FANUC Robotics America GFK-1535A - Manual (Page 89)

A-4 VersaMax™ System Genius® Network Interface Unit User's Manual – November 2000 GFK-1535A
A
Maximum Bus Length
Three effects limit the maximum length bus available at any baud rate:
1. Voltage attenuation
2. Waveform distortion (frequency dispersion)
3. Propagation delays
Attenuation
The transmitter output levels and receiver thresholds determine the maximum
attenuation that can be tolerated. This is the principal determinant when using
recommended cables.
Distortion
Waveform distortion is due to the limited bandwidth of wire media, which causes
the various frequency components of a pulse waveform to travel at different speeds
and arrive separately in time (called dispersion). As a result, the received pulse
appears rounded and distorted. The signal at the extreme end from the transmitter
may look rounded and skewed as shown below. Distortion is most apparent near the
beginning and end of a pulse train where it may appear as a change in phase or a
frequency shift. Critical timing for a logic 0 transmission is shown below in a more
detailed version of the waveform:
Tw Tw
+Vr
-Vr
Tp/2 Tp/2
Note the first and last half-cycle look wider. The most critical to operation is the
first full cycle of the first start bit of the transmission. Detection of this pulse
establishes the time synchronization of the receiver to the incoming waveform.
Missing this first pulse does not cause the data to be missed, but may compromise
the noise immunity with respect to extra or missing pulses. The frequency of the AC
pulse is 3X the baud rate as noted earlier. This means the normal period Tp(normal)
is:
T 2.17 microseconds at 153.6 Kb
T 4.34 microseconds at 76.8 Kb
T 8.68 microseconds at 38.4 Kb.