HP 8016A Word Generator

This is a continuation of an effort started a few years back. I had purchased one of these off eBay, but found that only every other bit would get stored. The troubleshooting steps in the manual indicated that this meant either a bad A3 buffer or A4 controller card. Although the schematics are included, there are no board-level troubleshooting instructions, or detailed theory of operation. Furthermore, HP says to not even return these boards for repair, simply discard them and they will send replacements. I because they’re made up of inexpensive 74LSxx series chips, even back in the late 70’s when the device was built, it would not be economically viable to have a technician spend their time diagnosing and repairing these boards. Yet that’s what I’m going to attempt.

When I first dug into this, I soldered dozens of little jumpers of magnet wire to various pins so I could probe the inner workings while the card was inserted. This got me nowhere, and so I shelved the project.

A year or two later, I saw another one come up on eBay, and one it for a surprisingly reasonable sum. It was listed “as-is, for parts”, but I figured I’d take the risk. Although in worse physical shape, this unit actually was fully functional. So now armed with a set of working cards, I was able to troubleshoot the bad unit.

Swapping cards, I was able to confirm that A3 was the problem. Fortunately, this card is accessible in place if you remove the HPIB (GPIB) card, which will not hinder operation. This finally let me probe the device in action.

The A3 board is essentially the edit buffer. Bits are read in, displayed on the front panel, edited, and then placed back into memory. The read & write is done serially, so there’s a shift register made up of 7474 flip-flops, and 7451 And & Nor gates that allow for the bits to be modified. The circuit is split up into even & odd sections, each with their own clock, serial in & out lines, and a few other control signals. (note the odd symbol for a NOR gate in the schematic below).

I first suspected the the flip-flops, so I unsoldered all of them, and installed sockets. I tested them in a little chinesium chip-tester, and one of them was bad, but the problem remained. I turned my attention to the and / nor gates, which unfortunately my little tester didn’t support. I wired up my 16500 Logic Analyzer to the output of each of those gates, and found that U32 was glitching. After replacing that I had what seemed like two working units.

Maybe this happened during repair, or maybe it was an issue that I didn’t catch at first, but now on the second unit (the one I thought was perfect), The 16th bit now mirrors the 2nd bit, on all words. This problem seems to follow card A4, which is not accessible when installed. It’s also a more complex circuit.

A4 card, partial schematic

I flipped the unit on it’s side, so I could start probing the edge connectors. On the analyzer, I’ll designate the last pin on the pod as PROBE, so I can move it around looking for interesting signals. When I find one, I’ll add a grabber to the next free line on the pod, and add it on the trace. I have it in continuous capture mode, to trigger on the clock signal (which only runs during a load or fetch). I turned my attention to J1-8 & J1-5, which are odd & even lines out to the RAM card. I expected them to look similar, but they don’t.

The odd line, which is working, seems to be in phase with it’s clock (this is showing no bits active), while the even one looks different.

I expected to see a similar pattern as the odd line on both lines of the good unit. I was wrong.

It looks totally different. Not shown here, but when there are bits selected on the bad unit, I’m seeing short glitch pulses on both the odd & even lines. Those are not evident on the working unit.

So without being able to dig deeper into the circuit while it’s working, I’m at a bit of a loss. For the time being, I’m going to be content with one working unit and one parts unit.

I decided the next step would be to build an extender card so I can freely probe both good & bad units while in operation. The edge connectors are 3.96mm pitch (?!), and I found 4 on eBay at a reasonable sum. They’re right angle connectors, but I’ll make ’em work. The bigger annoyance is there are jumper cables between cards along the top, so I both need to make an extender for those to connect to card A4, and I need to accommodate cables that need to pass across A4, without having to extend a bunch of other jumpers. I think I’ll design this card with a large enough hole in it. It’ll be mildly annoying, and I think a job left to Future Paul (screw that guy…).

HP 16500B Logic Analyzer

A collection of notes & research on the HP 16500 series logic analyzers.  I’m buying one in a few days, and trying to understand the differences between versions, and the myriad of features & options.

