Lighting Controls Blue Box LT Series Installation, Operation & Maintenance Manual page 64

64 THE BLUE BOX LT TROUBLESHOOTING WITH AN OSCILLOSCOPE
USING AN OSCILLOSCOPE TO TEST
A DIGITAL BUS
In troubleshooting a system or a bus, the "Final Activation
Checklist" is always the first and most important step . An Os-
cilloscope is a visual aid to troubleshoot the bus better .
The Oscilloscope will let one test the stability of the bus
while everything is powered up . This will show if the bus
is getting noise, how clean the bus signal is, if the bus is
properly terminated, if there is a bad device present on
the bus, and give an indication of crimp quality .
Below are some examples where the use of an Oscillo-
scope is definitely needed:
1 . The Hardware Activation Tests included the "System
Startup and Cabling Guide" passed, but when the sys-
tem is powered up, devices appear/disappear on the
bus scan .
2 . The Hardware Activation Tests passed, but some de-
vices don't seem to be working if the bus is connect-
ed, and the same devices work perfectly fine when
connected directly to a master panel .
3 . Everything else was checked (The Hardware Activa-
tion Tests" passed and all devices are good) but the
bus is still unstable .
HOW TO SETUP THE SCOPE FOR TESTING
Make sure the Oscilloscope is not earth grounded in any
way . A battery powered scope is best .
1 . Connect the probe leads to the system's A & B ter-
minals; A "Bus Checker Card" may be used to make
the testing easier . Attach the alligator clip (which is the
ground lead of the probe) to the bus checker card's
B terminal, then connect the main probe lead (which
is usually a retractable hook tip) to the bus checker
card's A terminal .
2 . Adjust the vertical & horizontal controls accordingly
and set the input coupling to DC . You should be able
to see your bus signal which is a "Square Wave Sig-
nal . "
3 . The advisable setting would be "1 . 0 volts/div" & "(20
µs to 50 µs) sec/div" this should be able to let one see
a decent size waveform on the screen . (10-1 Probe) .
4 . A completely "Stable Bus" should show a clean & bal-
anced square wave signal . The baseline is centered at
"0" and never changes .
5 . If the bus is not terminated properly, some "ringing"
may be seen . With less loading, the Vpp is higher . If
there are not bad devices, the baseline is still at Zero .
Ideal wave form; Square waves with no ring or dampen-
ing means the bus is terminated. +V = -V means no bad
devices.
Problematic: Dampened square wave means the bus is
missing one or both terminators. +V = -V means no bad
devices.
+V = -V
+V = -V
+V = - V