TechStep not required: Using the built-in "test mode" to fix a dead Mac

In my last SE/30 repair video, I was totally stuck with what to do next because the machine crashed as soon as I turned it on. That led me on a path to discover the internal ROM based Macintosh diagnostics which we can leverage to figure out what exactly is wrong with these broken SE/30 motherboards!
Part 0: • 0089 Troubleshooting M...
Part 1: • This Mac SE/30 was sev...
Part 2: This part!
Part 3: Coming soon
-- Links
Using internal Mac diagnostic modes:
docs.google.co...
Original document on the Diagnostic protocol:
web.archive.or...
Replicated Schematics of the SE/30:
github.com/mis...
Mac SE and SE/30 PicoATX PSU adapter:
github.com/dek...
www.tindie.com...
RGB2HDMI:
github.com/Ian...
TechStep Photos:
appletothecore...
TechStep Replica:
ko-fi.com/s/aa...
Adrian's Digital Basement Merch store:
my-store-c82bd...
Adrian's Digital Basement ][ (Second Channel)
/ @adriansdigitalbasement2
Support the channel on Patreon:
/ adriansdigitalbasement
My GitHub repository:
github.com/mis...
-- Tools
Deoxit D5:
amzn.to/2VvOKy1
store.caig.com/...
O-Ring Pick Set: (I use these to lift chips off boards)
amzn.to/3a9x54J
Elenco Electronics LP-560 Logic Probe:
amzn.to/2VrT5lW
Hakko FR301 Desoldering Iron:
amzn.to/2ye6xC0
Rigol DS1054Z Four Channel Oscilloscope:
www.rigolna.co...
Head Worn Magnifying Goggles / Dual Lens Flip-In Head Magnifier:
amzn.to/3adRbuy
TL866II Plus Chip Tester and EPROM Programmer: (The MiniPro)
amzn.to/2wG4tlP
www.aliexpress...
TS100 Soldering Iron:
amzn.to/2K36dJ5
www.ebay.com/i...
EEVBlog 121GW Multimeter:
www.eevblog.co...
DSLogic Basic Logic Analyzer:
amzn.to/2RDSDQw
www.ebay.com/i...
Magnetic Screw Holder:
amzn.to/3b8LOhG
www.harborfrei...
Universal ZIP sockets: (clones, used on my ZIF-64 test machine)
www.ebay.com/i...
RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)
www.retrotink.com/
Plato (Clone) Side Cutters: (Order Five)
www.ebay.com/i...
Heat Sinks:
www.aliexpress...
Little squeezy bottles: (available elsewhere too)
amzn.to/3b8LOOI
--- Instructional videos
My video on chip removal without damage:
• How to remove chips wi...
--- Music
Intro music and other tracks by:
Nathan Divino
@itsnathandivino

[@neonhomer]

Raise your hand if you wouldn’t mind a 2+ hour video from Adrian…. ✋

[@ByteDelight]

Will for sure make me non-active at work, but I love every second of it

[@alexmirica]

Or a live video, no cut scenes, uncensored.

[@imqqmi]

I'll just watch it in multiple chunks, so bring it on!

[@jandjrandr]

This is by far one of your most revolutionary videos for Mac II/SE diagnostics thus far. Kudos to you Adrian for this investigation!

[@scramble45]

love these types of moments when you just happen to stumble across an old txt file that unlocks forgotten knowledge

[@HaveYouTriedGuillotines]

Given Apple's tendencies, this is more like forbidden knowledge.

[@horusfalcon]

I"m glad for you and all the Mac mavens out there that you captured this before the Archive went down. They're still down as I type, and this is an excellent example of they kind of stuff we'll be missing without it.

[@adriansdigitalbasement]

Yeah and I even saved the archived page locally which is something I've learned to do from experience. You just can't always rely on things to be up when you need them.

[@TheGreatAtario]

@@adriansdigitalbasement Archive of an archive, truly the way of the archivist!