I believe I’m getting the following configuration:

  • 16500B Mainframe
  • Hard-disk (the owner says he added, and thought that made this essentially a C model, but that’s not quite true)
  • Network option (not sure which one)
  • (1) 16530 400 Megasample per second (MS/s) scope timebase
  • (3) 16531 400 MS/s dual channel scope module
  • (1) 16555A 100 MHz state / 500 MHz 68 Channel logic analyzer 1MS depth

There’s a lot of documentation, and the mainframe and cards have their own manuals.  Generally, everything has a User Guide, a Service Guide and a Programmers Guide.  The mainframe also has a Setup Guide & a User’s Reference.  The User’s Reference covers the basic operation of the mainframe itself. Unfortunately, the only available PDF it has a font issue, making it very difficult to read.
To understand how to use the logic analysis & oscilloscope functions, refer to the manuals for the cards themselves.

Other references online:


The 16500L card gives you a 15 pin AUI jack, for which an adapter (or Media Access Unit) is required for 10Base-T Ethernet connectivity.
The 16500H card has an RJ-45 jack built in, as well as the expansion card connector (also on the 16500L?) and a “High Speed Port” for connection to the 16505A Prototype Analyzer (is this SCSI?)

The 16500C has an RJ-45 jack built in. X-windows connectivity is supported on the B & C models, but only later cards are fully supported.  For example, earlier scope cards will have a visible interface, but won’t show their trace; only the 16532A Oscilloscope Module will.

Programming via LAN can either happen via sockets at port 5025, echoing commands to \system\program over NFS, or copying a file containing a list of commands to \system\program.


16500B ships with 8MB, and has been found to support up to 64MB of RAM, though earlier reports from HP say it tops out at 32MB.  It’s unclear if 64MB makes a difference vs 32MB. From Phil’s findings, it seems the mainframe is tolerant of EDO or FPM RAM, and though the spec says parity, you can get away without it.


16500B & 16500C both run a Motorola 68EC030, per their Service Guides.  One of the links above claims that the ‘B’ runs a 68020

Hard Drive

Turns out the hard drive on this unit is no good (got a few bucks off because of it). These machines are of the IDE era, and the list of supported drives is small, and mostly Quantum Fireballs.  Ko4bb’s page seems to have the most comprehensive write-up, and I already have a Syba SD-CF-IDE-A adapter on the way.  I’m hoping a 128MB CF card that I already have lying around will be supported, but I’ve also ordered a 256MB card that was explicitly on Ko4bb’s list.

Other things to look at

  • 16505A Prototype Analyzer.
  • 16534A 2GS/s, 500MHz dual channel scope (no extra timebase card needed)
  • 16521A 48 channel pattern generator (I don’t know if this works on it’s own, or is only an expander to the 12 channel 16520A).

First Light

I opened it up, gave it a good cleaning, and reseated all of the connectors.  It was mentioned that the hard drive may be problematic, and so is the touch-screen.

  • With the mouse connected, I get an ‘Impaired’ error about the touchscreen, but it seems to work.
  • The hard drive does not work
  • I can boot off either the original Composite System Disk or the hand-made ‘boot disk’, but in either case, none of the cards are recognized, and it doesn’t even see the network card.

I believe the mainframe need to see the modules for the installed cards on the boot disk, so lets see about making our own.

  • If I make a copy of the boot disk by copying files in windows I get an error on boot.
  • Windows complains about formatting a floppy
  • The mainframe can format floppies.  I’ve had mixed success – it formatted one floppy fine, and whined about another one after having just formatted it.
  • I was able to boot with a disk I made by formatting on the mainframe, and copying files over from Windows, however I’m still seeing unrecognized cards.
  • I removed all but one timebase & one scope card, and still not recognizing the cards.

Hard Drive, revisited

The original hard drive finally spun up to life after a number of reboots and I got a few days of playing around with it until the parts came to replace the drive.  I largely followed the instructions on Ko4bb’s page, but had a few difficulties:

  • When copying files (individually, unfortunately) from the floppy to the drive, you have to go into the name field, and hit clear so that it automatically picks up the name of file. If you don’t do this, it still has the name of the last filed you copied, and you’ll end up writing over it.
  • I had a hell of a time getting file writes to stick at first.  My first few attempts gave me an empty hard drive upon reboot.  I wasn’t sure if it was a bad card, or my process (I followed the instructions), but what finally worked was a combination of the last CF card in my inventory, and rebooting after formatting, and then rebooting again after making the /SYSTEM folder, just in case.  That did the trick and I am now able to boot off the CF card to the latest OS.


