HP 59401A GPIB Bus System Analyser - Page 1
Forum section: electronics / repair

author: rob.manderson
author_rank: Regular Contributor
timestamp: August 11, 2018, 05:47:19 pm
content: I bought one of these recently (I have an entry in the Test Equipment board under 'List your test equipment "scores" here! - post # 2095 on page 84 (yikes!)).It's been suggested I do a teardown but I won't for 3 reasons.1 - I'm a lazy bastard!2 - My camera is sadly lacking in ability to take good photos of the kind expected here (or maybe I'm just no good at taking photos!)3 - There's not a lot to see. Just two boards full of TTL. And this is your grandfathers first generation TTL. None of this new-fangled LS TTL!The instrument I received seems to have been manufactured in 1979 - the chips have 78 and 79 date codes. It seems to have been originally released in 1975 - at least the manuals have a release date of January 1975. They didn't make all that many of these. Mine, from 1979, is serial 1914A00955. On the other hand, the manual is confused as well - the HP serial number scheme back then was that the first two digits indicated the year of the latest revision to which this item was built, yet the 1975 manual says it applies to instruments with serial prefix 1914A.Anyway, as received it didn't work. The front panel contains a bunch of toggle switches, a 6 position LED matrix display and 10 other leds. The LED matrix display didn't display anything, no matter what switch settings were chosen. The individual LED's did work and their state followed that of the appropriate toggle switch. I.e., set EOI to on and the EOI LED lit up. And so on.One Artek manual purchase later and I'm ready to start. If the matrix display isn't working the place to start was on the display board and work backward. And so it proved. No activity whatsoever anywhere within the multiplexers driving the display. I traced it back to lack of a clock signal driving that part of the instrument.They used a SN7414 hex schmitt inverter to generate the clock. Well, one sixth of one anyway. I'd completely forgotten that one can generate a clock with such a simple circuit. A feedback resistor between the input and the output and a capacitor from the input to ground. Simple. Anyway, there was no output from this chip. Short the input to ground and I'd expect the output to go high but it didn't. Short the input to Vcc and I'd expect the output to go low. It didn't. So there it was. A dead SN7414. (The chip in question is U113).To confirm it whilst I waited for Digikey to ship the replacement chip I set up my 3325A to a 2.5V p-p 6 KHz square wave and injected that signal into the output side of the inverter. The display sprang into life.They actually use two of the inverters in the same configuration to generate two clocks. Neither one was clocking.Now this is generating clocks to drive a display multiplexer and one wouldn't expect that it needs to be particularly accurate. So I had a chuckle at the resistor value they used in the feedback. 392 ohms! Not 390, nope, 392. Yes, that's a value in the E96 range but really, does it need to be 1%? Couldn't they have gotten by with 10%? And no, I don't think it needs to be in sync with other clocks on board - there's no attempt in the circuitry to sync with the primary clock.Interestingly enough, there's a second SN7414 on the board and someone has been there before me and replaced it. It's date code is from 1984 and the soldering job, whilst neat enough, was let down by them leaving burnt flux all around it. I cleaned that off.So now the instrument is working again. Another dinosaur rescued. So it displays everything in octal. I can cope with that. It's easy to forget that these date from the early era of desktop computers and they were designed to accommodate the conventions of the time. I'm sure if they'd been designed in 1983/4 instead of a decade earlier they'd have been in Hex. They'd probably have more than 32 bytes of memory as well. I half expected to find a small core matrix memory instead of semiconductor based memory when I opened it up!Notes to anyone wanting to repair one of these. Don't remove the front panel from the display assembly unless you really need to. It's one of those deals where you seem to need 5 hands to get it back on. The problem is that whilst most of the toggle switches have 3 positions and can be easily lined up with the holes, there are 4 that only have two positions and this makes it difficult to line things up so that they'll pass through the holes. Add in the fact that the power switch is mounted on the front panel and the amount of slack in the leads is minimal and it adds up to a mildly frustrating job getting it back together again.Other notes - the manual indicates that the 1914A serial range above number 490 should have an A3 assembly p/n 59401-66503 Rev B. Below that serial it should be 59401-66503 Rev A. However my instrument contains 59401-66505 Rev A. It has the same layout as the 66503 Rev B board and it seems to match the circuit diagrams but it has a later part number. Is this significant? No idea!