[@logiclust]

used to use a techstep back in the days of being a certified mac tech, however, our board-level repair techs could spot problems faster than the tool could expose them so rarely used the thing. You helped bring back a lot of fond memories of those years. it was the dawn of the PPC and the clone wars; we we had a lot of interesting hardware rolling through the shop. i loved apple's toolless industrial design back in those days, everything was so easy to work on.

[@adriansdigitalbasement]

I'd love to know more about Mac board level repair happening back then. Who was doing this work and where did the spare parts come from? All I ever knew is Apple didn't allow the certified dealers to do this kind of work, or was this a bad assumption?

[@JanusCycle]

Great find. I absolutely love complicated obscure inbuilt test modes like this. Thanks for exploring this and showing us. I hope the archive comes back up soon.

[@snooks5607]

16:19 glad you got to IA before they went down. I guess this video might not have happened without it, goes to show how critical of a service it is

[@adriansdigitalbasement]

What is interesting -- is I never take for granted that documentation I use in a repair will be available in the future, so I always save a local copy and file it away. I really hope the Internet Archive comes back online soon.

[@Plarndude]

@@adriansdigitalbasementI really hope IA is put on Hyphanet, or Pirate Bay torrents.

[@Davide0033]

@@adriansdigitalbasement yeah, it's almost crazy to see the internet archive down that's one place everyone took for granted will always be there

[@CyranoJones509]

The Archive got hit by a DDOS attack. They're currently offline while they upgrade their security. According to their Twitter channel, they expect this to take a few days to get everything right

[@rommix0]

@@adriansdigitalbasement Yeah. From what I've heard, the people at IA say it might be days until it comes back as they are fixing all the security issues that the hackers exploited. Pretty smart on their part. Thankfully, data didn't get wiped.

[@sahajsarup]

So much

[@falecore8585]

This series is excellent and confirms many of the frustrations I have encountered over the years when working on these boards. Thank you Adrian, you are an incredible resource to the community!

[@TonyHoyle]

It's wild to me that people knew it was a serial protocol for 30 years and nobody actually tried to use it (except that one guy who wrote the document) until now.

[@OtakuXP2]

Just imagine the number of boards thats gone to the great e-waste in the sky before this info became more public 😮

[@adriansdigitalbasement]

I'm hoping that people can dust off their broken boards they have hopefully held onto to try poking around at again.

[@mariestarlight]

As a former systems admin/network admin, you made me nostalgic for null modem cables. When I was young using those to transfer files and even play Doom with another person was possible. As an admin, we used Serial all the time to configure devices, and these days you use usb to serial adapters. I still have a frankencable similar to your laplink cable, but it allows me to turn "null modem" off and on, and has both male and female adapters on both sides, so I have a 1-cable solution for any serial ports I come across. I even have an RJ45 adapter for all the networking equipment that uses serial over an RJ45 port.

[@chuckthetekkie]

I would imagine that someone probably could create an inexpensive good enough TechStep using a Raspberry Pi that could send the appropriate commands to the Mac via a touchscreen GUI. It just needs to be good enough for basic troubleshooting like you are doing here. Plus it wouldn't be stuck to only Macs as you could have Boot select as part of turning the device on. Just a thought for more talented people.

[@adriansdigitalbasement]

Eric (of the Bluescsi project) already hit me up regarding about this. :-) There is still a ton of work to get done before this could be automated since it appears what you can do varies from Mac to Mac, but once all of this is understood and documented then something open source could easily be developed. For now, I'm just happy to know how to do this myself so I won't be sitting around guessing. Guessing is no way to fix something! :-)

[@christopherdecorte1599]

​@adriansdigitalbasement documenting the input strings like you are seems to be the best first step, and then someone can create scripts for each task.