The ‘impaired’ status was ultimately fixed by removing the face panel and cleaning off the bezel on both sides, and brushing off the IR emitter / receivers.  One of the links above incorrectly states that this is an acoustic touchscreen – it’s not: It’s an IR grid, similar to some Tek scopes of the same era.  Works great now.

Pod interface

I’ve got this connected up to the RC2014 bus with the flying leads into a strip of header pins, but it’s not something I’d like to repeat.  Instead, I should build a breakout board.  This guy made some inline adapters for various 8 bit platforms, but hasn’t updated his site with any detailed info.

Here’s the pod pinout, from this post that also has some cool X-rays of the pod and leads.

There are no electronics in the pods, but there is an RC network in the probes.  I’m clearly not the first person who wanted to connect their logic analyzer to a board without using a bunch of leads, and HP has a whole document detailing their series of connectors and accessories dedicated to just that.  This document describes an isolation network that should be built into the connector interface, and is I believe pretty much what’s in the probe body of the flying leads.

This is also the circuit in the isolation adapters, which reduce down to a 20 pin connector.  If these are available cheap enough, it may be better than soldering a bunch of SMD resistors & caps.

The plus side is there only 12 – 15 bucks each, the downside it’s it’s more dangley shit hanging off the board, with a fragile flex PCB lead, and more connection points.   Still, it saves a hundred SMD components.

Update – new cards arrived

the owner contacted me and said he found some other cards:

  • (2) 16510B: 35 MHz State / 100 MHz Timing, 80 Channel, 1K Sample buffer Logic Analyzer
  • (1) 16550A: 100 Mhz State / 500 MHz Timing, 102 Channel, 4K Sample buffer Logic Analyzer
  • (1) 16520A 50 Mbps 12 Channel Pattern Generator

Unfortunately, none of the new logic analysis cards work with the existing one, in so much that they couldn’t stack to add channels, rather they’d function as their own separate logic analysis engine.  I could stack the two 16510B cards for a total of 160 channels, albeit at a rather anemic sample depth & speed, or I could use the 16550 for 102 channels at 500MHz, but still shallower depth than what’s on the 16555.  Ideally, I’d get another 16555 card, which I might if the price was right.

Definitely looking forward to messing with the pattern generator though!

Currently Installed:

  • A: 16520A Pattern Generator
  • B: 16531A 2 channel 100 MHz Oscilloscope
  • C: 16530A Oscilloscope Timebase
  • D: 16550A 102 Channel Logic Analyzer
  • E: 16555A 68 Channel Logic Analyzer

In storage:

  • (2) 16510B 80 Channel Logic Analyzer cards
  • (2) 16531A 2 channel 100 MHz Oscilloscope cards

Ideal Configuration

  • A: 16555A 68 Channel Logic Analyzer
  • B: 16555A 68 Channel Logic Analyzer (bonded to above)
  • C: 165621A 48 Pattern Generator Expansion (needs 16520A)
  • D: 16520A 12 Channel Pattern Generator
  • E: 16534A 2 channel 500 MHz Oscilloscope (built-in timebase)

Update 1/5/2021:

Thanks to Alex C who pointed out that if you’re using multiple 16555A cards, you need to be mindful of the setting of the Master jumper – as if they’re both set to master, the mainframe gets confused. See the manual for the jumper settings.

HP 5300A + 5301A, another hamfest find

At 5:50AM April 12th, I awoke, inexplicably on a Saturday morning.  By 6:45, it was clear that sleep was a thing of the past.  “hey, its Spring, I wonder if there any hamfests”, thought my foggy brain.  Turned out there was one in northwest Bumblefuck NJ.

OK, the thing about hamfests is:

  1. They’re always at at ass-o’clock: Doors at 8:00 AM, with many folks starting to wrap up as early as 11:00 AM.
  2. With the exception of the one at the New York Hall of Science in Queens, They’re always in Bumblefuck, New Jersey, or ohChristTheTraffic, Long Island.

