The bit stream is organized into 8-bit bytes that are further grouped into messages. A HART transaction consists of a master command and a slave response. Media access consists of token passing between the devices connected to the channel. The passing of the token is implied by the actual message transmitted. Timers are used to bind the period between transactions. Once the timer expires, control of the channel is relinquished by the owner of the token.
The Application Layer, Layer 7 defines the commands, responses, data types and status reporting supported by the Protocol. In addition, there are certain conventions in HART (for example how to trim the loop current) that are also considered part of the Application Layer. While the Command Summary, Common Tables and Command Response Code Specifications all establish mandatory Application Layer practices (e.g. data types, common definitions of data items, and procedures), the Universal Commands specify the minimum Application Layer content of all HART compatible devices.
To learn more about the HART protocol, you can complete the on-line tutorials and Download the HART Application guide on the HART Foundation web site.
What’s New in HART 6
HART 6 adds the following new features to the existing HART protocol:
- Device Variable Classification (Required)
Provides master applications with a simple mechanism to determine the number and type of process related variables (pressure, temperature, etc.) that are available within a device. A key enabler for many HART 6 improvements, this mechanism provides master applications with considerable information about device capabilities. (Commands 0, 8)
- Extended Device Status ( Required)
One additional byte of well defined status information with Command 0 and new cyclic data access Command 9. This new Extended Device Status byte is in addition to the current device status byte returned with each command response. Provides additional device status alerts such as “Device Needs Maintenance” plus more. (Commands 0, 9)
- Device Variable Status (Required)
One byte of well defined status for each Device Variable returned by new cyclic data access Command 9. Enables field devices to self-validate and report on the quality of the data in the command response (good, poor, bad, fixed) plus more. (Command 9)
- Long Tag (Required)
This new Long Tag with international (ISO Latin 1) characters allows consistent implementation of the longer tag names required by many industry users. The specifications currently reflect the length of this tag to be 32 characters as outlined in the Long Tag proposal approved by HCF members in January 1998. However, the recent discovery that some existing I/O systems will not “pass-through” the 32 character tag, may cause the number of characters to be reduced in the final specification. The need for this new Long Tag is unquestioned. Only the number of characters is being reconsidered. (Commands 20, 21, 22)
- Configuration Change Counter (Required)
Improved mechanism for master applications to determine that a field device configuration has been changed. Protects integrity of plant configuration databases. (Command 0)
- Device Families ( Optional)
Establishes standard commands and status indicators for devices based on the type of process connection. Standard commands allow simple master applications to more fully communicate and configure devices without having to rely on the complexity of DDL. Initial Device Families for Temperature and PID Control are included in the specification package. The Valve Positioner / Actuator Device Family is in final preparation.
- Transducer Trim Commands (Optional)
New Common Practice commands for performing transducer trim (calibration) operations. Reduces need for device specific commands to perform this common function. Standard commands make it easier for master applications such as field instrument calibrators to perform calibration functions on all HART devices.
- Sub-Devices (Optional)
A simple mechanism using Common Practice Commands to support “HART device within a HART device” functionality. Potential uses include flow computers and multi-channel temperature devices.
- Block Data Transfer (Optional)
An updated mechanism to support the movement of large blocks of data between masters and field devices. Potential uses include the movement of diagnostic information stored in the field device and upload/download of device configurations.
- Catch Device Variable (Optional)
A simple mechanism using Common Practice Commands to support the sharing of process data between field devices on the same HART network. Allows a listening field device to capture process data from another field device to be used in calculations such as tank gauging, flow computers or PID control functions.
- Write Device Variable (Optional)
New Common Practice Commands to support forcing the digital value for any Device Variable to a specific value to aid in commissioning and troubleshooting.
- Lock Device (Optional)
New Common Practice Commands allow a master application to lock the “local” front panel of a field device while performing remote configuration functions.
- Squawk and Find Device (Optional)
New Common Practice commands to support commissioning and troubleshooting of HART devices in multi-drop and multi-pair cable installations.
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