Regarding HART fieldbus, A typical signal waveform is shown below. Given that th

Question

Regarding HART fieldbus,

A typical signal waveform is shown below. Given that the FSK for digital encoding has 1200Hz for logic value “1”, what is the baud rate for HART communication? Explain your answer.

: Regarding HART fieldbus, a. A typical signal waveform is shown below. Given that the FSK for digital encoding has 1200Hz for logic value "1", what is the baud rate for HART communication? Explain your answer. 20 mA Digital Signal Analog Signal 4 mA Note: Note to Scale Time b. An example of a data frame is shown in the HART Protocol analyzer (See attachment). i. Is this a "command" or a "response" message? ii. What the specific address? FFFFFFFFFF 06 | 86 01 | 07|0000|0843FO0000 76 'Read' PVcommand data field(5+status (2) Fig. 4 HART frame data for a flow measurement of 30 kg/minute

Explanation / Answer

(a)

In Hart protocol or Fieldbus communication (Highway Addressable Remote Transducer Protocol) a hybrid analog+digital industrial automation protocol has been used. This is basically based on the phase continuous frequency shift keying (FSK) technique where in generally For the digital FSK signal 0 bit has been nodulated in 2200Hz sinusoidal signal and the bit 1 is modulated in 1200Hz sinusoidal signal ,where in general the BAUD rate is 1200bps . The most advantage of HArt devices is they used  4-20 mA analog instrumentation current loops.

Now what is baud rate
BAud rate is the rate at which information is transferred in a communication channel.
Here 1200 bps means , the Hart transducer is capable of transferring a maximum of 1200 bits per second
It can be define with simple formula
B= D.(b+n)/n
where B is units of "total bits per second"
D with units of "application data bits per second"
b to be the total number of parity, start, and stop bits
n to be the total number of "application data bits per application data byte"

By calculation we will get the Baud Rate for FSK signal of a Hart communicator is 1200BPS

(b) In Hart protocol analyzer the value of different data frame means

P means preamble which lies between 5 and 20 bytes of hex FF (all 1’s), helps the receiver to synchronise to the character stream. SC means start character , AD is address field includes both the master address (a single bit: 1 for a primary master, 0 for a secondary master) and the slave address , Then  expansion field allows for up to 3 additional bytes to be inserted between the address and command fields .

; common practice commands are in the range 32 to 126; device-specific commands are in the range 128 to 253. HART rev. 6 introduced 16-bit “extended commands” for device family commands. These put 31 (hex 1f) in this byte, and the 2-byte command number as the first 2 bytes in the “data” field. The command byte contains the HART command for this message. Universal commands are in the range 0 to 30.The byte count byte contains the number of bytes to follow in the status and data bytes . then S means he status field (also known as the “response code”) is two bytes, only present in the response message from a slave

As here the S is 0 so it is not a response message rather it is a command one.

And we can see the Adress is 86 hex.

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