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GFK-0825F Appendix C The Genius Serial Bus C-5
C
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.
“0”
“0”
“0”
“1”
“1”
+Vp
-Vp
+Vr
-Vr
SERIAL 1 VOLTAGE RELATIVE TO SERIAL 2
t =
baud rate
1
Use caution when connecting instrumentation to the bus. A differential probe or a summation of
two probes relative to 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
above. 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 the baud frequency. These cause no problem if
they do not cause violation of the amplitude criteria of the previous paragraphs.
The Serial 1 and Serial 2 lines should always have a termination resistor equal to the characteristic
impedance of the cable connected at each extreme end. When testing a Genius block or other
device using a Hand Held Monitor, when no serial bus is present, a terminating resistor will
improve integrity. 75 Ohms is recommended.