Suitable for industrial control application isolation PLC digital input Industrial control -

Number/binary system of sensors and switches to signal monitoring and control is essential, is widely used in industrial control, industrial automation, electrical control and process automation. All the output of the sensor need to be the central processing unit testing and monitoring. To achieve this purpose, usually using programmable logic controller ( PLC） Digital input module of the two high power resistor voltage divider detection sensor output voltage. To isolate each sensor channels, need to use an independent light coupling. Depending on the complexity of a system often need to use multiple light coupling ( FIG. 1) 。

figure 1: the traditional industrial sensor monitoring system schematic diagram, including resistor voltage divider and optical coupling is used for monitoring and detection sensor output signal to the PLC system.

in this kind of traditional architecture, the consumption of resistor voltage divider power is larger, form a circuit board ( PCB） “ Hot & throughout; Support to design high temperature work, and increase the radiator. Hot spots may even reduce the system reliability. In addition, for the high number of channel module, optical coupling design increases system cost and power consumption, waste valuable circuit board space. Obviously, compact and simple isolation digital input interface will be good for industrial production.

simplify PLC digital input

integration can satisfy this requirement. Say it easy to difficult! First, increase the input channel, extend the system capacity, but still keep the interface simple. Now, to consider digital serialization, and seek to save segregation light coupling way. Using configurable current limit in order to reduce power consumption ( See figure 4) 。 To improve the error detection function, make the same simple interface of data transmission is very reliable. Integration of these features, make the digital input function to be more perfect and reliable, produce less heat, lower power consumption, save a space, and the cost is greatly reduced, this is the goal.

isolate the realization of the digital input interface design

design goals above solution is Corona isolated subsystems reference design, the design USES the digital input converter/serial, and digital isolator. Corona is designed to provide the front-end interface circuit of PLC controller digital input module, support high voltage input ( The highest 36 v) , the power supply and data isolation & ndash; — All integration in 90 mm & TImes; 20 mm smaller sizes of packaging. The eight channel design integrated digital input level converter/serializer, six channels data isolator and used for the isolation power supply design ( If the scene without power supply) H bridge transformer drives. We further discuss the design of hardware and software. Hardware description

Corona input module as shown in figure 2, the system block diagram as shown in figure 3.

figure 2: Corona reference design circuit boards, MAXREFDES12#） 。

figure 3: the digital input subsystem reference design block diagram.

diagram MAX31911 eight channels of U1 level translator/serializer, U3 isolator MAX14850 6 channels of data.

, in the design of the industrial digital input serial device ( U1) Will be 24 v digital output of sensor and switch level conversion, signal conditioning and serialization, shift to meet the requirements of the micro-controller of CMOS compatible signal. The device provides the front-end interface circuit of PLC digital input module, compared with the traditional discrete resistance partial pressure solution, the input current limit can effectively reduce the power consumption of the scene. As shown in figure 4 for the two kinds of methods of current - single input channel Voltage. Can choose on the piece of low-pass filter flexibly to shake and filtering of sensor output. On 8 to 1 serialization saves the optical coupling isolation needed. Every 8 bits of data through the SPI port to send over a CRC check, ensure high reliable communication industrial environment noise. To achieve greater flexibility, on-chip integration of 5 v voltage stabilizer for the external light coupling, digital isolator or other external power supply 5 v circuit.

figure 4: the traditional design scheme and the Corona ( MAX31911） Design scheme of single - input channel current Relationship between voltage comparison.

U3 ( MAX14850) The size of compatible with Pmod 6 channel data isolation is realized. Pmod specification allows 3. 3 v and 5 v module, as well as a variety of pin assignment. In Pmod side, the power supply voltage is 3. 3 v or 5 v. U1 side voltage of 5 v. Support data isolation is 600 VRMS.

in most cases, the U1 ( MAX31911） By 24 v ac power supply; If without the power supply is available, U1 can by the controller side of power supply. In the latter case, Corona H bridge circuit board transformers drive ( U2, MAX13256) And the transformer provides MAX31911 level use isolated power supply. Description

software Corona Nexys3 and ZedBoard platform design validation. Currently offers both platform project files, device drivers, and sample code. Due to the onboard Pmod compatible connector is very simple, so Corona design is easy to use it for any micro controller or FPGA development board.

summary

in this paper, the Corona ( MAXREFDES12#） How subsystem reference design for industrial control and automation applications provide compact structure and easy isolation digital input interface. Corona design eight-way digital input channels. Through a single SPI interface simple cascade multi-disc eight channels of digital input IC— — No additional line selection, is easy to increase the number of channels in multiples of 8. A single SPI interface the sensor data can be transmitted to PLC, without additional channel isolation, sharply reduced the number of isolator input module. This design greatly reduces the cost, to take up less space, higher channel density per unit area of PCB. The design is based on Nexys3 or ZedBoard platform example of software.

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