[@LeftoverBeefcake]

Yup, it's super simple using Python to access the serial port of a Pi, and from there to send the proper commands over to the Mac, and then read and translate the various responses into something more human readable. You could also run the same Python code, with a GUI for the desktop, compiled into an .exe for any Linux, Windows or Mac. It could also be adapted for the Amiga since the DiagROM the Amiga has can also work over serial.

[@thenerd6192]

I remember in the last video you mentioned in passing that there was some activity on the serial controller as if you could attach a serial debugger, and I wondered what would happen if you tried-I never imagined it would be this sophisticated though! Amazing work!

[@memadmax69]

This video not just for the SE/30, but for all those early 90s Mac machines... Even the freakin printers.

[@adriansdigitalbasement]

I am pretty amazed those early LaserWriter printers might have this too. The Machine Identifier is there but I'm not sure the diagnostics actually work.

[@AviDrissman]

​@@adriansdigitalbasement I’m not surprised at all. The early LaserWriters had a 12 MHz 68000 (and the IIf/g were 030s), and if I were a hardware engineer building one, I sure as heck would reuse this external diagnostics protocol given that there was all this hardware support to drive it.

[@memadmax69]

@@adriansdigitalbasement I would love to see a vid experimenting on all the various 90s apple stuff! =P

[@dave928]

@@adriansdigitalbasement maybe it would at least let you be able to check the RAM. the LW IInt runs off a 68000 and only has 1 SIMM slot, for 2MB total, while the IIntx runs off a 68020 and has *12* SIMM slots for a whopping 12MB total (like mine).

[@shmehfleh3115]

You might want to turn local echo on in your terminal program. It makes life easier when the remote machine doesn't echo.

[@JoesComputerMuseum]

Great work Adrian! I had tried similar reverse engineering of the boot diagnostic sequence a couple years ago, but you actually got it done and figured out. Congrats!

[@BobDarlington]

I have no particular interest or love for old macs, but I do appreciate your walk through of the troubleshooting process. Thanks.

[@robertfear7021]

Sir, never dissapointed by your videos. Good stuff. Appreciate the content. I have a working VIC-20 now (just passed to my youngest son,) because of the motivation your videos gave me.

[@Vvardenfell_Outlander]

Loving this series. Was hoping for a quick bodge wire and a test at the end but I can't wait for the conclusion.

[@pauldourish]

This. THIS is why we watch ADB. Fabulous stuff, Adrian!

[@nilz23]

Am I the only one that would be fine with a 2 hour video?

[@vladimus9749]

Detecting an error but not giving any indication of what it could be, thus requiring an expensive trip to Apple, is such an Apple thing to do.

[@IlBiggo]

LOL, you've not seen a lot of vintage Apple machines. Or pc clones of the same era.

[@TonyHoyle]

The laplink cables I remember were bodged parallel cables with 4 bits going in one direction and 4 bits going in the other. I'm not surprised serial ones existed, it makes sense.. just never seen that until you held one up!

[@gwholdom]

Nooooo… “To be continued”. Nice diagnosis, skills, and storytelling Adrian - can’t wait for episode III!

[@MetalJesusRocks]

Hey dude! Very nice meeting you at PRGE. You seem like a super cool dude and I love your videos. 🤘

[@adriansdigitalbasement]

Thanks! Same! I just watched your PRGE wrap up last night. I forgot to really take any pictures of the whole event. (Typical of me, LOL)

[@tony359]

It took me months to get a loan Techstep recreation to try and diagnose my Colour Classic - then comes Adrian Black with some serial commands I can run! Amazing! This will open so many doors. I still haven't diagnosed the CC, I suppose if the TechStep will come up with some unhelpful errors like the Bus Error, I might be able to dig a bit more into it. Thanks for that Sir! I'll mention your work in my video of course.

