FANUC Robotics America GFK-1535A - Manual (Page 88)

GFK-1535A Appendix A Operation of the Genius Bus A-3
A
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 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 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.