Definition
HART (Highway Addressable Remote Transducer) overlays FSK digital signals on 4~20mA analog loops, enabling remote configuration, diagnosis, and multivariable reading of smart instruments. Pressure transmitters, flow meters, etc., often support HART, facilitating integration with DCS systems. HART multi-hosts must be paired with multiplexers.
Technical Principle
HART overlays 2200Hz (logic 1) and 1200Hz (logic 0) FSK signals on 4~20mA DC, with amplitudes of about 1mA, without affecting analog transmission. Uses master-slave polling; the master can be a handheld device, DCS, or data acquisition device. Supports point-to-point (single master and single slave) and multi-point (multiple slave parallel, requires 250Ω resistor) modes. Communication rate is 1200bps. Typical commands include reading master variable, reading multiple variables, writing range, write damping, self-check, etc.
Application Value
While retaining 4~20mA analog output, the HART instrument can remotely read multiple variables (such as pressure, temperature, density), equipment status, diagnostic information, and remotely configure parameters such as range, units, and damping, without the need for on-site wiring or lid opening. It is suitable for process industries such as petrochemical, power, and water treatment. The HART gateway can convert HART data into Modbus, , OPC UA, and access SCADA , DCS, and IoT platforms.
Relationship with 4~20mA
4–20mA is the standard for analog transmission, with HART superimposing digital communication on it. Pure 4–20mA instruments have no digital capability; HART instruments are compatible with 4–20mA, can be connected to traditional DCS analog inputs, and also support digital configuration and diagnostics via a HART master. When both analog requirements and intelligent configuration needs exist on-site, HART is the ideal choice.
Loop resistance and the frequency-shift overlay
The analog current still represents the primary variable. Digital communication is a small frequency-shift keying signal laid on that current. The master reads those frequencies as an alternating voltage across the load resistor. If the resistor is too small, the alternating voltage is not enough and a handheld gets no reply. If the resistor is too large, the loop cannot supply the transmitter with enough voltage and the analog current starts to fall. The resistor therefore has to satisfy both digital reception and the instrument's minimum supply. In point-to-point mode the address is usually zero and the current follows the process. Multidrop puts several instruments in parallel, the current no longer represents the process, and the address must be non-zero, or several instruments answer at once. Polling is master and slave. There is no carrier-sense contention on the bus. The master timeout has to cover the slowest instrument.
Cable capacitance is another condition. The longer the cable, the rounder the waveform. On a long run or in multidrop, calculate capacitance. Do not look only at conductor size. A safety barrier, an isolator, and some input cards filter the frequency-shift signal away. The analog value stays normal and the digital communication is simply gone. That loop cannot be commissioned with a handheld on the control-room side of the barrier. It can only be reached on the field side. A write changes range, damping, and units. Rights and the password should follow the instrument manual. A read usually does not change the process value. Declare the units of the primary variable and the secondary variables on the master. Do not store a bare number.
Multidrop addresses and the analog value together
The most common mistake is to keep measuring 4–20 mA in multidrop as if it were point-to-point. The current sits near a small fixed value, and the control system still interlocks on it as a process quantity. The opposite mistake is a point-to-point instrument written to a non-zero address. The current no longer follows the measurement, and the temperature on the graphic looks frozen. Another mistake is to change the digital range and not the input range in the control system. The two engineering values differ by a step. The communication reading is right and the analog value used for control is wrong.
Two masters polling at once interrupt each other. After a handheld is plugged in, digital acquisition in the control system starts to time out. Stagger them, or use an arrangement that allows more than one master. If damping is set very long, the digital value moves very slowly and an operator thinks the instrument has failed. When the sensor fails, some instruments hold the current at the last value or at a fault current, while the status byte on the digital side is already bad. Collecting only the current misses the fault. After an instrument is replaced, check the address, the write-protect jumper, and the range again. Do not assume they match the old instrument. When parsing a frame, the preamble length changes with the device and the cable. Always stripping the same number of bytes shifts the status byte.
Where to measure, before the barrier and after it
An isolating barrier and some input cards filter the digital communication away, and the analog current is still there. A handheld has to be connected on the side where the alternating component can be seen. Measuring no waveform on the control-room terminals does not mean the field instrument has no digital capability. In multidrop the current no longer follows the process, and an interlock must not use that current anymore. If point-to-point is set to a non-zero address by mistake, the current freezes and the graphic looks like a failed instrument. It follows again only after the address is set back to zero.
Change the range on both sides together. Changing only the instrument, or only the input card, leaves the engineering values a step apart, and the digital reading and the analog reading do not match. When damping is very long the digital value moves slowly. Tell that apart from a sensor fault: a fault is in the status byte, and damping is in the configuration. Two masters polling interrupt each other. Before inserting a handheld, confirm whether the control system is already reading the digital variables. Preamble length changes with the cable. Parsing cannot always drop the same number of bytes. After an instrument is replaced, check the address, the write protection, and the range. None of the three is inherited automatically.
Use a resistor accurate enough for the loop, and place it where the master can pick off the alternating voltage. If an input card's internal resistance takes the place of that resistor, confirm that the internal resistance both supplies the instrument and keeps the digital signal large enough. When cable capacitance is over the limit, the waveform rounds off. A handheld nearby can still communicate and a master farther away cannot. That is not an address error. If the write-protect jumper or the password is not open, reading a variable succeeds and writing the range fails. Keep the two commands separate in the record. A secondary variable's unit may differ from the primary variable. Parsing the secondary variable in the primary unit produces the wrong temperature or density. When the instrument's fault current overlaps the normal current range, the fault can be judged only from the digital status. It cannot be judged only from whether the current sits between 4 and 20.