HART communication communicates in a half-duplex communication mode through carrier transmission. HART communication is based on the Bell202 communication standard and uses the FSK frequency shift keying signal to superimpose a digital signal with an amplitude of 0.5mA on the low-frequency 4~20mA analog signal. For two-way digital communication, since the average value of the FSK signal is 0, it will not affect the transmission of the current signal, allowing the two signals to share hardware without conflict.

HART protocol instruments have increasingly become the first choice for customers, and HART equipment's certification by the Foundation has become one of the identification standards for HART product performance. Therefore, HART equipment certification is increasingly valued by major instrument manufacturers. The following is a brief description of the various contents of the physical layer test of HART equipment for your reference.

HART certification physical layer testing mainly includes the following items: HART waveform test, carrier start/stop and delay time test, HART signal instantaneous start and stop test, output noise test at rest, simulated change rate test, input impedance test, noise Tolerance testing and carrier detect testing.

Let’s look at the first three items today: HART waveform test, carrier start/stop and delay time test, and HART signal instantaneous start and stop test.

HART waveform test:

HART waveform test mainly includes HART amplitude test, frequency test, rise time and fall time test. The picture below shows the waveform test chart of the HART signal.

● HART signal amplitude:

Low resistance device: 400-800mVpp

High resistance device: 400-600mVpp

● Measurement frequency:

The qualified measurement frequency of 2200Hz is between 2178-2222Hz

The qualified measurement frequency of 1200Hz is between 1188-1212Hz

● Rise time and fall time:

Rise time is 75-100μs, fall time is 75-100μs at 2200Hz frequency

Rise time is 75-200μs, fall time is 75-200μs at 1200Hz frequency

Carrier start/stop and delay time testing:

This test is mainly to detect the minimum time required by the HART signal in the carrier start time, carrier stop time, and carrier delay time. This test needs to elicit the control signal (RTS signal) of the HART chip.

● Carrier start time

Requires less than 4.2ms

● Carrier stop time

Requires less than 2.5ms

● Carrier delay time

Requires less than 5ms

Instantaneous start and stop testing of HART signals:

When a HART device starts or ends its transmission, it does not create instantaneous spikes in the network. If the transient is too large, it will cause interference to the HART signal received by other devices, or affect the analog signal of the main device. This test requires the use of the Foundation's testing tools.

● Transient waveform when HART signal is started

Required to be less than 100mV

● Transient waveform when HART signal stops

Requires less than 100mV

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