Propagation delay – FANUC Robotics America GFK-1535A User Manual

Page 90

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GFK-1535A

Appendix A Operation of the Genius Bus

A-5

A

The half cycle pulse width, when measured between the positive and negative
receiver thresholds, denoted as Tp/2 in the figure, will vary along the waveform due
to dispersion, and resembles a frequency shift.. The digital input filter essentially is
a band pass filter which looks at the half cycle timing Tp/2, and the duration above
the thresholds, Tw. The limits are:

T

Tp/2 = 0.6 Tp(normal) maximum

T

Tw = 0.188 Tp(normal) minimum

These measurements can be taken when evaluating the maximum length of an
unspecified cable. Dispersion is much less of a problem with fiber optic links since
the media is much wider bandwidth, and therefore has less distortion.

Propagation Delay

The propagation delay is caused by travel time of the signal down the cable. Typical
signal velocity in data grade cables is around 65- 78% of the speed of light. This
requires about 3 microseconds to travel a 2000 foot long bus. This is about half a
bit time at 153.6 Kb. This skew could affect the bus access sequence since only one
bit of quiet bus (skip) time is usually allocated between transmission of adjacent
addresses. The signal must reach all devices on the bus within the period of one bit.
Propagation delay causes the ultimate limitation in bus length, even with ideal
media. Propagation speed through fiber optic is not significantly different than wire,
and delays through the interfaces must be accounted for.

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