Serial Bus Waveforms - Emerson PACSystems VersaMax User Manual

PACSystems VersaMax Genius Network Interface User Manual
GFK-1535D
A-2

Serial Bus Waveforms

The actual waveforms seen on the cable depend on the cable impedance
and the distance from the station presently transmitting. A data "0" is a
series of three AC pulses, while a "1" is no pulse.
Figure 72: Example Serial Bus Waveform
Use caution when connecting instrumentation to the bus. A differential
probe or a summation of two probes relative to the ground is required.
Inadvertent grounding of one side of the bus can cause loss of data or
data errors.
The pulse frequency is three times the baud frequency, for example,
460.8 KHz at 153.6 Kb.
The peak transmitted voltage Vp and the receiver thresholds Vr are per
the electrical specification in this section. The peak voltages measured
will decline with distance along the cable from the transmitting station,
so different stations will have varying amplitudes. The wave shape will
also become more rounded with distance.
The minimum amplitude pulses seen during a "0" should exceed the
receiver threshold Vr of 900 millivolts by 50% (about 1.4 volts) for best
reliability. An occasional pulse at or below the threshold may still not
cause the bit to be missed, due to a voting algorithm in the logic,
however.
Likewise, no pulses greater than Vr should exist during logic "1" intervals.
Occasional extra pulses during this interval are also rejected by the logic.
Line reflections will show up as notch distortion during the pulse or low-
level pulses during "1" intervals, and their appearance is synchronized to
Operation of the Genius Bus
Appendix A
Jan 2023
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