The Plexus P/20 is now fully operational!

It's been quite a journey that I've been on with this Plexus P/20 computer. Is this going to be the video where I can finally say that the machine might actually be fully operational?

Part 5: This part

-- Links

Plexus P/20 Discord channel at Usagi Electric:

General Adrian's Digital Basement channel:

My Github Repo (with ROM dumps and images)

Plexus P/20 Brochure:

Plexus Unix:

MFM emulator:

Fujitsu Hard Drive Brochure:

Omti 5200 Manual:

Adrian's Digital Basement Merch store:

Adrian's Digital Basement ][ (Second Channel)

Support the channel on Patreon:

My GitHub repository:

-- Tools

Deoxit D5:

O-Ring Pick Set: (I use these to lift chips off boards)

Elenco Electronics LP-560 Logic Probe:

Hakko FR301 Desoldering Iron:

Rigol DS1054Z Four Channel Oscilloscope:

Head Worn Magnifying Goggles / Dual Lens Flip-In Head Magnifier:

TL866II Plus Chip Tester and EPROM programmer: (The MiniPro)

TS100 Soldering Iron:

EEVBlog 121GW Multimeter:

DSLogic Basic Logic Analyzer:

Magnetic Screw Holder:

Universal ZIP sockets: (clones, used on my ZIF-64 test machine)

RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)

Plato (Clone) Side Cutters: (order five)

Heat Sinks:

Little squeezy bottles: (available elsewhere too)

--- Instructional videos

My video on damage-free chip removal:

--- Music

Intro music and other tracks by:
Nathan Divino
@itsnathandivino

[exidy-yt]

Usagi's discord crew are seriously some of the smartest retro-tech people in the world when it comes to these 70s and 80s minis and server machines. Absolute brilliance and it's so good that they are around helping people like you preserve the less well documented aspects of our digital toddlerhood. Great stuff Adrian!

[nix-consulting]

I used to service Plexus machines in the 80s. I never came across a P/20 though. P/30, P/35, P/55 and P/75, but not a P/20. They were quite good machines. Very reliable.

[hardlyworgen]

"Fully operational" reminds me of Emperor Palpatine saying "witness the power of this fully armed and operational battle station."

[Cherijo]

I'd still really like to see a diagnosis of the power supply reset circuit. One of your other commenters and I on the last video had a debate about whether your fix was potentially bypassing a potentially more serious problem in the power supply That is actively happening, or if it wasn't that big a deal and your reset circuit wasn't an issue. To that end, I'd love to see the power supply revisited and rescoped out both to check the voltages again as the other user suggested should be done, and to see if we can get an understanding of The reset circuit and how it monitors the power. It looked like a complicated circuit and it would be interesting to see what it's responding to or if it's just broken.

[yourneighborhood]

This is really a fun series. Almost like an archival treasure hunt. Every new episode has surprises.

[jandjrandr]

It has been quite the roller-coaster ride of a series for the P/20. So awesome that the community real came through to help out making this such a successful adventure!

[kepamurray]

I had zero interest in this plexus machine because I thought "what can you possibly do with it with no documentation" until I heard what was on it.
That was a game changer! Well done to you and the discord team. That is a once in a lifetime score.

[necro_ware]

Such a lovely series, it was very exciting detectives work! Thank you for not giving up and sharing. Also thanks a lot to the community and all the help.

[packetman]

P20 is a handsome machine and just reconfirms my belief that all machines need more blinking lights on them. Love this series.

[stonent]

I think the reason /stand commands didn't run in Unix is that they are not compiled for Unix but compiled directly for the CPU. They probably try to talk directly to the peripherals rather than speaking to the kernel. Sort of like an ATTiny doesn't have an operating system, it just runs the raw code. So even if the program could execute, it might try to access a protected memory space in Unix since it thinks it has full control of the memory map, Unix might intercept that and force the program to dump and exit.

[adamboggs]

Paused the C compiler test just to verify you had it print Hellorld as expected. Nice work reviving this super interesting machine!

[ivantod]

Truly these Plexus videos are some of your best work ever, Adrian! Thank you for making them!

[bthjf]

Unix System V svr2 commercial source license was for $43, 000, with three months of support, and a $16, 000 price per additional CPU…. That’s some expensive system you just resurrected !

[erinwiebe]

This has been one of my favourite series on the channel. Congratulations on getting it running! The community really came through with this one. Love it!

[Jody_VESAR]

Every time I heard "BATT Signal", my brain filled in "Na na na na na na na na na na na na na na na na... " 🙂

[rj]

the transistor circuit that un select cal.ce- when in reset / power down is probably against content corruption of the registers; The bus is in a unknown state and stray voltages/signals may be present on the bus that could cause a random write to one of the registers is the chip is selected.

[Kevakazii]

I had no idea what you were talking about 95% of the time but I really enjoyed this series nonetheless. I get enjoyment out of seeing people talk about things they are deeply knowledgable about.

[MarkBender]

Adrian: “Well, that pretty much tells us one thing”

Also Adrian: *immediately lists at least three things*

[Powertampa]

The com port loses connection because in balanced power mode unused devices are shut down when the machine goes to sleep mode. You can change individual power saving settings for devices though to stop it from putting it to sleep due to inactivity. Putty also has an option for how often it should send keep alive and on lower baud rates that often needs to be increased so the emu can keep signal sync. I know it's convenient, but sleep mode isn't the greatest option for hardware. Low c states still have current bleed issues even on modern CPUs... yeah be a cold day in hell before that ever gets fixed. Congrats on getting the old girl running :)

[ferrarik]

I find it so interesting that these old unix device names still are part of being used today. If you use a LTO tape drive, the drive is like /dev/st0 and the "no rewind" function is /dev/nst0, notice the extra "n". And that is found in the device names here as well, it's simply fascinating :)