Tag Archives: 16500B

HP 16534A triggering problems

DSC_0404

One of my HP 16500B logic analysis systems has a 16534A digital oscilloscope module installed. The module has a great specification: two channels, 2Gsamples/sec, 500MHz analogue bandwidth, and 32k record length. I would find it a really useful debugging tool, if it worked properly. Sadly it doesn’t.

Though it displays a trace successfully, and the controls work, it won’t trigger. The only way I get a trace is by auto-trigger. In this case the trace isn’t synchronised, so it’s impossible to catch a particular event at a particular time. Here it is displaying a square wave (well, it’s nearly square – I hadn’t adjusted the probe compensation so it’s a bit curvy on the top and bottom). Adjusting the trigger level just doesn’t make any difference.

DSC_0409

The service manual for the 16534A isn’t very helpful. It describes roughly what the self-tests do, and how to run the self-calibration procedure, but has no detailed circuit information. The self-test reports a ‘D/A’ failure:

DSC_0405and the self-calibration procedure fails on the Trigger and Hysteresis sections:

DSC_0406

The service guide describes the D/A test like this:

Test DAC This test verifies the correct operation of the D/A convertor on the board.
Both the offset and trigger level D/A convertors for each channel are set to a reference
level and then changed. The logic trigger IC is programmed to detect the changes. The
detection of a correct trigger indicates that the D/A convertor is operating normally.

It seems clear that the offset D/A converter is working, since the offset control works and moves the trace up and down as it should. However, the trigger level control doesn’t seem to be doing what it should. I had a look at the board, looking for components likely to be the DAC or trigger IC. Unfortunately, the semiconductors on there seem to be mostly either standard ECL logic, trivial things like op-amps, or magic custom HP chips with dozens of pins. One exception was this:

DSC_0410

It’s an AD96687 from Analog Devices, which is a dual high-speed comparator with ECL outputs. If I was designing a high-speed trigger circuit using 1994-vintage components, I might use a device like this. I’ve had a look at all its pins with the power switched on and a signal applied and tests running, and none of them seem to change at all. One noninverting input is stuck at +5V, and the other inputs are near ground. One set of outputs isn’t even showing valid ECL logic levels, but it may be unused and thus have no load resistors on it.

Maybe it’s a red herring and nothing to do with the triggering at all, but I’m suspicious that I never see anything change. Without more circuit information, it’s hard to get much further, sadly.

 

 

Advertisements

HP 16500A and 16500B logic analysers

Recently I’ve been putting a couple of HP logic analysis systems into action. Accumulated from various places over the last couple of years I have both a 16500A and a 16500B. They’re remarkable units which take a whole variety of plugins for doing high-speed digital and analogue measurements. They must have been eye-wateringly expensive when new and, as a result, have such high specifications that they’re still useful even though they’re around 20 years old. For example, the 16500B boasts a colour touch-screen, a built-in hard drive and Ethernet. It was launched in 1992 when many desktop PCs didn’t have those features, so it must have seemed awesomely high-tech at the time.

Here’s a picture of the 16500B. The 16500A is similar but looks…older.

DSC_0247s

There are a few quirks to using these machines in 2013, and I’m intending to write about some of my experiences here.