Table of Contents

Introduction

The RediGate Configuration Manual describes the configuration of many of the RediGate's standard features using the ACE program. This document gives additional instructions for configuring the RediGate to use the following protocols:

• HART Master

See the following Quick Start example configurations:

• HART to RediGate Master

HART Protocol Description

This document is not intended to provide a detailed description of the protocol(s) involved, nor to disclose proprietary information that may belong to other parties. Depending on the protocol, it may be necessary to refer to other vendor protocol documentation or device configuration details to understand how the RediGate should be configured to interface with it. This section provides a brief discussion of the protocol for the purpose of understanding the RediGate's configuration objects.

HART® (Highway Addressable Remote Transducer) and WirelessHART® registered trademarks of the HART Communication Foundation, Austin, Texas, USA (http://www.hartcomm.org). The HART Communication Foundation owns and manages the specification for the HART protocol. HART was designed to leverage compatibility with traditional 4-20 mA analog devices, while adding a low-power digital communication layer that allows for enhanced data acquisition and operational capabilities. HART digital signaling uses a 1200 bps Frequency-Shift-Keying (FSK) small amplitude current superimposed on the relatively low-frequency 4-20 mA analog current signal. HART protocol communication typically involves using a modem that translates the 1200 baud FSK signal from the current loop (seen as voltage across a low impedance loop resistor) to a serial port of a microcontroller, such as the RediGate. (There is also provision for a C8PSK physical layer that can transmit at up to 9600 bps.)

HART is a master-slave protocol and may include read or write operations. The HART specification defines three categories of commands:

• Universal Commands – must be implemented by all HART compatible devices according to specification.
• Common Practice Commands – optional, standardized commands that must be used according to specification when implemented.
• Device Specific Commands – commands defined by device manufacturers according to individual device requirements.

Traditionally, HART devices were addressed by a numeric Short Address from 0 to 15. Address 0 indicates the transmitter is in 4-20 mA current signaling mode, while addresses 1-15 indicate the device is in multi-drop mode (with a fixed analog current, and all device data is obtained via the digital protocol). Devices may also be queried using the device's Tag name instead of the Short Address. The structure of a HART message is shown below:

• Preamble is three or more bytes of hexadecimal 0xFF characters allowing the receiving modem to synchronize its frequency detection circuit.
• Start Character has various values depending on the frame format used and source of the message.
  Master to slave (0x02 for short frame message, 0x82 for long frame)
  Slave to master (0x06 for short frame message, 0x86 for long frame)
• Address contains the field device address for the message in either single byte form (short frame) or five bytes (long frame). MSB indicates whether the host is a primary master (1) or secondary master (0).
• Command indicates which HART command is being transmitted.
• Byte Count indicates total number of bytes in the Status and Data portions of the message.
• Status (2 bytes) is only present in a slave response and indicates various status conditions of the device. The RediGate stores the Status bytes into an RTDB register.
• Data may be empty or contain relevant data bytes depending on the command request or reply. The Data portion of the message is important to understand when configuring the RediGate, because each HART command must be defined in the RediGate configuration, with data fields mapped to RTDB registers.
• Checksum is a single byte XOR (longitudinal parity) of all preceding bytes beginning with the Start Character.

The following sections describe the ACE objects used for HART Master, object properties (including constraints on the Instance number), and object fields and their possible values required to configure for the given protocol(s). The object structure in ACE is hierarchical, with each object existing under a certain parent object. For instance, the FieldUnit is the child of one of several types of Circuit objects, either serial or network type (" System>Clients>Master Channels>Master Channel>Circuit").

The Description and Enabled properties are included in ACE as part of each object but are not mentioned here. The "UFF External" property is only mentioned for the objects where it is typically used, but should normally be left unchecked.

HART Master Channel

The structure of ACE objects for a Master Channel used for HART protocol is shown below. Note the configuration must include the special HartCircuit and one or more HartCommand objects:

In the HART FieldUnit's Poll Table, every command must be included that is intended to be supported by the RediGate. But as with all other FieldUnit types, it is not required to scan all of those polls in real-time. For instance, a typical Poll Table is shown below that includes seven polls defined in the HART Command objects.

Although the Poll Table might contain many Command Index entries, the Master Channel might be configured to poll only for the 3rd Poll Record (CMD 03 - Read PVs as floating point), as shown below. This allows the primary process variables to be read continuously for each of the configured HART devices.

Note that a Scan Period of 0 milliseconds (and also Response Timeout and Interpoll Delay of 0) is used to obtain the maximum HART scan rate, but actual communication delays are determined by built-in timing required by the HART protocol, which makes the effective scan rate about 2-3 polls per second. Any non-zero Scan Period or Interpoll Delay will add additional poll delay beyond the timing required by the HART protocol, and a non-zero Response Timeout will increase delay on timeouts.

Although in the example above the Master Channel is only scanning for HART CMD 03 continuously, the other polls in the HART FieldUnit's Poll Table may be requested on demand using the device's Command Register (see HART RTU Extra).

