computer science

Highway Addressable Remote Transducer Protocol

Highway Addressable Remote Transducer Protocol is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Highway Addressable Remote Transducer Protocol rather than just read about it. In short: The HART Communication Protocol (Highway Addressable Remote Transducer) is a hybrid analog+digital industrial automation open protocol. Its most notable advantage is that it can communicate over legacy 4–20 mA analog instrumentation current loops, sharing the pair of wires used by the analog-only host systems.

Key takeaways

Highway Addressable Remote Transducer Protocol belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Highway Addressable Remote Transducer Protocol to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Highway Addressable Remote Transducer Protocol from memory before moving on to harder problems.

Reference excerpt

The HART Communication Protocol (Highway Addressable Remote Transducer) is a hybrid analog+digital industrial automation open protocol. Its most notable advantage is that it can communicate over legacy 4–20 mA analog instrumentation current loops, sharing the pair of wires used by the analog-only host systems. HART is widely used in process and instrumentation systems ranging from small automation applications up to highly sophisticated industrial applications. Based on the OSI model, HART resides at Layer 7, the Application Layer. Layers 3–6 are not used. When sent over 4–20 mA it uses a Bell 202 for layer 1. But it is often converted to RS485 or RS232.

According to Emerson, due to the huge installation base of 4–20 mA systems throughout the world, the HART Protocol is one of the most popular industrial protocols today. HART protocol has made a good transition protocol for users who wished to use the legacy 4–20 mA signals, but wanted to implement a "smart" protocol.

History The protocol was developed by Rosemount Inc., built off the Bell 202 early communications standard in the mid-1980s as a proprietary digital communication protocol for their smart field instruments. Soon it evolved into HART and in 1986 it was made an open protocol. Since then, the capabilities of the protocol have been enhanced by successive revisions to the specification.

Modes

There are two main operational modes of HART instruments: point-to-point (analog/digital) mode, and multi-drop mode.

Point to point In point-to-point mode the digital signals are overlaid on the 4–20 mA loop current. Both the 4–20 mA current and the digital signal are valid signalling protocols between the controller and measuring instrument or final control element. The polling address of the instrument is set to "0". Only one instrument can be put on each instrument cable signal pair. One signal, generally specified by the user, is specified to be the 4–20 mA signal. Other signals are sent digitally on top of the 4–20 mA signal. For example, pressure can be sent as 4–20 mA, representing a range of pressures, and temperature can be sent digitally over the same wires. In point-to-point mode, the digital part of the HART protocol can be seen as a kind of digital current loop interface.

Multi-drop In multi-drop mode the analog loop current is fixed at 4 mA and it is possible to have more than one instrument on a signal loop. HART revisions 3 through 5 allowed polling addresses of the instruments to be in the range 1–15. HART revision 6 allowed addresses 1 to 63; HART revision 7 allows addresses 0 to 63. Each instrument must have a unique address.

Packet structure The request HART packet has the following structure:

Preamble Currently all the newer devices implement five byte preamble, since anything greater reduces the communication speed. However, masters are responsible for backwards support. Master communication to a new device starts with the maximum preamble length (20 bytes) and is later reduced once the preamble size for the current device is determined. Preamble is: "ff" "ff" "ff" "ff" "ff" (5 times ff)

Start delimiter This byte contains the Master number and specifies that the communication packet is starting.

bit 7, if high use Unique (5 byte) address, else use Polling (1 Byte) addresses. bit 6 and 5, Number of Expansion bytes normally it set if Expansion field is used, normally 0. bit 4 and 3, Physical layer type 0=Asynchronous, 1=Synchronous bit 2, 1 and 0, Frame type 1=BACK Burst Acknowledge send by Burst-mode Device 2=STX Master to Field Devices. 6=Slave Acknowledge to STX frame.

Address Specifies the destination address as implemented in one of the HART schemes. The original addressing scheme used only four bits to specify the device address, which limited the number of devices to 16 including the master. The newer scheme utilizes 38 bits to specify the device address. This address is requested from the device using either Command 0, or Command 11.

Command This is a one byte numerical value representing which command is to be executed. Command 0 and Command 11 are used to request the device number.

Number of data bytes Specifies the number of communication data bytes to follow.

Status The status field is absent for the master and is two bytes for the slave. This field is used by the slave to inform the master whether it completed the task and what its current health status is.

Data Data contained in this field depends on the command to be executed.

Checksum Checksum is composed of an XOR of all the bytes starting from the start byte and ending with the last byte of the data field, including those bytes.

Manufacturer codes Each manufacturer that participates in the HART convention is assigned an identification number. This number is communicated as part of the basic device identification command used when first connecting to a device.

References

External links FieldComm Group .NET Open Source project Archived 2017-12-23 at the Wayback Machine

Worked examples

Example 1 — a first encounter with Highway Addressable Remote Transducer Protocol

Start with the simplest possible case. Write down what Highway Addressable Remote Transducer Protocol claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Highway Addressable Remote Transducer Protocol before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Highway Addressable Remote Transducer Protocol ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Highway Addressable Remote Transducer Protocol

In research: Highway Addressable Remote Transducer Protocol appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Highway Addressable Remote Transducer Protocol in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Highway Addressable Remote Transducer Protocol is common in secondary-school and first-year university syllabi. It links to neighbouring topics Industrial automation, Industrial computing, Network protocols, so understanding it makes those chapters shorter.
In everyday life: Look for Highway Addressable Remote Transducer Protocol outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Highway Addressable Remote Transducer Protocol in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Highway Addressable Remote Transducer Protocol means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Highway Addressable Remote Transducer Protocol out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Highway Addressable Remote Transducer Protocol in simple terms?

Why does Highway Addressable Remote Transducer Protocol matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Highway Addressable Remote Transducer Protocol?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Highway Addressable Remote Transducer Protocol.