How to design an analog I/O system to realize industrial automation?

It is suitable for PLC/DCS applications, supports HART and Modbus connection analog I/O system, evaluation and design support.

Applicable to PLC/DCS applications, analog I/O systems supporting HART and Modbus connections

Evaluation and design support

Circuit Evaluation Board

CN-0414 Circuit Evaluation Board (EVAL-CN0414-ARDZ)

CN-0418 Circuit Evaluation Board (EVAL-CN0418-ARDZ)

CN-0416 Circuit Evaluation Board (EVAL-CN0416-ARDZ)

ADALM-UARTJTAG board

Arduino size development platform (EVAL-ADICUP3029)

Design and integration files

PLC/DCS Wiki User Guide

Schematics, layout files, bill of materials, software

Circuit functions and advantages

Programmable logic controllers (PLC) and distributed control systems (DCS) are used to monitor and control intelligent (HART) and analog field instruments in industrial automation applications.

The circuit shown in Figure 1 is a simple DCS system consisting of a host, a single node, two 4-channel isolated analog input boards and two 4-channel isolated analog output boards. These boards are locally controlled by an Arduino-sized substrate . The RS-485 transceiver is connected to a PC or other host, so that the user can exchange data with the node using the Modbus protocol.

The analog input data is read locally and provided through a serial interface using the industry standard Modbus protocol, ensuring data integrity and compatibility in a series of software applications and libraries. Similarly, the analog output is set by writing to the Modbus register, and then converted into an analog voltage or current signal.

Each node can support a combination of 4 analog input and output boards. As shown in Figure 2, a multi-node system with up to 16 nodes can be designed using the provided hardware and software infrastructure. The circuit supports point-to-point HART communication, and can be expanded into a multi-substation HART network to integrate multiple HART devices on the same channel.

The analog input and analog output of each board (a group of 4) are electrically isolated, and the analog input supports open circuit detection, which simplifies the process of fault detection and diagnosis. When used in severe industrial automation environments, these features can enhance reliability and durability.

How to design an analog I/O system to realize industrial automation?

Figure 1. PLC (or single node DCS) Modbus system functional block diagram

How to design an analog I/O system to realize industrial automation?

Figure 2. Functional block diagram of multi-node DCS Modbus system

Circuit description

The application focuses on the development of PLC/DCS systems controlled by the Modbus host, and uses examples to illustrate how to use the latest functions of important components. Single-node systems are often referred to as PLCs, and larger systems are often referred to as DCS.

Each node can manage up to 16 analog field devices, sensors or drives (compatible with HART or only support simulation), the system can be expanded to include up to 16 individual nodes. The system can also be used for general precision analog data acquisition applications, such as instrumentation, analog data logging, or test and measurement.

PLC/DCS topology

Supports multiple connection topologies. In a single-node (PLC, or single-node DCS) system, the host can use a micro-USB cable to directly connect to the USB serial port of the EVAL-ADICUP3029 platform board, which is very suitable for experiments that require the distance between the host and the node to be less than 2 meters Laboratory test and measurement applications.

In this point-to-point topology, the four analog inputs and outputs of the circuit board are still isolated from the host. Although generally not related to laboratory equipment, the Modbus protocol provides a convenient and standard method of communicating with nodes. The HART connection supports the configuration of smart sensors and drives.

When the distance between the host and the node increases to more than 2 meters, signal integrity, noise pickup and electrical failure will become more serious hidden dangers. In these cases, EVAL-CN0416-ARDZ provides a reliable RS-485 connection for the host. In a single-node, point-to-point system, it supports full-duplex or half-duplex communication over a distance of more than 1 km, depending on the baud rate.

For multi-node systems (called DCS is more appropriate), EVAL-CN0416-ARDZ provides daisy chain ports, supports switchable half/full duplex operation and switchable terminal electrodes, so the system can integrate 2 to 16 nodes.

Because Modbus is used as a serial communication protocol to send information between devices through a serial link, a simple, reliable and durable system can be realized regardless of scale. The hardware protocol stack for PLC/DCS applications contains three different reference designs.

Analog input board

The CN-0414 shown in Figure 3 is used to measure 4 fully differential signals, or 8 single-ended voltage and 4 current signals. The core of the circuit is the AD4111 low-power, low-noise 24-bit Σ-Δ analog-to-digital converter (ADC), which integrates a ±10 V and 20 mA analog front end.

Its voltage input supports an input range of up to ±10 V. AD4111 has unique characteristics, supports open circuit detection on ±10 V voltage input, and uses a single 5 V or 3.3 V power supply. The previous solutions generally require a power supply greater than ±10 V.

