In master address and slave address is incorporated in

In HART protocol data can be
transmitted in two modes:

• Poll (AKA: Response) mode

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• Burst (AKA: broadcast)
mode (liPták, Date Unknown)

Poll Mode

In the poll/response mode, the master polls each of
the field devices in the network and requests for required information. (liPták,
Date Unknown)

Burst Mode

In burst mode, the field device uninterruptedly transmits
process data without any request message. (liPták, Date Unknown)

The HART Message Structure and Format

There are 9 fields in a HART message format. The message
structure of HART communication is shown in the figure below:

Figure
3 :HART Structure FSK (Mishra, 2011)

Field 1: Preamble

The preamble field consist of 5 to 20 bytes. In hex it
is FF i.e. all are 1. Synchronization with character stream is initiated
through this field. As mentioned above all the 5 to 20 bytes are only 1 here
and it offers delay between the synchronization and the starting of start bit
as well. (Mishra, 2011)

Field 2: Start Character

Second field is Start Character which consist of 1
byte (8 bits). Start character can have various values and its function are:

–         
Specify the message type.  

–         
Indicates, is it slave to master or
master to slave?

–         
Designate the mode of the
transmission, is it in burst mode or poll mode?             (Mishra, 2011)

Field 3: Address

Both the master address and slave address is
incorporated in this field. If it is 1 that indicates it is a primary master
and 0 indicates for a secondary master. There are two frame formats of message
like short format and long format. For short format, the polling address of the
slave is 4 bits. On the other hand, it is 38 bits in long format for the same. (Mishra,
2011)

Field 4: Expansion

Expansion field purpose is to make a space between
command and address field. It allows extra 3 bytes (24 bits) between those
fields. The number of bytes (max 3 byte) is specified by the 6 and 5 bits of
start delimiter. (Mishra, 2011)

Field 5: Command

Common field are two types:

Universal Command: Range 0 to 30

Common Practice Command: Range 32 to 126.

Sometimes there are device specific commands also
which range is 128 to 253. All type of command field consist of 1 byte (8
bits). The command field is the instruction for field device to what to do.
(Mishra, 2011)

Field 6: Byte Count

Since HART message format doesn’t has any “end of
message” character, Byte count field is used to confirm the receiver about the
message completion. How many bytes of status and data bytes should be followed
by the receiver is confirmed by this field. The byte count field contains the
number of bytes to follow in the status and data bytes. And this way, it helps
the receiver to know when the message is going to complete. (Mishra, 2011)

Field 7: Status

The other name of status field is “response code”. It
consists of two bytes (16 bits). Only the response message from the slave
contain this field. The purpose of the field is:

–         
To confirm about any error in the
outgoing message.

–         
To declare the status of received command
form master.

–         
To inform about its own (field
device) status as well. (Mishra, 2011)

 

Field 8: Data

As mentioned earlier, there are several type of
command in HART communication protocol. And all messages don’t have the data
filed. It depends on the type of command. In universal and common practice
command, they use 33 bytes for reasonable message duration. Whereas some device
specific command uses longer data field up to 253 bytes. (Mishra, 2011)

Field 9: Checksum

The last field is checksum field which contains byte 1
(8 bits). This field is used to detect any communication error. Starting from
start character to end, checksum field work as a longitudinal parity for all
the bytes and thus check the error. (Mishra, 2011)

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