HART (Highway Addressable Remote Transducer) provides digital communication to microprocessor-based (smart) analog process control instruments. Originally intended to allow convenient calibration, range adjustment, damping adjustment, etc. of analog process transmitters; it was the first bi-directional digital communication scheme for process transmitters that didn’t disturb the analog signal.
Since the HART signal band is essentially the same as the band available to voice signals in telephone networks, a telephone network can be used to transmit HART. In the United States and Canada the HART FSK signal frequencies are OK as is. In Europe or other countries that use CCITT standards, HART can still be used except that the frequencies must be changed to 1300 Hz (logic 1) and 2100 Hz (logic 0). These frequencies are acceptable in the U.S. and Canada. And,fortunately, most HART and Bell-202 modems will accept these two frequencies. This leads to some simplification in an interface device.
A typical HART-over-phone-lines application is shown in figure
The computer and office modem constitute the HART Master. The office modem is a standard Bell-202 or CCITT V.23 telecom modem. There is no point in trying to adapt a HART modem at the office end, since commercially available telecom modems already have the desired certification and are directly compatible with the telephone network. They just need to be set up to work with HART When used with the Public Switched Telephone Network (PSTN) the computer and office modem can “call up” any number of Field Instruments. The size of this network is virtually unlimited. And, of course, there can still be up to 15 HART Field Instruments served by each Telecom-HART Interface, so that there can be up to 15 Field Instruments at each phone number.
The “Telecom-HART Interface” is a necessary part of the scheme, since a process instrument isn’t directly compatible with the telephone network. Even when a leased line is purchased from the phone company, direct connection of a process instrument isn’t advised because signal levels and impedances will not be correct. If the process instrument is a 2-wire device, there is also the question of how to power it.
Block Diagram of Telecom/HART Interface
At the start of a transaction the telco carrier will come ON and the upper path will become active. As soon as the path becomes active, both carrier detects will be on and the upper path will remain active. The Master will finish its transmission so that the telco carrier goes away. At this point, the carrier at the HART side might also go away if there is no immediate reply by a HART Slave.Then both paths would become inactive. When the Slave finally replies, there will be a HART carrier and no telco carrier so that the bottom path will become active.Another possibility is that after the telco carrier stops, the HART side carrier stays active because the HART Slave has already begun to reply. Then the bottom path will be made active at the same time that the upper path becomes inactive.At the telephone end, the interface device provides a data access arrangement (DAA), so that the device may be legally connected. Limiters in both paths control the amplitude of signals that are applied to the respective networks. An entire modem is added if the device is to be used in Europe. This modem accepts the HART signal frequencies of 1200 Hz and 2200 Hz and converts them to CCITTcompatible frequencies of 1300 Hz and 2100 Hz.
Diadvantages: A potential problem with trying to use conventional Master software in this telephone application is the delays in the telephone network