PLCs) and Motor Drives to receive 24V digital inputs from field sensors and switches. Isolation is used to manage ground potential differences. The input signal is interpreted as a logic high or low using a voltage comparator with hysteresis. Some form of current limit is also implemented to avoid excess current draw from the 24V inputs; power dissipation is a critical concern in high-density compact multichannel designs.
The IEC 61131-2 standard for programmable controllers specifies three different types of digital input receivers: Types 1, 2, and 3. Designed for implementations for Type 1 and 3 need to draw as close to 2mA as possible in the ON-state, and have voltage transition thresholds between 5V and 11V.