A bagel, some coffee, and a ZipCar later, I was in… Somewhere off Interstate 80, about an hour away.  It was lovely.  First buy was a few little round Weston panel meters.  If nothing else, I can always grab a few meter movements at a fest.  They’re usually abundant, and pretty cheap – I scored 2 for $3. After admiring the WW2 frequency meter (which I later succumbed to).  I saw this HP frequency counter, and Bell & Howell multimeter.  Price-tag said ‘make offer’

The HP on the bench: DSC_0059


The meter already assimilated into the shelf above, under the Eico model 147 signal tracer.   Mmm… nixie: DSC_0087

“$20 for both?” I said.

It was way low, but the guy just wanted them off his table and out of his basement, and said ‘yes’ to that effect.

I’ve never actually seen this flavor of meter from HP before, and the only other frequency counter I own is a rather large, loud, power-hungry, rack-mount, nixie-tube unit from the 60’s (which don’t get me wrong, is awesome), so I was happy to add this to my collection.

Rear shown, with fuse cover open – you can’t expose the fuse while the unit is plugged in.  Well played HP.  ‘OSC’ is the output of the 10MHz internal reference oscillator, ‘OSC ADJ’ is to calibrate that oscillator, and ‘DIGITAL RECORDER’ is for… digitally.. recording.  to something.DSC_0063

pulling out the kajiggers on the back of the unit releases the upper readout portion from the lower acquisition module.  You swap out the acquisition module for others, and optionally add a battery pack, DAC, or GPIB module in between the top & bottom, varying the height of your HP sandwich.



After shucking, the two halves reveal their secrets.  Bound together only by some plastic bits and a Centronics connector; it’s pretty economic for an early 70’s era piece.  On the right is the top module, the 5300A Measuring System.  On the left is the bottom portion, the 5301A 10MHz counter. Only one screw and 4 plastic tabs keep the 5300A’s circuit board affixed to the shell.  With the top shell removed, we can re-assemble the two modules, and perform some proper brain surgery.



Power supply to the lower right (including the riser card), 10MHz reference oscillator to the right, display driving gak to the front, and timebase & control in the middle.



Display portion –  A row of 12 transistors, 2 per digit, driven from the 1820-1060 chip directly above, and a row of 7 transistors below, 6 per digit + 1 common,  marked !DP1 through !DP6 and !DP_COMM in the schematic, these are for the decimal points.  (I’m using ‘!’ to denote for NOT / Inverted signals, in the schematic, they have the line atop the name.. an overscore? what’s that called). I’m focusing on the details of the display, because it went wonky about an hour after being powered up.



DSC_0083 the display should read ‘040347’, and it mostly does, but the left most digit is dim, the left half of the next digit is even dimmer, and the right half of that same digit is a bit brighter.

This dot display is driven differently then your run-of-the mill common-cathode (or anode) 7 segment displays.  Each character has a pair of common anodes, one for the left, one for the right, split mirrored down the center of the display.  Here’s a diagram of one segment:


Because of this, the display driver runs two clock cycles for every digit. The decimal point is actually driven directly by the counter module. Here’s the relevant bit of the display schematic: HP5300 display The whole schematic is here: HP 5300 Schematic

Interestingly (to me anyway) is that U1, the display scanner is free-running.  It cycles through the 12 halves, tells U2, the character generator the current digit & half, and tells the counter module the current digit so it can pass the correct value for that digit.  It also passes the digit half to the counter module, though I suspect it rarely (if ever) actually cares. I didn’t document it well, but I did actually read the data lines on my glitchy old Tektronix 7D01 logic analyzer.  The output looked something like this:



So back to the failing display.  While I was prodding around with a meter, I shorted the left & right select pins of a digit: DSC_0084

Bingo.  The other half of the display lit up, so it’s not the display, it’s not the driver transistor, it’s U1.  I was lucky enough to score a replacement chip (HP 1820-1060) on Ebay for $10 or $20.  Swapped it out and it worked like a charm.  I was a little concerned that their might be some other, overarching problem which would cause the same failure in the new chip, but I metered around and checked the temp on the chip for a while and felt pretty confident that all was good.

It’s useful enough for the money, the repair was easy, and it was an interesting circuit to get to know.  This early digital stuff is pretty accessible, as specially when the documentation is available, and this thoroughly detailed, as it often is with the old Tek & HP gear.  Full manual here

Thanks for reading!