HART Circuit

The HART Circuit object is a special communications path for one or more HART devices from a common master channel. Use this circuit instead of the generic Async Circuit when configuring a HART device under a Master Channel.

The HART Circuit should have at least one Field Unit HART child object defined under it.

AsyncPort

Make sure to include an COM port (System → Networks → AsyncPort) object in the configuration with the instance number matching the physical COM port, and the baud rate corresponding to the field device’s setting. For a port connected to a HART modem device, the settings of the AsyncPort should typically be:

See the RediGate Configuration Manual for information on configuring the AsyncPort.

HART Field Unit

The FieldUnitHART object is used to configure polling of a particular HART device that is on a HART circuit.

The FieldUnit must have an RTDB and should also typically have a HART RTU Extra child object defined under it. 

Before defining the Poll Table, you must also define one or more HART Command objects.

HART RTU Extra

The HART RTU Extra object

A Modbus FieldUnit object contains unique information for each physical field device that uses the Modbus communication protocol, defining parameters for how data is read and written for the device.

HART Commands

The HART Commands object is a placeholder, under which one or more HART Command objects will be included.

HART Command

The HART Command object specficies a particular HART command and defines data types and RTDB registers in which to store the data used in communication with the HART device. HART Command objects are dynamically referenced by HART Field Unit Poll Tables. The name of the object is what is shown in the dropdown list of the Field Unit HART → Poll Table → Command Index column.

Examples of HART Command Definitions

The definition of a HART Command within the ACE configuration requires knowledge of the command structure (Data portion only), as published by the HART Communication Foundation or manufacturer device-specific command documentation. Several examples are given below.

HART Command 3 – Read Dynamic Variables and Loop Current

HART universal command #3 returns the loop current and up to four process variables with their engineering units codes (the 2nd, 3rd, and 4th variables are optional). Documentation for command #3 describes the following Data structure:

Request Data Bytes

Response Data Bytes

To create a HART Command definition in ACE that will read up to four process variable and the milliamp current (but discard the unit codes), a Command definition would be constructed as shown below. The minimum required RTDB configuration is also shown. This command would be scanned in the Master Channel using the Unit Address of the device (as configured in ACE) and the Poll Record number (row number of this HART Command in the device's Poll Table).

However, as a variation on the example, to poll and store all four process variables, milliamp current, and the units codes into the RTDB registers, a different HART Command and additional RTDB registers would need to be defined, as shown below:

HART Command 0 – Read Unique Identifier

Although the RediGate automatically uses HART command #0 (or #11) to identify the device initially, so that Command #0 doesn't need to be defined in the HART Command objects, it may still be valuable to define a Command #0 to be polled as data that can be stored into the RTDB. Command #0 includes device information that might be important to acquire in order to report to a host system.

Below is an example HART Command definition for command #0. In this example, there are no request (Write) Data fields. Response data bytes are processed as shown in the following bullets:

• Byte 0 – discard
• Bytes 1-2 → 40003 (expanded device type, Integer type reads 2 bytes)
• Byte 3 → discard
• Byte 4 → 40005 (HART revision number)
• Byte 5 → 40006 (Device revision level)
• Byte 6 → 40007 (Software revision level)
• Byte 7 → 40008 (Hardware revision level, physical signaling code)
• Byte 8 → 40009 (Flags)
• Bytes 9-11 → 41001 (Device ID, three bytes stored to UINT32 register)
• Bytes 12-13 → discard
• Bytes 14-15 → 40012 (Configuration change counter)
• Byte 16 → 40013 (Extended device status)
• Bytes 17-18 → 40014 (Manufacturer ID code)

Some older devices won't have bytes 17 and 18, so the command processing will end after all available Data bytes have been processed. Other devices will have additional bytes 19 through 21, which will be automatically discarded, because they aren't included in the HART Command definition in the RediGate.

HART Command 18 – Write Tag, Descriptor, Date

This example shows the definition of a command that writes data to the HART device. HART command #13 reads the 8-character Tag, 16-character Descriptor, and a 3-byte Date code. HART Command #18 writes these values from the RTDB to the HART device and reads the same data back into the RTDB from the device response. The example configuration shown below uses the following registers:

• Bytes 0-5 – Registers 41002-41003 (two 32-bit integers)
• Bytes 6-17 – Registers 41004-41007 (four 32-bit integers)
• Bytes 18-20 – Register 41008 (uses three of the four bytes from 32-bit integer)

To trigger the operation of HART Command 18, first set the new Tag, Descriptor, and Date code into registers 41002-41008. Then write the Poll Index (row number in the Poll Table) of the command into the Command Register of the HART FieldUnit.

HART Command 6 – Write Short Address

This example shows the definition of a command that sets the Short Address of a HART device. An example configuration is shown below. Enter a new Short Address (0-15) into register 40020, then write the Poll Index (row number in the Poll Table) of the command into the Command Register of the HART FieldUnit.