KROHNE CORIMASS MFC 85 Interface EN - Manual (Page 6)

6
2. The HART
®
Protocol
2.1 Method of Operation
The HART
®
protocol operates using the frequency shift keying (FSK) principle, which is based
on the Bell 202 [1] communication standard. The digital signal is made up from two
frequencies - 1200 Hz and 2200 Hz, representing bits 1 and 0 respectively. Sine waves of
these frequencies are superimposed on the DC analog signal cables to give simultaneous
analog and digital communications. Because the average value of the FSK signal is always
zero, the 4 - 20 mA signal is not affected.
This produces genuine, simultaneous communication with a response time of approximately
500 ms for each field device, without interrupting any analog signal transmission that might be
taking place.
Up to two master devices may be connected to each HART
®
loop. The primary one is
generally a management system or a PC while the secondary one can be a hand-held terminal
or laptop computer. A standard hand-held terminal - called the HART
®
Communicator - is
available to make field operations as uniform as possible. Further networking options are
provided by gateways.
approx. +0.5 mA
approx. +0.5 mA
0
1200 Hz
“1”
2200 Hz
“0”
analog signal
Simultaneous analog and digital signals
Because the mean harmonic signal value is zero, digital communication makes no difference
to any existing analog signal as demonstrated in the figure above.
2.2 Point to point operation
The figure below shows some examples of point-to-point mode. The conventional 4 - 20 mA
signal continues to be used for analog transmission while measurement, adjustment and
equipment data is transferred digitally.
The analog signal remains unaffected and can be used for control in the normal way. HART
®
data gives access to maintenance, diagnostic and other operational data.
4 20
mA
Up to 2 updates/sec
Multiple Masters
Analog
Digital
MFC 081/085
HART
interface