7 ms·

Fixing the Ethernet Board from a Vintage Xerox Alto

138 points by mkeeter 9y ago

deleted 9y ago
[deleted]
sgt 9y ago
That is a very interesting use of a BeagleBone. He made an interface to the Alto's 3Mbit/sec Ethernet using the BeagleBone's PRU (a programmable real-time unit of which I believe it has two). Someone also used the PRU to build a video card for a Macintosh SE: https://hackaday.com/2014/02/05/the-30th-anniversary-macintosh/ https://hackaday.com/2014/02/05/the-30th-anniversary-macinto...
CodeWriter23 9y ago
A word from experience: an extender card can introduce noise, which can lead to observations of self-induced problems due to the introduced noise. I don't think this is the case for your situation, but symptoms the runt edges you observed could be caused by the extender. Also, I have on occasion observed the magic of an extender card causing a flaky card to function properly. If you get stumped, you might want to depopulate some non-essential cards to open up a space to get your hands and probes on the card under investigation, while it is directly plugged into the slot. Excuse me, I mean get your hands and probes in there while the system is powered off.
purplezooey 9y ago
What job can I get where people pay me to restore vintage gear like this? :) (answer: "already be rich")
- ChuckMcM 9y ago
  Actually if you have the skills people will seek you out. The trick is to repair things and demonstrate said skills. Since most of the vintage gear is easily covered by a 100Mhz oscilloscope, a DMM, and a 10Mhz signal generator you can do a lot of that. Fixing things you pick up at thrift shops will help you develop the mindset and 'bodging' skills, then work your way up to more interesting things. You will be amazed at what you find in the bin at some places.
sillysaurus3 9y ago
I started probing the Ethernet board's input circuit with the oscilloscope. The board was receiving the input okay, but a few gates later the signals looked kind of sketchy, as you can see above. Is it even possible to do this kind of debugging on modern hardware? Or is this a lost art?
- al2o3cr 9y ago
  Sorta? Not all the signals are available, depending on the level of integration. OTOH, modern hardware also frequently has test support via things like JTAG, which can give you a ton of information and/or control the device directly. For instance, there's ChipScope from Xilinx (https://www.xilinx.com/products/design-tools/chipscopepro.html https://www.xilinx.com/products/design-tools/chipscopepro.ht...) which allows you to debug your FPGA hardware from outside without wasting a ton of pins. It's halfway between poking the board with an oscilloscope and running GDB, as it uses the metadata from your design to label things.
- subway 9y ago
  If you can find the relevant documentation, sure. Modern hardware is made from black boxes on top of black boxes.
- sigstoat 9y ago
  > Is it even possible to do this kind of debugging on modern hardware? Or is this a lost art? EE's still have to diagnose hardware problems. Keysight will be happy to sell you diagnostic equipment for investigating problems with your 100Gbps ethernet devices. Your bank probably won't care to fund the necessary loans, though.
- SAI_Peregrinus 9y ago
  It's possible, but harder. A modern Ethernet card would probably be doing this step entirely on one chip. A quick search shows the Microchip ENC28J60 as a pretty typical part. The result is that you'd be able to probe the input to the chip and the output, and see the failure at the output, and replace the chip. Effectively the same as what was done here, just with more components built into the chip. Louis Rossmann[1] does macbook component-level logic board repairs, though 99% of the time he doesn't need to break out the oscilloscope to find the problem. It's almost always a power supply or liquid damage problem, or something similar that can be found with nothing more than a multimeter. [1] https://www.youtube.com/playlist?list=PLkVbIsAWN2lsHdY7ldAAgtJug50pRNQv0 https://www.youtube.com/playlist?list=PLkVbIsAWN2lsHdY7ldAAg...
ChrisGammell 9y ago
Had a chance to chat with Ken, really enjoyed hearing about the history of the Alto and the process they take for troubleshooting what is effectively priceless hardware (given the fact that so few are remaining): https://theamphour.com/361-an-interview-with-ken-shirriff/ https://theamphour.com/361-an-interview-with-ken-shirriff/
linker3000 9y ago
Noted that Ken had an issue when a 74S TTL part was replaced with a 74LS because the characteristics are different. Back in the day (mid 1980s), when I was going through an electronic engineering apprenticeship with a flight simulator company and many boards were end-to-end TTL, it was generally understood that power-hungry 74S series TTL were used for a reason (generally speed) and should not be substituted lest 'funny things' happen. Mind you, there was one time when debugging a glitch led to one chip being replaced with the same type from a different family - something like a 74LS replacing a 74F - and the propagation delay difference was enough to fix an edge-case timing issue. Due to project constraints, the root cause wasn't tracked down and so the board bill of materials made it clear that the 'odd one out' was correct and should not be changed. PS: Ken - I think I still have a quantity of 74S TTL 'pulls' from the day, so if all else fails sourcing a part, look me up!
- WalterBright 9y ago
  Back in the 70s when I designed TTL circuits, the rule was to do the math to work out all the worst case timings. The design was done with the expectation that any part could be replaced with a faster part, and the design would still work. (Because parts got faster over time, not slower, and the slower parts would become unavailable.)
- Declanomous 9y ago
  Wouldn't replacing the 74S86 with a 74VHC86 have worked? I'm a hobbyist, so my knowledge is pretty limited[1], but I was under the impression that the VHC series chips were drop-in replacements for the S series in both timing and logic levels. [1] Read as 'I know just enough to be dangerous, but not enough to understand why what I'm doing is dangerous.'
  - CodeWriter23 9y ago
    The way to confirm your assumption is to look at the different typical timing waveform charts from the data sheet for each part. I once traced hundreds of thousands of dollars worth of bunk boards back to some RAM chips substituted on the basis of being a "drop in replacement" by the procurement department. The timing diagrams revealed the replacements possessed different timing characteristics. Enough so that the chips would occasionally leave the bus floating when the memory was read from the host side.

