9.2.1 Sinking Inputs

Figure 16 shows the design for a typical multichannel, isolated digital-input module with sinking inputs. Push-button switches, proximity sensors, and other field inputs connect to the host controller through an isolated interface. The design is easily scalable from a few channels, such as 4 or 8, to many channels, such as 256 or more. The R_SENSE resistor limits the current drawn from the input pins. The R_THR resistor is used to adjust the voltage thresholds and limit the peak current during surge events. The C_IN capacitor is used to filter noise on the input pins. For more information on selecting R_SENSE, R_THR, and C_IN, see the Detailed Design Procedure section.

The ISO121x devices derive field-side power from the input pins which eliminates the requirement for a field-side, 24-V input power supply to the module. Similarly, an isolated dc-dc converter creating a field-side power supply from the controller side back plane supply is also eliminated which improves flexibility of system design and reduces system cost.

For systems requiring channel-to-channel isolation on the field side, use the ISO1211 device as shown in Figure 17.

Figure 16. Typical Application Schematic With Sinking Inputs

Figure 17. Single-Channel or Channel-to-Channel Isolated Designs With ISO1211