Masoneilan SVI II AP Installation and Maintenance ... - GE Energy

GE Oil and Gas
SVI II AP Instruction Manual
HART Physical Layer Compliance of the Control System
Communications to a SVI II AP requires a HART-compliant communications loop. The
HART protocol specifies the noise level, impedance requirements, and configuration of
the loop. The controller or output card of the control system must comply with the
Physical Layer Specification.
Impedance Constraints
Noise Constraints
HART communication is based on the talking device generating an AC current
superimposed on the 4 - 20 mA control signal. Two frequencies are generated; 1200 Hz
representing the digital value 1 and 2200 Hz representing the digital value 0. The listening
device responds to the voltage generated when the AC current flows through the loop
impedance. In order to generate a voltage from a current there must be impedance.
HART Protocol requires that this impedance be at least 220 Ohms at the tone signaling
frequencies.
HART compliant current sources are supplied with the correct Impedance Versus
Frequency Characteristic. In Non-Compliant Current Sources there may be a noise
reduction capacitor across the output that lowers the impedance at higher frequencies
and thus lowers the signaling voltage. To be certain that at least 220 Ohms of impedance
is presented by the current source a resistor can be added in series with the current
source. This reduces the effective compliance voltage of the current source by 20 mA
times the value of the series resistor. An added resistor is unnecessary during tests with
high impedance current calibrators such as the Altek Model 334 Loop Calibrator.
HART Communication depends on converting two frequencies (1200 and 2200 Hz) into
digital values 1 and 0. Noise can cause errors in the conversion. Conventional good
wiring practice, such as use of twisted shielded pair cable with the shield and signal loop
grounded at only one point, minimizes the effects of noise.
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