HP 4262A - Page 196

Model
4262A
Section
VIII
Figure
8-12
Digital Section Troubleshooting Using Signature Analyzer.
The advantage of troubleshooting based on ‘‘Signature Analysis’’ is accuracy and ease in
finding failures. It is generally difficult to search for an error by means of observing
waveforms on an oscilloscope for the reason that bit trains in a digital circuit seem to be
much the same whichever is observed. Specifically, to find the errors in stream of a large bit
size (or word length) data takes much time and requires the use of an instrument such as a
logic state analyzer. Hewlett-Packard has proposed a method called ““Signature Analysis’’
which recognizes the bit pattern measured in a 4 digit hexa-decimal code (signature) for
running an easy diagnostic test program. With the Signature Analyzer (HP 5004A), the
signatures are displayed in a readable 4 digit-figure set of alphanumeric figures (0123456
789 AC F HP U). The signature analysis is based the usual signal tracing method followed
in troubleshooting an analog circuit. According to signature analysis, devices in a digital
circuit are checked with the signal analyzer by comparing signal input and output signatures
to and from each device for the “correct’ signature denoted in the service manual signature
map. If a signature is not identical, the troubleshooter need only trace the bit train in
opposite direction to the signal flow and, when a device is noted which generates an erratic
signature despite a correct input, the component may be regarded as faulty. One additional
important consideration, since the actual program ROM board (P/N: 04262-66523) in the
4262A does not include a self-test program for signature analysis (as part of the program
ROM), a troubleshooting board is required when diagnosing with the Signature Analyzer.
When the troubleshooting board is installed in the instrument, a test program is written out
from a special ROM which activates overall the digital control circuit, and, if included, any
optional circuits. For convenience in troubleshooting the 4262A, this signature test board is
supplied as Service Kit (04262-87002).
HOW TO USE THE SIGNATURE ANALYZER TEST BOARD.
Note
Use either procedure 1 or 2 depending upon
instrument serial number.
1. Serial numbers 1710J00340 and below.
a.
Remove
All,
A12,
A13
and
Al4
boards
from
instrument.
b.
Take
out
A23
Board.
¢.
Disconnect
A23U16
(ROM)
from
socket
J2
and
put
aside.
d.
Disconnect
signature
program
ROM
from
socket
J3
(labeled
TEST
ROM)
on
test
board
and
install
the
ROM
in
place
of
A23U16.
e.
Reinstall
A23
Board
in
its
normal
position.
Note
When
testing
ROM’s
with
A23
board
as-
sembly,
install
the
ROM
in
socket
J1
(labeled
2708A)
on
the
test
board.
Install
the
test
board
in
place
of
A13
board
assembly.
Ob-
serve
signatures
at
test
points
DO
through
D7
on
the
board
and
follow
troubleshooting
pro-
cedures.
Test
board
flat
cable
need
not
be
connected
anywhere.
Figure
8-12.
Signature
Analysis
Guide
(sheet
1
of
3).
8-23