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Signature Analysis
The easiest and most efficient method of troubleshooting microprocessor based instruments is signature analysis (S.A.). The
S.A. technique is similar to signal tracing with an oscilloscope in linear circuits. Part of the microcomputer memory is
dedicated to signature analysis and a known bit stream is generated to stimulate as many nodes as possible within the circuit.
However, because it is virtually impossible to analyze a bit stream with an oscilloscope, a signature analyzer is used to
compress the bit stream into a four character signature. By comparing signatures of the IC under test to the correct
signatures for each node, faults can usually be isolated to one or two components.
Signature analysis tests are provided for most of the digital circuits in the primary and secondary interface circuits of the
Electronic Load. There are four primary interface S.A. tests given in Tables 3-4 through 3-7, and five secondary interface
tests given in Tables 3-8 through 3-12. Refer to "Firmware Revisions" for information about the valid firmware revisions for
the signature analysis tables.
References are made to the appropriate S.A. test table from the troubleshooting flow charts or procedures. The following
general rules apply to signature analysis testing of the primary and secondary interface circuits.
1.
Be sure to use the correct test setup connections for the specific test. See "Test Setup for Signature Analysis".
2. Note the signatures for Vcc ( + 5V) and common on the IC being examined. If an incorrect signature is the same as that
of Vcc or common, that pin (or point in the circuit) is probably shorted to Vcc or ground.
3. If two pins have identical signatures, they are probably shorted together.
4. If two signatures are similar, it is only a coincidence.
5. If a signature is incorrect at an input pin, but is correct at its source (output of previous IC), check for printed circuit
track or soldering problems.
6. An incorrect signature at an output could be caused by a faulty component producing the output. It can also be caused
by an input short circuit in another component on the board.
Firmware Revisions
The primary interface ROM chip (U205) and the secondary microprocessor chip (U301) are identified with labels that
specify the revision of the Electronic Load's firmware.
The signatures given in Primary S.A. Tables 3-4 through 3-7 are valid for ROM chip U205 firmware revision "Rev
A.02.01''. You can also identify the revision of the U205 firmware using the *IDN? query in the program listed below.
10 OUTPUT 705;"*IDN?"
20 ENTER 705;
30 DISP L$
40 END
The computer will display the Electronic Load Agilent part number and the firmware revision of the U205 primary ROM
chip.
The signatures given in Secondary S.A. Tables 3-8 through 3-12 are valid for secondary interface microprocessor chip
U301 revision "Rev A.02.01" . Note that the U301 revision is only identified by the label; it cannot be read back using the
*IDN? query.
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