What are the types of digital inputs?

The three input types according to the IEC standard

The selection of digital inputs depends on the characteristics of the inputs and is important for the various sensors. The IEC 61131-2 standard defines three types of current-consuming digital inputs. Current sinking modules are those that have the property of consuming current. The three digital input types are described below.

Type 1: Mechanical switching contacts (2-wire connection) and semiconductor sensors (3-wire connection only)

Type 1 digital inputs convert signals from electromechanical switching devices (relays, buttons, etc.) with two states into a binary number (one bit). However, these inputs cannot be used for the 2-wire connection of semiconductor switches (sensors, proximity switches, ...). Type 1 was defined in the standard at a time when mainly mechanical contacts were used.

Type 2: Semiconductor sensors (2-wire connection)

Type 2 digital inputs convert signals with two possible states from semiconductor switches into a binary number (one bit).

Type 2 inputs

• have a higher current consumption and are more suitable for modules with low channel density,
• can be used for 2-wire proximity switches when connected according to IEC 60947-5-2.

Type 3: Semiconductor sensors (2-wire/3-wire connection) - reduced power consumption

Similar to the type 2 digital inputs, the type 3 digital inputs convert signals with two possible states from semiconductor switches (2-wire proximity switches) into a binary number (one bit).

Type 3 inputs

• have a lower electrical power consumption than type 2 inputs (even at high voltages),
• have lower power consumption and are therefore very suitable for small modules with high channel density,
• have low heat dissipation, allowing a higher channel density per module,
• provide zero signal current to the sensors,
• can be used for type 2 and type 1 inputs,
• are compatible with IEC 60947-5-2 devices (with a lower current in the off-state).

The growing demands on control technology in the industry required a higher packing density for digital inputs. The type 3 definition in the standard makes it possible to optimize the space in the control cabinet and dispense with the zero signal current.

Current and voltage limits for digital inputs

The following table shows the specified limit values for the current-carrying digital inputs. The current and voltage limit values are taken from the EN 61131-2:2003 standard: