Programmable logic controllers (PLCs) and distributed control systems (DCSs) are used to monitor and control intelligent (HART-enabled) and analog field instrumentation 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, which are managed locally by an Arduino form factor baseboard . 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.

Analog input data is read locally and provided via a serial interface using the industry standard Modbus protocol, ensuring data integrity and compatibility across a range of software applications and libraries. Likewise, the analog output is set by writing to a Modbus register, which is then converted to an analog voltage or current signal.

Each node can support a combination of 4 analog input and output boards. As shown in Figure 2, multi-node systems 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 extended to a multi-drop HART network, integrating multiple HART devices on the same channel.

The analog inputs and analog outputs of each board (group of 4) are electrically isolated, and the analog inputs support open circuit detection, simplifying fault detection and diagnosis. These features enhance reliability and durability when used in harsh industrial automation environments.

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Figure 1. Functional block diagram of PLC (or single-node DCS) Modbus system

Figure 2. Multi-node DCS Modbus System Functional Block Diagram Circuit Description This application focuses on demonstrating the development of a PLC/DCS system governed by a Modbus master, with examples of how to use the latest capabilities of key components. Single-node systems are often referred to as PLCs, and larger systems are often referred to as DCSs.

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

The PLC/DCS topology supports multiple connection topologies. In a single-node (PLC, or single-node DCS) system, the host can be directly connected to the USB serial port of the EVAL-ADICUP3029 platform board using a micro-USB cable, ideal for experiments requiring less than 2 meters between host and node room test and measurement applications.

In this point-to-point topology, the board's quadruple analog inputs and outputs remain isolated from the host computer. Although not generally associated with laboratory equipment, the Modbus protocol provides a convenient, standard method of communicating with nodes. HART connectivity enables configuration of smart sensors and drives.

Signal integrity, noise pickup, and electrical failures become more serious concerns as the distance between the host and the node increases beyond 2 meters. In these cases, the EVAL-CN0416-ARDZ provides a reliable RS-485 connection to the host. In single-node, point-to-point systems, full-duplex or half-duplex communication is supported over distances greater than 1 km, depending on the baud rate.

For multi-node systems (better known as DCS), the EVAL-CN0416-ARDZ provides daisy-chain ports that support switchable half/full duplex operation and switchable termination, so the system can integrate 2 to 16 nodes.

Because Modbus is used as a serial communication protocol to send information between devices over a serial link, simple, reliable, and robust systems can be implemented regardless of size. The hardware stack for PLC/DCS applications consists of 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. At the heart of this circuit is the AD4111 low power, low noise 24-bit sigma-delta analog-to-digital converter (ADC) with an integrated ±10 V and 20 mA analog front end.

Its voltage input supports input ranges up to ±10 V. The AD4111 has a unique feature that supports open-circuit detection on ±10 V voltage inputs while operating from a single 5 V or 3.3 V supply, whereas previous solutions typically required supplies 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 the AD4111), and all inputs are referenced to isolated ground. A 250 Ω input resistor is required on the current input so that the HART compliant AD5700-1 modem can be used with the AD4111.

The analog front end of the circuit, the AD4111 and AD5700-1, is isolated from the processing side by the ADuM5411 and ADuM3151, resulting in significant space savings compared to discrete transformer-based solutions.

The CN-0414 board is powered by a 9.5 V to 36 V DC power supply, which is typical in industrial automation systems, so it can be easily integrated into your system.

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 a 4 mA to 20 mA current output, as well as a unipolar or bipolar voltage output (±10 V). The board also features the AD5700-1 HART modem, which provides a complete analog output solution that supports HART connectivity. Also includes external transient protection circuitry, which is extremely important for applications in harsh industrial environments.

Current and voltage outputs are provided on separate pins, with only one output active at a time, allowing two output pins to be tied together and to a single port. The analog outputs are short-circuit and open-circuit protected.

The AD5755-1 integrates a dynamic power control function based on a DC-DC boost converter circuit to reduce power consumption in current output mode.

The AD5755-1 has four CHART pins, which correspond to the four output channels. 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 enables easy data transfer between multiple systems or nodes, especially over long distances.

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

The circuit uses on-board RJ-45 jacks, so it can be quickly and physically connected to the node using common CAT5 Ethernet cables. Termination resistance is set by default to the characteristic impedance of CAT5 cable 100 8486 ;, but after configuration, it can support 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 adjusted differential voltage thresholds. It uses the iCoupler data channel, which provides 5 kV signal isolation, and the isoPower integrated DC-DC converter, which provides 5 kV power isolation.

Data rates of the LTC2865 can reach 20 Mbps, providing a truly secure receiver input. An internal window comparator determines the safe condition without adjusting the differential input voltage threshold.

Figure 5. RS-485 Transceiver Board

Wiring HART Compatible Field Devices

Figure 6. Wiring for HART Compatible Field Devices

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

In point-to-point mode, a 4 mA to 20 mA signal is used to transmit one 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 is not affected by the HART signal and can be used for control. The HART protocol provides access to auxiliary variables and other data that can be used for operation, commissioning, maintenance and diagnostics.

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 of exchanging data with a single node, ensuring data integrity through CRC error detection. As an open standard, there are numerous open source and commercial Modbus software libraries available for various platforms (eg Windows®, Linux®, embedded platforms, etc.).

The software also provides a simple command line interface (CLI) mode that enables the system to be manually authenticated from the serial port without any additional software loaded on the host.

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

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 (eg C, Python, MATLAB) and host platform (eg Linux, Windows, embedded platform). Then, a simple test application must be written to convert the analog and HART parameters into Modbus register addresses and values.

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

In addition, several top-level applications based on open source Modbus libraries are provided, including:

Check system configuration: query all Modbus nodes and display the configuration.

Read and write output holding registers: Check or change the state of the output holding registers of all tested boards.

Read analog input registers: Check the status of the input registers of all tested boards.

Read Analog Data: Read a single analog input or all analog inputs and display the data on the console.

Write Analog Data: Write the 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 the analog voltage or current to the analog output board.

common changes
The CN-0435 software reads analog input values and writes analog output values without local processing. The software can be extended to include functions such as fault detection and response, or to include closed-loop PID control loops, offloading these functions from the host, saving bandwidth on the communication bus.

The Raspberry Pi can be used as a compact and inexpensive host solution. The Raspberry Pi offers wired or wireless Ethernet connectivity and can be connected directly to the EVAL-ADICUP3029's USB-UART.

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 the three Modbus types is how the messages are encoded.

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 the Modbus RTU or Modbus ASCII data protocol, the maximum number of nodes that can be addressed is 32, however, if the Ethernet physical layer is used with the Modbus TCP data protocol, the addressable nodes to 247.

Device addresses are numbers from 0 to 247. Messages sent to address 0 (broadcast messages) are accepted by all slaves, but the numbers 1 to 247 are device-specific addresses.

The Arduino form factor of the 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), Ethernet for Control Automation Technology (EtherCAT), EtherNet /IP, Modbus Plus, and other protocols.