Installation, Operation and Maintenance Manual
MDE 254 Rev. 1 March 2021
HART Protocol Previews 7
Section 5: HART Protocol Previews
Section 5: HART Protocol Previews
The HART bus combines the familiarity of using the 4 - 20 mA signals with the benefits of the bus
technology. In fact, by means of the simultaneous analog and digital signals, additional information
can be carried out on the same pair of wires together with the analog 4 - 20 mA signal. The digital
communication signal has a response time of approximately 2 - 3 data updates per second without
interrupting the analog signal.
HART is typically a request-response communication protocol, which means that during normal
operation (2 - 3 data updates per second), each field device communication is initiated by a host
communication device. Two hosts can connect to each HART loop. The primary host is generally a
distributed control system (DCS), programmable logic controller (PLC), or a personal computer (PC).
Our actuator interface is a field device.
The ICON HRT2000v4 Module support the optional burst communication mode. Burst mode enables
faster communication (3 - 4 data updates per second). In burst mode, the host instructs the field device
to continuously broadcast a standard HART reply message (e.g. the value of the process variable).
The host receives the message at the higher rate until it instructs the device to stop bursting.
The HART Communication Protocol is based on the Bell 202 telephone communication standard
and operates using the frequency shift keying (FSK) principle. The digital signal is made up of two
frequencies 1,200 Hz and 2,200 Hz representing bits 1 and 0, respectively. Sine waves of these two
frequencies are superimposed on the direct current analog signal cables to provide simultaneous
analog and digital communications. Because the average value of the FSK signal is always zero,
the 4 - 20 mA analog signal is not affected.
Figure 3 Digital over Analog
More information about HART are in the official website http://www.hartcomm.org.
Digital
Signal
Analog
Signal
Time
4 mA -
"1"
"1"
"1"
"1"
"1"
"0"
"0"
"0"
"0"
Note: Drawing not to scale
20 mA -
To Next Page
Loading...
Need help?
General
