Digital/binary sensors and switches are crucial for signal monitoring and system control, and are widely used in industrial control, industrial automation, motor control, and process automation. The outputs of all sensors need to be detected and monitored by a central processing unit (CPU). To achieve this, two high-power resistive voltage dividers in the digital input module of a programmable logic controller (PLC) are typically used to detect the sensor output voltage. Independent optocouplers are required to isolate each sensor channel. Depending on the complexity, a system often uses multiple optocouplers (Figure 1).
Figure 1: Schematic diagram of a traditional industrial sensor monitoring system, in which a resistor divider and an optocoupler are used to monitor and detect the signals output by the sensor to the system PLC.
In this traditional architecture, the resistive voltage divider consumes a significant amount of power, creating "hot spots" on the PCB and requiring designs to support high-temperature operation and additional heat sinks. These hot spots can even reduce system reliability. Furthermore, for modules with a high number of channels, multiple optocouplers increase system cost and power consumption, wasting valuable PCB space. Clearly, a compact and simple isolated digital input interface would be beneficial for industrial production.
Simplify PLC digital input
Integration can meet this requirement. Easier said than done! First, increase the number of input channels to expand system capacity while keeping the interface simple. Now, consider digital serialization and find ways to eliminate the need for isolation optocouplers. Use configurable current limiting to reduce power consumption (see Figure 4). Improve error detection to make data transmission on the same simple interface highly reliable. The goal is to integrate these features to make digital input functionality more complete and reliable, generate less heat, consume less power, save space, and significantly reduce costs.
Implementation of isolated digital input interface design
The solution to the above design goals is the Corona Isolation Subsystem Reference Design, which utilizes digital input level converters/serializers and digital isolators. The Corona design provides the front-end interface circuitry for PLC digital input modules, supporting high-voltage input (up to 36V), power and data isolation—all integrated into a small 90mm x 20mm package. The design integrates an eight-channel digital input level converter/serializer, a six-channel data isolator, and an H-bridge transformer driver for isolated power supply designs (if no field power is available). We will further discuss the hardware and software of this design.
Hardware Description
The Corona input module is shown in Figure 2, and the system block diagram is shown in Figure 3.
Figure 2: Corona reference design board (MAXREFDES12#).
Figure 3: Reference design block diagram of the digital input subsystem.
In the diagram, U1 is a MAX31911 eight-channel level converter/serializer, and U3 is a MAX148506-channel data isolator.
In this design, the industrial digital input serializer (U1) performs level conversion, signal conditioning, and serialization of the 24V digital outputs from sensors and switches, transforming them into CMOS-compatible signals that meet the requirements of microcontrollers. This device provides the front-end interface circuitry for PLC digital input modules, and compared to traditional discrete resistor voltage divider schemes, input current limiting effectively reduces power consumption in the field. Figure 4 shows the current-voltage relationship for a single input channel in both methods. An optional on-chip low-pass filter flexibly debounces and filters the sensor outputs. On-chip 8-to-1 serialization eliminates the need for optocouplers for isolation. A multi-bit CRC check is sent every 8 bits of data via the SPI port, ensuring reliable communication in high-noise industrial environments. For greater flexibility, an on-chip integrated 5V voltage regulator can power external optocouplers, digital isolators, or other external 5V circuits.
Figure 4: Comparison of current-voltage relationship of single input channel in traditional design scheme and Corona (MAX31911) design scheme.
The U3 (MAX14850) implements 6-channel data isolation in a Pmod-compatible form factor. The Pmod specification allows for 3.3V and 5V modules, as well as various pin assignments. On the Pmod side, the supply voltage can be either 3.3V or 5V; the voltage on the U1 side is 5V. Supported data isolation is 600VRMS.
In most cases, U1 (MAX31911) is powered by a 24V field power supply; if no field power supply is available, U1 can be powered by the controller. In the latter case, the H-bridge transformer driver (U2, MAX13256) and transformer on the Corona board provide operational-grade isolated power to the MAX31911.
Software Description
The Corona design has been validated on Nexys3 and ZedBoard platforms. Project files, device drivers, and sample code for both platforms are currently available. Due to its simple onboard Pmod-compatible connectors, the Corona design can be easily used with any microcontroller or FPGA development board.
Summarize
This article describes how the Corona (MAXREFDES12#) subsystem reference design provides a compact and simple isolated digital input interface for industrial control and automation applications. The Corona design offers eight digital input channels. Multiple eight-channel digital input ICs can be easily cascaded via a single SPI interface—no additional chip select lines are required, and the number of channels can be easily increased in multiples of eight. Sensor data can be transmitted to the PLC via a single SPI interface, eliminating the need for additional isolated channels and significantly reducing the number of isolators required in the input module. This design significantly reduces cost, occupies a smaller footprint, and achieves high channel density per unit PCB area. Sample software based on the Nexys3 or ZedBoard platform is provided.
