Signature analysis

Signature Analysis is a method to simplify (field) troubleshooting of complex digital systems, developed at Hewlett-Packard in the early 1970s.

The idea is to describe "known good" states of signals in the system – typically when in some diagnostic / troubleshooting mode – as a four-digit "fingerprint" of that signal, when referenced to a known clock and defined start/stop conditions. The concept was introduced in Hewlett Packard Journal May 1977 along with their first SA instrument, the HP 5004A.

The signature algorithm is a 16-bit cyclic redundancy check that can be efficiently implemented in hardware using a linear-feedback shift register. HP selected the polynomial x¹⁶+x¹²+x⁹+x⁷+1 instead of a standard CRC-16 or CCITT-16 polynomial, in order to scatter the missed errors as much as possible and to avoid selecting feedback taps that are evenly spaced or four or eight bits apart because the types of instruments [...] that we will most frequently be testing tend to repeat patterns at four and eight-bit intervals.

They also chose a particular "funny hex" format to display the resulting 16-bit number (four hexadecimal digits) to be read easily on seven-segment displays, as 0–9,A,C,F,H,P,U instead of 0–9,A,B,C,D,E,F. (Presumably, including "H" and "P" in the display was an extra benefit.)

Signature analysis was only popular for a fairly short time in the late 1970s and early 1980s before increasing integration made it ineffective. Some arcade game manufacturers used signature analysis to allow component-level repair by field technicians.

It is also possible to use signature analysis in a limited way in equipment that has not been designed for it, by inserting a "free run" or "NOP" adapter between the CPU and its socket. This adapter is wired to present a "no operation" opcode to the CPU, which then loops through all its addresses, generating specific and constant signatures on address lines, allowing these lines in the circuit to be tested or identified (see reference below).

to be expanded

History

Dennis Tillman says:

Signature Analysis was a very interesting troubleshooting concept that HP developed. It had several important benefits:

• It was easy and inexpensive to incorporate this concept into products under development.
• It made it possible for anyone, without prior knowledge or experience with digital electronics, to troubleshoot the new digital circuitry that was rapidly appearing in electronic instruments.
• HP freely shared the concept so others could incorporate Signature Analysis in their products.

Because Signature Analysis was developed by HP, Tektronix would never use it. This was the word from the very top at Tek. [...]

In 1978 Tek was developing a radically new analog/digital scope that had a microprocessor, A/D converter, digital memory, programming capability, and a GPIB interface (also originally developed by HP as the HPIB). All of this complex digital circuitry was going to make these scopes virtually impossible to be repaired in the Field Office Service Centers.

Tom Rousseau, the 7854 Project Manager, realized this was going to be a huge problem for the Service Centers. When Tom read the May 1977 Hewlett Packard Journal which was entirely devoted to Signature Analysis, Tom realized this concept would make it much easier for the Service Centers to troubleshoot the 7854.

But there was a problem: Howard Vollum was adamantly opposed to using anything from HP. Tom eventually got Howard to budge a little when Tom made it clear how difficult this scope would be to troubleshoot in the service centers without a massive training program for all the field technicians.

Howard reluctantly agreed to allow Tom to incorporate Signature Analysis in the 7854 provided Tek did not have to buy the HP 5004A Signature Analyzer. Unfortunately there were no other signature analyzers.

As a result Tek developed their own 067-1090-00 Signature Analyzer single-wide plugin for the TM500 mainframe. It was later renamed to the SA501.

Signature Analyzers

• HP 5004A (10 MHz)
• HP 5005A Signature Multimeter (20 MHz) / 5005B (25 MHz)
• HP 5006A (25 MHz)
• Tektronix 308 (20 MHz)
• Tektronix SA501 (40 MHz)
• Atari CAT BOX arcade tester / Schematics
• Kurz-Kasch Signature II (15 MHz) (1979)
• robotron 31020 (10 MHz) (East Germany 1987)
• Signature analyzer project @ EEVBlog Forum
• Signature Analysis support in sigrok

Tektronix products designed for signature analysis

• 468
• 1240, 1241
• 5D10
• 7854
• 7A42
• CG5001 / CG551AP
• SA501
• ...

Links

• HP Application Note 222, A Designer's Guide to Signature Analysis
  • AN-222-1, Implementing Signature Analysis for Production Testing with the HP 3060A Board Test System
  • AN-222-2, Application Articles on Signature Analysis, 1979
  • AN-222-3, A Manager's Guide to Signature Analysis, 1980
  • AN-222-4, Guidelines for Signature Analysis – Understanding the Signature Measurement
  • AN-222-5, Increasing Productivity in Manufacturing and Service with a Logic Troubleshooting System, 1982
  • AN-222-6, Troubleshooting with Composite Signatures, 1983

• Troubleshooting Microprocessors with "Free Run" Fixtures / A 6502 NOP adapter

Patents that may apply to Signature analysis

Page	Title	Inventors	Filing date	Grant date	Links
Patent US 3976864A	Apparatus and method for testing digital circuits	Gary B. Gordon • George A. Haag • Jan R. Hofland • Daniel I. Kolody	1974-09-03	1976-08-24	Signature analysis • 308 • SA501
Patent US 4192451A	Digital diagnostic system employing signature analysis	Stephen Swerling • Steven R. Smith • Thomas W. Wiesmann	1978-05-30	1980-03-11	Signature analysis
Patent US 4527272A	Signature analysis using random probing and signature memory	Michael G. Reiney	1982-12-06	1985-07-02	Signature analysis
Patent US 4752928A	Transaction analyzer	David D. Chapman • Donald C. Kirkpatrick	1985-05-06	1988-06-21	Signature analysis