CN0435

The application focuses on demonstrating the development of a PLC/DCS system controlled by a Modbus master and exemplifies how the latest features of key components are used. A single node system is often referred to as a PLC, whereas a larger system is often referred to as a DCS.

Each node can control up to 16 analog field devices, sensors or actuators, either HART-compatible or analog-only, and systems can be extended to include as many as 16 individual nodes. The system can also be used for general-purpose precision analog data acquisition applications such as instrumentation, analog datalogging, or test and measurement.

PLC/DCS Topologies

Several connection topologies are supported. In a single-node (PLC, or single-node DCS) system, the host computer can be connected directly to the USB serial port on the EVAL-ADICUP3029 platform board via a micro-USB cable, which is appropriate for laboratory test and measurement applications where the distance between the host and node is less than 2 m.

In this point-to-point topology, groups of four analog inputs and outputs per board are still isolated from the host computer. Though generally not associated with lab equipment, the Modbus protocol provides a convenient and standard method for communicating with the node. HART connectivity allows configuration of smart sensors and actuators.

As the distance between the host and nodes increases beyond 2 m, signal integrity, noise pickup, and electrical faults become a greater concern. The EVAL-CN0416-ARDZ provides robust RS-485 connectivity to the host in these situations. In a single-node, point-to-point system, either full or half-duplex communication is supported, over distances up to 1 km, depending on baud rate

For multinode (better described as a DCS), the EVAL-CN0416-ARDZ includes daisy-chain ports, switchable half/full-duplex operation, and switchable termination, allowing systems of between 2 and 16 nodes to be assembled.

Because Modbus is used as a serial communication protocol to transmit information between devices over a serial link, a simple, reliable, and robust system can be obtained regardless of the scale. The PLC/DCS application hardware stack is composed of three different reference designs.

Analog Input Board

The CN-0414 shown in Figure 3 is designed to measure four fully differential or eight single-ended voltage and four current signals. The heart of the circuit is an AD4111 low power, low noise 24-bit, Σ-Δ analog-to-digital converter (ADC) with integrated ±10 V and 20 mA analog front ends. 

The voltage inputs support an input range of up to ±10 V. The AD4111 incorporates a unique feature that enables open wire detection on ±10 V voltage inputs while operating on a 5 V or 3.3 V single power supply, while previous solutions would typically require a supply greater than ±10 V.

The current inputs support input ranges of 0 mA to 24 mA. The input impedance of the circuit is 250 Ω (60 Ω internal to the AD4111), with all inputs referenced to isolated ground. The 250 Ω input impedance on the current inputs is necessary to make the AD5700-1 HART-compliant modem to work in conjunction with the AD4111.

The analog front end of the circuit, the AD4111 and the AD5700-1 are isolated from the processing side through ADuM5411 and ADuM3151, to offer significant space savings over discrete transformer-based solutions.

The CN-0414 board is powered by a 9.5 V to 36 V dc power supply that is typical of industrial automation systems, and therefore makes it easy to retrofit into your system.

Analog Output Board

The CN-0418 shown in Figure 4 is a quad 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 outputs, as well as unipolar or bipolar voltage outputs (±10 V). The board also includes the AD5700-1 HART modem, to give a complete analog output solution with HART connectivity. External transient protection circuitry is also included, which is important for applications located in harsh industrial environments. 

The current and voltage outputs are available on separate pins, but only one is active at a time, thus allowing both output pins to be tied together and connected to a single terminal. Analog outputs are short-circuited, and open-circuit protected.

The AD5755-1 contains integrated dynamic power control using a dc-to-dc boost converter circuit, allowing reduced power consumption in the current output mode.

The AD5755-1 has four CHART pins, corresponding to each of the four output channels. The HART signal can be coupled into these pins and appears on the corresponding output if that output is enabled.

RS-485 Transceiver Board

The CN-0416 shown in Figure 5 is an isolated and non-isolated RS-485 transceiver board, which allows easy implementation of data transmission between multiple systems or nodes, especially over long distances.

The circuit uses the ADM2682E RS-485 transceiver for isolated communications and the LTC2865 for non-isolated RS-485 communications. Both can be configured to either full duplex or half-duplex operation and with open or terminated transmission lines.

The circuit has on-board RJ-45 jacks, which allow the use of common CAT5 Ethernet cables for fast physical wiring of nodes. The termination resistance is set by default to the CAT5 cable characteristic impedance of 100 Ω but can be configured to support the standard RS-485 cable impedance of 120 Ω. 

The ADM2682E is capable of a data rate up to 16 Mbps and has true fail-safe receiver inputs with adjusted differential voltage threshold. It provides 5 kV signal isolation using the iCoupler data channel and 5 kV power isolation using the isoPower integrated dc-to-dc converter.

The LTC2865 is capable of a data rate up to 20 Mbps and has full fail-safe receiver inputs. An internal window comparator determines the fail-safe condition without the need to adjust the differential input voltage thresholds.

HART-Compatible Field Devices Wiring

HART Networks

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

In point-to-point mode, the 4 mA to 20 mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using 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 gives access to secondary variables and other data that can be used for operations, commissioning, maintenance, and diagnostic purposes.

Modbus Protocol

The software running on the EVAL-ADICUP3029 implements the Modbus protocol, a de-facto and open industrial communication standard. Modbus provides a robust means of exchanging data with individual nodes, with CRC error detection ensuring data integrity. Being an open standard, there are numerous open-source and commercial Modbus software libraries available, targeting various platforms (such as Windows^®, Linux^®, embedded platforms, among others).

The software also provides a simple command-line interface (CLI) mode, allowing systems to be verified manually from a serial terminal, without requiring any additional software on the host.

Hardware and Software Stack

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

After configuring the PLC/DCS hardware, the user would typically select an appropriate Modbus library according to the language (such as C, Python, MATLAB) and host platform (such as Linux, Windows, embedded). A simple test application must then be written, providing translation of analog and HART parameters to Modbus register addresses and values

The CN-0435 User Guide provides a complete description of the Modbus register map for this application, and Modbus compliance is verified using an open-source Modbus debugger

Several example top-level applications are also provided, built on an open-source Modbus library, and include the following:

• Detect system configuration: query all Modbus nodes and display configuration.
• Read or write output holding registers: check or change the state of output holding registers off all detected boards.
• Read analog input registers: check the state of input registers off all detected boards.
• Read analog data: read a single analog input or all analog inputs and display data to the console.
• Write analog data: write analog outputs to generate a voltage or a current.
• Analog echo: read analog voltage or current from an analog input board and write the same analog voltage or current to an analog output board.