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Industry’s First High-Speed Multi-Channel PLC Input Isolators

Published: Aug 04,2016

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Silicon Labs has introduced the industry’s first high-speed, multi-channel digital isolators designed to meet the demands of programmable logic controller (PLC) applications.

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The new Si838x PLC field input isolator family offers an unmatched combination of high-speed channels (up to 2 Mbps), exceptional channel integration (up to eight channels per device), bipolar input flexibility, high noise immunity and 2.5k VRMS safety isolation. The Si838x 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 ideal for process automation controllers (PACs) used in distributed process control systems.

Widely deployed in harsh factory floor environments, 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 for the first time by providing a flexible, high-speed, multi-channel bipolar isolation solution in a compact QSOP package with much longer service life and higher reliability than opto-based solutions.

Silicon Labs’ Si838x isolators are highly integrated single-chip devices providing eight channels of 24 V 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 a serial-out configuration.

Leveraging Silicon Labs’ groundbreaking CMOS-based LED emulator technology, the Si838x isolators feature a flexible bipolar input capability that gives developers a choice of sinking or sourcing inputs. The LED emulator input circuit supports up to eight sinking or sourcing inputs in a single isolator device with no power supply required on the field side. The output interface to the controller supports low-power operation with 2.25 V operation capability.

The Si838x isolators provide a high-speed capability of 1 MHz (2 Mbps), which is essential for servo motor control applications. The isolators also offer best-in-class noise immunity on high-speed channels (50 kV/µs CMTI typical) to enhance system reliability.

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