[@8bitwiz_]

56:11 Learning that BERR is externally generated is not obvious, but an important thing to know about the 68K hardware. Normally the 68K waits until it is told that memory is ready. This is done by asserting DTACK after whatever wait state time that the memory needs to respond. Since some memory is slow, and I/O ports even slower, the CPU only has to wait as long as is necessary for a particular address. But if the address is in some wasteland that has nothing there, and the chipset doesn't handle it, the 68K will wait forever for either a DTACK or a BERR. In order for BERR to happen, something external to the CPU either has to know all the invalid memory ranges, or count a bunch of idle clocks with no DTACK. For instance, on the Sega Genesis, there is no circuitry at all to generate BERR. If you try to access certain invalid memory ranges, the system locks up waiting for DTACK. In the case of the Sega, it could probably be bodged in with one or two chips, but is only necessary for a development system. Real games don't have a problem with this because they've already been debugged to not access invalid addresses. Conversely, there is a meme of "DTACK grounded", where the DTACK pin is hardwired to always be asserted. This causes the 68K to run as fast as possible. For the original 8MHz 68000, if you use good fast SRAM and fast I/O chips, you can get away with this.

[@adriansdigitalbasement]

Thanks, great insight there. And the Genesis analogy is perfect -- it's not like it printing a BUS ERROR crash message on a production system would be useful, so I can see why they didn't even bother with it. The 68k really did bring some cool advancements. I hardly ever work on any system based on them, so I'm just not familiar with the system design but indeed, it is kind of essential to learn about these signals. Reading Apple docs on the Mac II line (which the SE/30 is) it introduces wait states for particular regions of the memory map, so I guess the GLUE does this by holding back the DTACK and counting cycles. Awesome!

[@seancurtin6103]

Came here to basically say this. And on 32 bit systems _DTACK got split into _DSACK0 and _DSACK1, to acknowledge the various transfer sizes that could be initiated. RAM itself does not generate any kind of acknowledge signal, so it was up to the DRAM controller to do this. With static RAM and ROM it was pretty much up to the address decoding hardware to guess based on the part timing used. Quick and dirty is to just OR (active-low) your _CS and _AS together and bump it through an inverter a couple times for the propagation delay. SRAM at least will likely have data ready long before the bus cycle is complete. For a bus error on a machine like this, the first place I'd look is the address decoding.

[@jasonatkin6787]

I passed on a complete Apple TechStep at a flea market for $40 about 15 years ago. Still kicking myself to this day for that one.

[@emily1743]

Can confirm. The school I was attending in 92-93 had an SE/30 that broke a lot. Like, in the two years I was there and using it, it broke three times and needed a motherboard swap. They asked the dealer what happened to the old boards and the dealer said they had to send them to Apple for credit.

[@martinh8481]

Apple still works the same way. An analogy would be the core charge that car part suppliers use, which gets refunded when you return the old part (core). The biggest difference is that Apple doesn't charge you the full replacement part price, unless you don't send the bad part back in their return window.

[@williamgreen5575]

This diagnostic mode through the serial port looks like a perfect job for a raspberry Pi or even an arduino! It only needs to send a series of serial commands and display the result. A cheap DIY Tech Step is only a small step from here.

[@MR-DCB]

This is really a fascinating video and some of your best work yet! Well done and thanks ever so much for sharing with the rest of us.

[@rvenden]

This is Adrian working at a whole new level! Excellent forensic work!

[@andrewb9830]

Adrian for the Win!!! You need what?!! An Unoptanium ridiculously expensive piece of diagnostics equipment. Adrian: "One sec. Hold my Beer"

[@douro20]

That tool probably cost the dealer as much as a Macintosh II back then.

