HP 5004 Signature Analyzer

© Brooke Clarke 2005 - 2007

Using analog test instruments like voltmeters or scopes to test digital circuits is less than satisfactory.  The digital signature test is a way to truble shoot digital circuits.  To do this a good working circuit needs to be characterized for the signature at a number of test points.  The inputs to the signature analyzer are a clock, start and stop pulses and the serial data stream on the pin under test.

This allows troubleshooting to the component level.  Most manufacturers today support troubleshooting to the board level rather than to the component level and so you can get an HP 5004 for little money.  It's only useful if you have equipment with signature analysis data like the Austron 2100T LORAN-C Timing Receiver.

The 4 display characters have 16 values but are not the common hexidecimal characters of 0123456789ABCDE, but are instead 0123456789ACFHPU.  The manual says the nonstandard characters are so that B and D can be different (but if the number 6 has a bar on top then it's different from the letter b, and a lower case letter d is different from all the other characters).  Note that a possible output is HP5A which looks like HPSA or Hewlett Packard Signature Analyzer.  There probably was some demo device that caused this signature to appear.

Shown at left connected to the Austron 2100T LORAN-C Timing Receiver MPU PCB.

There are two cables connected to the front panel.  One has the interface module which in turn has 4 leads ending with IC pin grabbers:
Black wire Ground
Yellow wire Clock
Green wire Start
Red wire Stop

Self Test

The IC grabbers can be unplugged from the colored wires and the wires can be plugged into the Start, Stop and Clock jacks on the front panel of the 5004.  Now with "Self Test" pressed in and the Start and Stop buttons pressed in (negative edge) (clock does not matter) the when the probe is inserted into the "Probe Test" jack the display will show UP73 then ACA2.  With the Start and Stop buttons out the display is 3951 then 2P61.

The other cable goes to what looks like the 545A Logic Probe.  It has a "reset" button and at the tip there's a translucent red plastic sleeve  and inside a red LED.  Touching the tip to ground causes the LED to turn off.  Leave the tip in air or on a tri-stated output and the LED is medium bright and steady.  Connecting the tip to Vcc causes the LED to be on bright and steady.  Connecting to any pulse train causes the LED to blink.  This is itself a great trouble shooting tool.

I think the 5004 is computing something like a Cyclic Redundancy Check sum from the start signal to the stop signal.  If the probe is grounded to connected to the start pin it reads "0000".  I connected to the Vcc pin it will read some value dependent on how many clock cycles are between start and stop.
It is a 16 bit CRC with taps at 1, 7, 9, 12 and 16.

The Austron Troubleshooting data includes where to connect the clock, start and stop probes as well as the polarity for the clock, start and stop signals.  To be sure you have those correct there are signatures for ground (0000) and Vcc (depends on the board).

I got the best results by pressing "HOLD" and using the "Reset" button on the probe.  The reset button needs  to be pressed each time a new test point is probed.  Sometimes the reset button needed to be pressed two times.

The 5006A is a newer version that has some more buttons and works up to 25 Mhz.