Hart® Physical Layer Compliance Of The Control System; Impedance Constraints; Noise Constraints - GE Masoneilan SVI II ESD Installation And Maintenance Manual

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HART Physical Layer Compliance of the Control System
Communicating with an SVI II ESD requires a HART
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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

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HART communication is based upon 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 has to be at least 220 Ohms at the tone signaling
frequencies.
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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 an impedance of at least 220 Ohms 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.

Noise Constraints

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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 shield grounded at only one point,
minimizes noise effects.
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-compliant communications loop. The
Masoneilan SVI II ESD Installation and Maintenance Manual
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