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Benefits of isolated PLC digital inputs

Posted: 17 Sep 2013 ?? ?Print Version ?Bookmark and Share

Keywords:industrial control? automation? CPU? optocouplers? programmable logic controller?

Digital/binary sensors and switches are crucial for signal monitoring and system control in industrial control, industrial automation, motor control, and process automation. All the sensors' outputs need to be detected and monitored by a central processing unit (CPU). To do this, typically two high-power resistor-dividers, which are located in the digital input module of a programmable logic controller (PLC), detect the sensors' output voltages. Individual optocouplers are required for isolating each sensor channel. Based on the complexity of the industrial system, many optocouplers might be in use simultaneously during normal operation (figure 1).

Figure 1: Schematic of a traditional industrial sensor monitoring system shows the resistor-dividers and optocouplers that monitor and detect sensor outputs into the system's PLC.

In this traditional architecture the resistor-dividers consume high power and generate PC board (PCB) "hot spots" that can require the design to support a higher operating temperature and add a heatsink. Hot spots can even reduce system reliability. For high-channel-count modules, moreover, the multiple optocouplers increment system costs and power consumption, while consuming valuable board space. Clearly the industry would benefit from a compact and simple, isolated digital-input interface.

Simplify the digital input to a PLC
Integration is the answer. Simple enough to say or plan, but not easy to do. First, increase the channel inputs to expand the system's capacity, but keep the interface simple. Now turn to data serialisation and find a way to eliminate the optocouplers used for isolation. Use configurable current limiting to lower the power dissipation (figure 3). Improve error detection and make data transfer very reliable over that same simple interface. Integrate all this functionality and you make the digital input far more versatile and robust, generate less heat and use less power, save space, and lower costs significantly. That is the goal.

Figure 2: The Corona reference design board, (MAXREFDES12#).

Implementing an isolated digital-input reference design
The touted solution to the above design objectives is the Corona isolated sub-system reference design utilising a digital-input translator/serialiser and a data isolator. The Corona design provides the front-end interface circuit of a PLC digital input module. The design accepts high-voltage inputs (36V, max) and features isolated power and data, all integrated into a small 90mm x 20mm form factor. The design integrates an octal digital-input translator/serialiser, a 6-channel data isolation device, and an H-bridge transformer driver for an isolated power supply if field power is not available. Let's take a closer look at the hardware and software.

Hardware description
The Corona input module is seen in figure 2.

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