High-speed multi-channel PLC input isolators for industrial automation

August 05, 2016 // By Graham Prophet
Silicon Labs presents its Si838x digital isolators as outperforming and outliving the alternative of optocouplers in programmable logic controllers for factory automation.

Silabs says its devices are the first high-speed, multi-channel digital isolators designed to meet the demands of programmable logic controller (PLC) applications. The Si838x PLC field input isolator family offers a combination of high-speed channels (up to 2 Mbps), channel integration of up to eight channels per device, bipolar input flexibility, high noise immunity and 2.5 kVRMS safety isolation. The family provides a purpose-built solution for PLC applications including industrial I/O modules, computer numerical control (CNC) machines and servo motor control. The Si838x isolators are also suitable for process automation controllers (PACs) used in distributed process control systems.


PLCs must be robust and compact; many PLC designs scale up to 128 channels and have multiple high-speed inputs. While optocouplers have provided the default isolation solution for decades, they present major challenges to PLC designers. Lacking high-speed capabilities, optos are not fast enough for high-performance PLC applications such as servo motor control. Optos also lack high noise immunity, and their performance degrades over time and temperature, posing reliability risks in high-temperature industrial applications. The Si838x family addresses these challenges by providing a flexible, high-speed, multi-channel bipolar isolation solution in a QSOP package with much longer service life and higher reliability than opto-based solutions.


These isolators are highly integrated single-chip devices providing eight channels of 24V digital field interface. This high level of integration enables significant bill of materials (BOM) and board space savings in PLC designs. By “daisy chaining” 16 Si838x isolators in a system design, developers can create PLCs with up to 128 channels supported by a single serial peripheral interface (SPI) port on the system’s embedded processor.


The controller interface can be eight parallel channels or a four-port SPI. Developers can configure the SPI for on-the-fly programming for custom de-bounce of noisy signals and for daisy chaining to support high-channel-density PLC designs. An optional de-bounce filter is either pre-programmed in a parallel configuration or programmable in