Protocol decoder:signature

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Name Signature analysis
Description Annotate signature of logic patterns.
Status supported
License GPLv2+
Source code decoders/signature
Input logic
Probes start, stop, clk, data
Optional probes
Options start_edge, stop_edge, clk_edge, annbits

The signature protocol decoder can decode the HP "signature" protocol used for troubleshooting logic circuits.

Signature Analysis (SA) gives you a four-character fingerprint (signature) from any logic pattern, which can be used to determine whether a node/IC is functioning properly or not. Some of Hewlett Packard gear provide valid signature in the service manual.


$ sigrok-cli -P signature:start=D0:stop=D1:clk=D2:data=D3 -i
signature-1: A3A5
signature-1: 6F9A
signature-1: 6F9A
signature-1: 6F9A

The first signature (A3A5 in this case) is often wrong. This is called an unstable state. HP 5004A and others have an unstable indicator, which we don't have. But clearly 6F9A is stable. You can determine stable/unstable by repetitivity.

signature-1: 13AA
signature-1: H344
signature-1: F405
signature-1: 8UP8
signature-1: 052H
signature-1: 5CH1
signature-1: U2FA
signature-1: 713P
signature-1: 0P5P

This is an entirely unstable example. Sometimes this is caused by a low samplerate. HP 5004A has 25MHz (equivalent) bandwidth. Samplerate should be at least 25MHz. In sigrok-cli --samplerate=24MHz is maybe OK, but sometimes not.

If you have a high enough samplerate but still get unstable value, maybe the circuit in that node is not working properly (or you forgot to switch to SA-Mode).

Another example of an unstable state (exact same probe point, but different samplerate):

Pv signature.png

CLOCK is below 1MHz in this example, however it doesn't mean 8MHz samplerate is enough. DATA is asynchronous with respect to CLOCK.


  • Signature is 0000 if all data are LO(0).
  • Signature is 0001 if all data are HI(1), and CLOCK is 65535 cycles during START to STOP.
  • Signature is 0003 if all data are HI(1), and CLOCK is 65536 cycles during START to STOP.
  • Usually the service manual tells you what signature of GND and V+(5V etc). From this, you can confirm START/STOP/CLOCK is working properly.
  • Do set up properly. Follow provided procedure (see manual). Edge mode of START/STOP/CLOCK may vary, even on the same board.
  • Because the logic threshold depends on the acquisition hardware, this decoder does not perfectly mimic the HP 5004A. However if it shows the same signature as a service manual provided, that node is most likely OK.

Known signature analyzers