author: SilverSolder
author_rank: Super Contributor
timestamp: October 14, 2018, 01:04:54 am
content: Nice job on rescuing this old item!I recently bought one as well, on the never ending journey into the quagmire of 70's technology. Mine worked "out of the box" so I haven't explored the internals.I do find it very useful for getting other oldies but goldies up and running over the GPIB. 31 bytes of memory is enough to be useful for that generation of equipment (and even up to 80's and early 90's era equipment). Even the ability to slow the bus down is useful.

author: rob.manderson
author_rank: Regular Contributor
timestamp: October 14, 2018, 12:30:39 pm
content: Quote from: SilverSolder on October 14, 2018, 01:04:54 amActually I didn't use the memory at all - what I wanted was the ability to single step HPIB bus transactions to see what was really happening, and the ability to drive the bus a byte at a time and, again, see what was really happening. This was all in the context of an HPIB problem in my HP8568B spectrum analyser. Now that's fixed I doubt I'll ever have another use for the machine. But it was an enjoyable repair job and I hate seeing things like that being thrown out. The build quality is little short of amazing.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 21, 2019, 03:47:18 am
content: It came in really handy today... one of the DMMs on the bus was behaving in unexpected ways... - so the question was, WTF is happening on the bus? Enter the 59401A. It was relatively easy to single step the bus communications and determine that the fault was with the DMM, not the controller (the DMM was reacting to messages not addressed to it). In one fell swoop, we ruled out bad cables, controller, and other devices on the bus!Once the faulty device was ascertained, it was easy to send some test messages study the behavior of the DMM - after some experimentation, it turned out that one of the bus address lines inside the DMM was stuck Low (false), causing it to appear multiple times in the bus address space!Both of these jobs would have been very tedious without something the 59401A !

author: Raindog
author_rank: Contributor
timestamp: April 29, 2020, 02:30:04 am
content: Just picked up a working, in good condition HP 59401A protocol analyzer off eBay today for $89.49 shipped. Been wanting one of these so I can learn more about the HPIB / GPIB buss. I have plenty of HPIB command documentation but no way to experiment with any of it till now. It's always nice to rescue this cool old gear before it heads off to be recycled.

author: SilverSolder
author_rank: Super Contributor
timestamp: April 29, 2020, 03:54:54 am
content: Quote from: Raindog on April 29, 2020, 02:30:04 amAwesome - welcome to the club of owners of this somewhat esoteric instrument! It is a fun "toy" and definitely good for using and learning GPIB.

author: Raindog
author_rank: Contributor
timestamp: April 29, 2020, 04:49:35 am
content: Thanks for the welcoming words. I can confirm that I purchased this instrument after watching Curiousmarc's YouTube video where he uses one of these to trouble shoot some HPIB issues he was having with his HP tape drive.

author: SilverSolder
author_rank: Super Contributor
timestamp: April 29, 2020, 10:20:01 am
content: Very cool video, thanks for posting. I have never actually used mine in "Talk" mode, but this has inspired me to give it a try!

author: PaulAm
author_rank: Frequent Contributor
timestamp: April 29, 2020, 05:44:16 pm
content: I've been looking for one of these off and on for a few years and stumbled across one at a local industrial auction. I get a lot of odds and ends from those guys even though the buyers premium is up to 18% now. Keeping that in mind keeps my bids low.I ended up paying about $35 after bp and tax and it seems to be 100% functional. Now if only I could find a box of gpib cables for $5

author: SilverSolder
author_rank: Super Contributor
timestamp: April 29, 2020, 08:55:26 pm
content: That's a good price for sure! Nothing like equipment that absolutely nobody other than total crazies would want, LOL!I've always wondered why GPIB cables are so expensive. Years of hunting for them has netted a reasonable collection, but only rarely could I call them a bargain...