Star Tech Journal

S*T J
MAY 1981
Signature Analysis is a technique based on data compression to provide a
unique fingerprint of each interconnection or test node in the unit under test (UUT).
The Signature Analyzer provides the technician with a test probe that can be used to
enter data to be recorded and read out for the test node.
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WORDS
Signature Analyzers convert serial bit streams into a 4 hexdigit "Signature".
Since a unique signature is generated for each data stream a prime requirement is
that the data stream passing a node be identical (unless faulty) for the same node on
each identical board. The second requirement is that this data stream repeat itself.
This repetition is assured by having a start and stop pulse which is time and polarity
related to the data stream.
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CLOCK
START/STOP
DATA
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START
STOP
FIGURE 1
If we start the Signature Analyzer by placing "Start" on the rising edge ( _r- )of
the first pulse on line 2 above and the "Stop" on the rising edge ( _r- ) of the 2nd
pulse labeled "Stop" we will allow clock pulses 2 thru 9 to enter the system. There is
one data bit entered into the compression circuit for each clock pulse.
In figure 1, the data stream entered would be: 01011011
This stream would be entered each time the window is generated.
To demonstrate how a single bit difference between two data streams affect the
"Signature" consider these two streams:
ll l ll 111100000111111 = D953
11111111100000011111 = 99F6
Either a bit difference or a displacement by time ( clock pulses) gives a totally
different signature - not one digit or segment difference.
Certain portions of a processor system not designed for Signature Analysis can
cause some problems but by and large following a few simple rules will overcome
these problems.
Next month we will take a processor system from the beginning and follow through to develop
signatures.
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