The current input supports an input range of 0 mA to 24 mA. The input impedance of the circuit is 250 Ω (60 Ω inside AD4111), and all inputs are based on insulated grounding. A 250 Ω input resistance is required on the current input so that the HART-compliant AD5700-1 modem can be used in conjunction with the AD4111.

The analog front ends AD4111 and AD5700-1 of the circuit are isolated from the processing side by ADuM5411 and ADuM3151, which can save a lot of space compared to solutions based on discrete transformers. The CN-0414 board is powered by a 9.5 V to 36 V DC power supply, which is very typical in industrial automation systems, so it can be easily integrated into your system.

How to design an analog I/O system to realize industrial automation?

Figure 3. Analog input board

Analog output board

The CN-0418 shown in Figure 4 is a 4-channel voltage and current output board based on the AD5755-1 DAC with dynamic power control.

This circuit provides 4 mA to 20 mA current output, and unipolar or bipolar voltage output (±10 V). This board is also equipped with AD5700-1 HART modem, which provides a complete analog output solution supporting HART connection. It also includes an external transient protection circuit, which is extremely important for applications in harsh industrial environments.

Current output and voltage output are provided through independent pins, and only one output is active at a time, thus allowing two output pins to be connected together and connected to a single port. The analog output is protected against short circuits and open circuits.

AD5755-1 integrates a dynamic power control function based on the DC-DC boost converter circuit, which can reduce power consumption in the current output mode. AD5755-1 has four CHART pins, which correspond to four output channels respectively. HART signals can be coupled to these pins and appear on the corresponding output (if the output is enabled).

Figure 4. Analog output board

RS-485 transceiver board

The CN-0416 shown in Figure 5 is an isolated and non-isolated RS-485 transceiver board that can easily implement data transmission between multiple systems or nodes, especially at long distances.

This circuit uses ADM2682E RS-485 transceiver for isolated communication, and LTC2865 RS-485 for non-isolated communication. Both devices can be configured for full-duplex or half-duplex operation, with open or terminated transmission lines.

The circuit uses on-board RJ-45 jacks, so it can use common CAT5 Ethernet cables to quickly connect to the node physically. The terminal electrode resistance is set as the characteristic impedance of CAT5 cable 100 Ω by default, but after configuration, it can support the 120 Ω impedance of standard RS-485 cable.

The data rate of the ADM2682E can reach 16 Mbps, providing a truly safe receiver input and an adjusted differential voltage threshold. It uses the iCoupler data channel to provide 5 kV signal isolation, and the isoPower integrated DC-DC converter provides 5 kV power isolation.

The data rate of the LTC2865 can reach 20 Mbps, providing a truly safe receiver input. An internal window comparator determines the safety condition without adjusting the differential input voltage threshold.

How to design an analog I/O system to realize industrial automation?

Figure 5. RS-485 transceiver board

Wiring of HART-compatible field devices

How to design an analog I/O system to realize industrial automation?

Figure 6. Wiring of HART-compatible field devices

HART network

HART devices can operate in one of two network configurations, point-to-point or multiple substations.

In point-to-point mode, a 4 mA to 20 mA signal is used to transmit a process variable, while additional process variables, configuration parameters, and other device data are transmitted digitally through the HART protocol. The 4 mA to 20 mA analog signal will not be affected by the HART signal and can be used to implement control. The HART protocol provides access to auxiliary variables and other data, which can be used for operation, debugging, maintenance and diagnosis.

Modbus protocol

The software running on the EVAL-ADICUP3029 uses the Modbus protocol-a de facto open industrial communication standard. Modbus provides a reliable way to exchange data with a single node, and ensures data integrity through CRC error detection. As an open standard, there are many available open source and commercial Modbus software libraries, suitable for various platforms (such as Windows®, Linux®, embedded platforms, etc.).

These softwares also provide a simple command line interface (CLI) mode, which enables the system to be manually verified from the serial port without any additional software being loaded on the host.

Hardware and software protocol stack

The PLC/DCS node system software and hardware protocol stack is shown in Figure 7.

How to design an analog I/O system to realize industrial automation?

Figure 7. PLC/DCS node system software and hardware protocol stack

After configuring the PLC/DCS hardware, the user generally selects the applicable Modbus library according to the language (such as C, Python, MATLAB) and the host platform (such as Linux, Windows, embedded platform). Then, a simple test application program must be written to convert the simulation and HART parameters into Modbus register addresses and values.

The CN-0435 User Guide provides a complete description of the Modbus register mapping for this application and uses the open source Modbus debugger to verify Modbus compliance.

