HART

HART (Highway addressable remote transducer) PROTOCOL

• It supports 4 - 20 mA loop current.
• It uses modulated communication.
• It supports two communication channels i.e., (analog + digital).
• It enables two way digital communications.
• HART modulates the 4-20mA current with a low-level frequency shift keying (FSK) sine-wave signal, without affecting the average analog signal.
• HART uses low frequencies (1200Hz and 2200Hz) to deal with poor cabling, its rate is 1200Bd. But it is sufficient.

Mixing of analog + digital data

Following picture tells you how data travels on HART protocol for two communication channels

Analog + Digital Data

Frequency shift keying

Following picture tells you the Frequency shift keying method to encode digital data or information on top of 4 - 20mA analog signal.

1. HART communicates 1200 bits/sec.
2. It allows 2 or more updates/sec from field devices.
3. Logical "1" value shown by frequency 1200 Hz.
4. Logical "0" value shown by frequency 2200 Hz.

Working of HART

Following picture shows the working of HART protocol, as you can see

1. Analog signal gets added with digital signal.

2. "C" means command from master device and "R" means response to that command.

3. Response shows the HART signal from device which consist of both frequencies either 1200 Hz or 2200 Hz as shown in picture.

Types of operation in HART

This HART protocol works two ways of
1. Master to slave communication - This is the normal condition when we directly connect our master to devices. (PLC- master, Device- slave)
2. Master master to slave communication - This is extra advantage in HART protocol means you can connect your handheld device in series with above communication so this becomes two master and one slave communication. (PLC- master, Handheld device - master, Device - slave)

Modes of Operation in HART

It have two modes

1. Normal mode - HART is a master-slave communication protocol, which means that during normal operation, each slave (field device) communication is initiated by a master communication device.

2. Burst mode - Burst mode enables faster communication (3–4 data updates per second). In burst mode, the master instructs the slave device to continuously broadcast a standard HART reply message (e.g., the value of the process variable). The master receives the message at the higher rate until it instructs the slave to stop bursting.

HART frame format (Character - oriented):

Following picture tells you how data framed in format

Comparison of field-buses 

	Foundation Fieldbus (H1)	ControlNet	DeviceNet	Ethernet
Speed	38.4 KBPS	5 MBPS	125 or 250 KBPS	10 or 100 MBPS
Length	1900m	250-1000m	500m	100m
Number of devices	32 devices	40 or less devices	64 devices	1024 nodes