138 points by mkeeter 9y ago

deleted 9y ago

[deleted]

sgt 9y ago

That is a very interesting use of a BeagleBone. He made an interface to the Alto's 3Mbit/sec Ethernet using the BeagleBone's PRU (a programmable real-time unit of which I believe it has two). Someone also used the PRU to build a video card for a Macintosh SE: https://hackaday.com/2014/02/05/the-30th-anniversary-macintosh/ https://hackaday.com/2014/02/05/the-30th-anniversary-macinto...

CodeWriter23 9y ago

A word from experience: an extender card can introduce noise, which can lead to observations of self-induced problems due to the introduced noise. I don't think this is the case for your situation, but symptoms the runt edges you observed could be caused by the extender. Also, I have on occasion observed the magic of an extender card causing a flaky card to function properly. If you get stumped, you might want to depopulate some non-essential cards to open up a space to get your hands and probes on the card under investigation, while it is directly plugged into the slot. Excuse me, I mean get your hands and probes in there while the system is powered off.

purplezooey 9y ago

What job can I get where people pay me to restore vintage gear like this? :) (answer: "already be rich")

ChuckMcM 9y ago
Actually if you have the skills people will seek you out. The trick is to repair things and demonstrate said skills. Since most of the vintage gear is easily covered by a 100Mhz oscilloscope, a DMM, and a 10Mhz signal generator you can do a lot of that. Fixing things you pick up at thrift shops will help you develop the mindset and 'bodging' skills, then work your way up to more interesting things. You will be amazed at what you find in the bin at some places.

sillysaurus3 9y ago

I started probing the Ethernet board's input circuit with the oscilloscope. The board was receiving the input okay, but a few gates later the signals looked kind of sketchy, as you can see above. Is it even possible to do this kind of debugging on modern hardware? Or is this a lost art?

