Troubleshooting — 9
Notes
Rev 3-97
* School
Bus cable or connectors, or it may mean an open opto-isolator at the input
device.
Crown Bus Wave Form Testing
When a loop problem exists and you have determined that the DC character-
istics of the Crown Bus are all good and that there are no breaks, it is time to
get serious and look at the data signal wave form. This check will tell you if there
is a capacitance or RFI problem on a loop.
General symptoms of a capacitance or RFI
problem on a loop would be partial or
inconsistant loop operation. In some cases it
may not be possible to establish communi-
cation at all, and in other cases ghost
components may appear at
a Roll Call that are not in the
system at all.
To perform this test you will
need to modify a loop cable
by adding a 100 ohm resis-
tor in series with the nega-
tive wire of the cable. You
will also need an oscillo-
scope to view the signal.
Signals may be generated
on the Crown Bus by run-
ning Roll Calls, loading
dataframes, or by discon-
necting a Crown Bus cable
at the input to a component
at the beginning of the loop.
Figure 5 shows a test rig
setup for wave form mea-
surement. Note that if an
ungrounded scope is used
it makes no difference
whether the resistor is in the
positive or negative leg, but
if a grounded scope is used
you must put the resistor in
the negative leg and the
negative lead of the scope
on the side of the resistor
electrically closest to the
output device.
Figure 6 shows what a good
data signal looks like. Note
that greater than 12 ma is
required for a current high
(data logic low) and current
level must drop below 4 ma
for a current low (data logic
high).
OUT
IN
+ – + –
SMX 1
OUT
IN
+ – + –
SMX 2
OUT
IN
+ – + –
SMX 3
OUT
IN
+ – + –
SMX 4
15 VDC
+
–
Loop is broken between SMX 2 and SMX 3.
Break Detect reports "Break Before SMX 2"
because SMX 2 senses loss of input continuity
and sends break report with component ID.
High voltage at the output driving open,
no voltage at input.
0 VDC
+
–
OUT
IN
+ – + –
SMX 1
OUT
IN
+ – + –
SMX 2
OUT
IN
+ – + –
SMX 3
OUT
IN
+ – + –
SMX 4
0 VDC
+
–
Loop is shorted between SMX 2 and SMX 3.
Break Detect reports "Break Before SMX 2"
because SMX 2 senses loss of input continuity
and sends break report with component ID.
No voltage is read at either connector due
to short. SMX 3 is protected by current limit.
0 VDC
+
–
Figure 3. Loop Voltage Check With Loop Open
Figure 4. Loop Voltage Check With Loop Shorted