In addition, it also provides a number of top-level applications based on the open source Modbus library, including:

• Check system configuration: query all Modbus nodes and Display configuration.

• Read and write output holding registers: check or change the status of the output holding registers of all boards under test.

• Read analog input registers: check the status of the input registers of all boards under test.

• Read analog data: read a single analog input or all analog inputs, and Display the data on the console.

• Write analog data: write analog output to generate voltage or current.

• Analog echo: Read the analog voltage or current of the analog input board, and then write the same value of analog voltage or current to the analog output board.

Common changes

CN-0435 software reads analog input values ​​and writes analog output values ​​without local processing. The software can be expanded to include functions such as fault detection and response, or to include a closed-loop PID control loop, to offload these functions from the host and save the bandwidth of the communication bus.

Raspberry Pi can be used as a compact and inexpensive host solution. Raspberry Pi provides wired or wireless Ethernet connection, which can be directly connected to the USB-UART of EVAL-ADICUP3029.

The three most commonly used Modbus versions today are: Modbus ASCII, Modbus RTU and Modbus TCP. All Modbus messages are sent in the same format. The only difference between these three Modbus types is the encoding of the messages.

The number of devices that can be connected via Modbus is determined by the physical layer and data protocol. If the RS-485 physical layer is used with Modbus RTU or Modbus ASCII data protocol, the maximum number of nodes that can be addressed is 32, but if the Ethernet physical layer is used with Modbus TCP data protocol, the nodes that can be addressed There are 247.

The device address is a number from 0 to 247. The message (broadcast message) sent to address 0 will be accepted by all slaves, but the values ​​from 1 to 247 are the addresses of specific devices.

The Arduino size of CN-0414 and CN-0418 ensures compatibility with development platforms that support a wide range of other automation communication protocols, including process field network (PROFINET), process field bus (PROFIBUS), control automation technology Ethernet (EtherCAT), EtherNet /IP, Modbus Plus, and other protocols.

Circuit evaluation and testing

The following section will introduce the equipment and general steps required to adopt the reference demonstration. The CLI option of the software can be used to assemble the DCS system and test its basic functions. For complete instructions and additional information, please refer to the Distributed Control System (DCS) Demo Wiki User Guide.

Equipment requirements

The following equipment is required:

• A PC with a USB port and Windows 7 (32-bit) or higher

• Serial terminal programs, such as TeraTerm or Putty

• One or more EVAL-CN0414-ARDZ circuit evaluation boards, and/or one or more EVAL-CN0418-ARDZ circuit evaluation boards, suitable for each node

• One or more EVAL-CN0416-ARDZ circuit boards, suitable for Modbus interface, and another EVAL-CN0416-ARDZ board, suitable for each node

• An ADALM-UARTJTAG evaluation board with an additional EVAL-CN0416-ARDZ board (or other half-duplex RS-485 adapter)

• An EVAL-ADICUP3029 evaluation board for each node

• Micro USB cable

• One RJ-45 cable for RS-485 interface and another cable for each node

• PLC system software or preset hexadecimal file

• 24 V DC voltage is used for 1 A power supply

start using

The following are the basic setup steps:

1. Plug the USB cable of the EVAL-ADICUP3029 into the PC, and then flash the firmware to each circuit board used.

2. Configure the hardware. Follow the distributed control system (DCS) demo Wiki user guide. Make sure to set the jumpers and switches on each board correctly. Or, for an analog input board, connect a sensor or signal source, for an analog output board, connect an actuator or a multimeter.

3. For each node, stack the platform and expansion board together in the following order:

• EVAL-CN0416-ARDZ (top)

• EVAL-CN0414-ARDZ or EVAL-CN0418-ARDZ (optional)

• EVAL-CN0414-ARDZ or EVAL-CN0418-ARDZ (optional)

• EVAL-CN0414-ARDZ or EVAL-CN0418-ARDZ (optional)

• EVAL-CN0414-ARDZ or EVAL-CN0418-ARDZ (optional)

• EVAL-ADICUP3029 (bottom)

4. Connect the RJ-45 cable between the node and the RS-485 adapter (may be ADALM-UARTJTAG and EVAL-CN0416-ARDZ).

5. Connect the RS-485 adapter to the host.

6. Press the 3029_Reset button, or restart the system.

For complete details, please refer to the Distributed Control System (DCS) Demo Wiki User Guide.

Functional block diagram

How to design an analog I/O system to realize industrial automation?

Figure 8. Single-node PLC analog I/O system

How to design an analog I/O system to realize industrial automation?

Figure 9. DCS analog I/O system

The Links:   LM150X08 MCC312-16IO1 THE IC INFO

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