Version: **
1 Overview
This article provides the classification and logic definition of digital inputs for a 24V system in accordance with IEC61131-2. In addition, this article also explains the advantages of using ISOFACE™ digital input switches.
Introduction
Generally, in an industrial environment, a digital input circuitry is used to receive input signals from the various sensors, switches, etc., and transfer them to a PLC or a microcontroller. These digital inputs are classified into three types according to IEC61131-2.
- Type 1:
Electromechanical switching devices such as relays and push button switches.
- Type 2:
2-wire semiconductor sensors with high power consumption and low channel density.
- Type 3:
2 / 3 wire semiconductor sensors with low power consumption and high channel density.
Figure 1 Switching characteristics of Type 1, 2, and 3 digital inputs according to IEC61131-2
Current and voltage ratings of different digital inputs with respect to a 24VDC system are as follows:
Table 1 Voltage ratings
|
Voltage (V) |
||||||
|
Type |
OFF state |
Transition state |
ON state |
|||
|
Min |
Max |
Min |
Max |
Min |
Max |
|
|
Type 1 |
-3 |
15 |
5 |
15 |
15 |
30 |
|
Type 2 |
-3 |
11 |
5 |
11 |
11 |
30 |
|
Type 3 |
-3 |
11 |
5 |
11 |
11 |
30 |
Table 2 Current ratings
|
Sinking current (mA) |
||||||
|
Type |
OFF state |
Transition state |
ON state |
|||
|
Min |
Max |
Min |
Max |
Min |
Max |
|
|
Type 1 |
ND |
15 |
0.5 |
15 |
2 |
15 |
|
Type 2 |
ND |
30 |
2 |
30 |
6 |
30 |
|
Type 3 |
ND |
15 |
1.5 |
15 |
2 |
15 |
2 Definition of digital input logic
Digital input circuitry is used in two different configurations:
- Positive logic
Figure 2 Positive logic
- Negative logic
Figure 3 Negative logic
3 ISOFACE™ digital input IC
ISOFACE™ digital input IC is used to interface sensors or switches with the microcontroller or PLC; in addition, ISOFACE™ provides galvanic isolation between the sensor and controller. ISOFACE™ digital input ICs have the following key advantages over the competition and a discrete solution:
- Supports commonly used positive logic configurations.
- Provides integrated galvanic isolation of 500V AC (EN60664-1, UL508).
- 8-channel input according to IEC61131-2 (Type 1/2/3).
- Power dissipation reduced by 2.5 times compared to a discrete solution.
- When compared with alternative solutions, ISOFACE™ digital input ICs enable up to four times more channels on the same PCB area.
- Provides digital filtering on the input signal.
- Provides diagnostics on IC supply and sensor connection integrity or wire break. (only ISO1I813T).
For more details on ISOFACE™, see ISOFACE™ isolated industrial interface.