al2o3cr 9y ago
Sorta? Not all the signals are available, depending on the level of integration. OTOH, modern hardware also frequently has test support via things like JTAG, which can give you a ton of information and/or control the device directly. For instance, there's ChipScope from Xilinx (https://www.xilinx.com/products/design-tools/chipscopepro.html https://www.xilinx.com/products/design-tools/chipscopepro.ht...) which allows you to debug your FPGA hardware from outside without wasting a ton of pins. It's halfway between poking the board with an oscilloscope and running GDB, as it uses the metadata from your design to label things.
subway 9y ago
If you can find the relevant documentation, sure. Modern hardware is made from black boxes on top of black boxes.
sigstoat 9y ago
> Is it even possible to do this kind of debugging on modern hardware? Or is this a lost art? EE's still have to diagnose hardware problems. Keysight will be happy to sell you diagnostic equipment for investigating problems with your 100Gbps ethernet devices. Your bank probably won't care to fund the necessary loans, though.
SAI_Peregrinus 9y ago
It's possible, but harder. A modern Ethernet card would probably be doing this step entirely on one chip. A quick search shows the Microchip ENC28J60 as a pretty typical part. The result is that you'd be able to probe the input to the chip and the output, and see the failure at the output, and replace the chip. Effectively the same as what was done here, just with more components built into the chip. Louis Rossmann[1] does macbook component-level logic board repairs, though 99% of the time he doesn't need to break out the oscilloscope to find the problem. It's almost always a power supply or liquid damage problem, or something similar that can be found with nothing more than a multimeter. [1] https://www.youtube.com/playlist?list=PLkVbIsAWN2lsHdY7ldAAgtJug50pRNQv0 https://www.youtube.com/playlist?list=PLkVbIsAWN2lsHdY7ldAAg...

ChrisGammell 9y ago

Had a chance to chat with Ken, really enjoyed hearing about the history of the Alto and the process they take for troubleshooting what is effectively priceless hardware (given the fact that so few are remaining): https://theamphour.com/361-an-interview-with-ken-shirriff/ https://theamphour.com/361-an-interview-with-ken-shirriff/

linker3000 9y ago

Noted that Ken had an issue when a 74S TTL part was replaced with a 74LS because the characteristics are different. Back in the day (mid 1980s), when I was going through an electronic engineering apprenticeship with a flight simulator company and many boards were end-to-end TTL, it was generally understood that power-hungry 74S series TTL were used for a reason (generally speed) and should not be substituted lest 'funny things' happen. Mind you, there was one time when debugging a glitch led to one chip being replaced with the same type from a different family - something like a 74LS replacing a 74F - and the propagation delay difference was enough to fix an edge-case timing issue. Due to project constraints, the root cause wasn't tracked down and so the board bill of materials made it clear that the 'odd one out' was correct and should not be changed. PS: Ken - I think I still have a quantity of 74S TTL 'pulls' from the day, so if all else fails sourcing a part, look me up!

WalterBright 9y ago
Back in the 70s when I designed TTL circuits, the rule was to do the math to work out all the worst case timings. The design was done with the expectation that any part could be replaced with a faster part, and the design would still work. (Because parts got faster over time, not slower, and the slower parts would become unavailable.)
Declanomous 9y ago
Wouldn't replacing the 74S86 with a 74VHC86 have worked? I'm a hobbyist, so my knowledge is pretty limited[1], but I was under the impression that the VHC series chips were drop-in replacements for the S series in both timing and logic levels. [1] Read as 'I know just enough to be dangerous, but not enough to understand why what I'm doing is dangerous.'
- CodeWriter23 9y ago
  The way to confirm your assumption is to look at the different typical timing waveform charts from the data sheet for each part. I once traced hundreds of thousands of dollars worth of bunk boards back to some RAM chips substituted on the basis of being a "drop in replacement" by the procurement department. The timing diagrams revealed the replacements possessed different timing characteristics. Enough so that the chips would occasionally leave the bus floating when the memory was read from the host side.