[@Mediateritorist]

Dear Adrian, this video was like a good crime thriller!!! You really are the Sherlock Holmes of computer hardware errors. Great - and many thanks from Germany! Georg

[@gvii]

I have my own opinions of Apple's whole repair/warranty system, but that's neither here nor there. But I always did have a huge fondness for their early and mid-life designs. They were pricey, but once you got into something and started farting around in there, you could see where a lot of the money went. Some of the case designs really were top notch. The old Power Macs I just adored. I had one I kept going long after it was useful just because I found it such a cool thing. I can't tell you how many times I had to repair that motherboard, which I did purely for the fun of it. At one point it did get to be past the point of being repairable, and I set it aside eventually tossing it when I finally sold my other home. But that's just babbling. It's incredibly cool that you were able to poke around and put this together to get out there. I'm sure it was a fair bit of work trying to figure out all that you did even with the help of that older document. But it's really fantastic that you were able to. I'm sure that's going to make a lot of hobbyist's lives MUCH easier being able to do some of the things that you would otherwise have use that ancient, insanely rare and expensive diag system to do. I always find this stuff incredible interesting, especially in the case of a company like Apple who has always kept their ecosystem played very close to the chest. That's why I really enjoyed the episodes on that dual processor Unix system. That whole thing of just happening to unknowingly acquire one of the company's internal systems that just happened to have one little typo somewhere in the past that accidently preserved so much information about everything on the hard drive was just an awesome thing to watch and learn about, even if the system is largely irrelevant in the grand scheme of things. So digging this diag stuff up is super fascinating to me. Plus, it's a huge win for everyone out there still tinkering with these machines.

[@AcheForWake]

Watched from beginning to send, great work sir! I feel you are on the road to a booting system. Looking forward to the next installment

[@reinoud6377]

37:58 as for the 24-32 bits being used for 8 bit devices comes straight from the processor being big-endian. It will store its MMSB in the 1st byte and its LLSB in the 4th. So an ldr.b instruction thus reads a byte from the 1st byte adressed and has thus to be on the bus as bits 24-31 as this is mapped to the LLSB. One consequence is that if you read a 8 bits from a 16 bits stored value you get the MSB where for little endian machines this would still give you the LSB part.

[@jjock3239]

I had a friend that was an Apple dealer in a fairly small market, and what you are describing about warranty repair work, , is the exact reason he got fed up, closed his business, and went to work for Home Depot.

[@vhm14u2c]

Thanks Adrian for putting this video together. I have a Mac classic that starts with zigzag line across screen. Hopefully I can find out more with a working pc running terminal software using null modem cable. Thanks again🥂

[@emq667]

I’m fine with 2 hour videos! Looking forward to the next part. Great work!

[@aetch77]

You ARE making a living repairing computers.

[@duncanny5848]

Really fascinating process. Love it. Looking forward to part 2

[@spewp]

Outstanding video and analysis. Kudos, Adrian! What a cliffhanger though! 😆

[@Mugenmasterman]

I'm fine with two hours. Can't wait for part 3! Awesome video!

[@yohannwilkerson6058]

Seems like that doc would make some good Wiki content.

[@adriansdigitalbasement]

I'm hoping the info here can be moved to something better (like a Wiki that can be updated by the community and augmented.) There is tons to discover here, like more snippets of assembly to do better and specific diagnostics and then also to better understand the differences in the ROM diagnostics from Machine to Machine. Lots of work to do and I feel I have only scratched the surface here.

[@mikebarushok5361]

​@@adriansdigitalbasement16:50 is an archive of a Wiki

[@EyeMWing]

Its particularly fascinating that Apple bothered shipping the techstep and diagnostic mode at all, because they were indeed basically useless outside of Apple’s own engineering departments because nobody was authorized to do board level repair anyway, and if they did some anyway they couldnt get parts and thus were limited to fixing solder joints and adding bodge wires (or replacing the occasional commodity chip). They had to build them for internal engineering purposes, but leaving them In the ROM is pretty fascinating.

[@lancenutter1067]

This Does Not Compute channel has the guy building a complete SE30, even with a new motherboard sourced from Germany. Pretty cool.

[@FrancisFjordCupola]

Awesome news. Although I never had Macs, I can imagine some being saved from the scrap heap.

[@twocvbloke]

That analogy of someone asking what's wrong with their car without having seen it, that actually happened a lot when I repaired PCs for a living, someone called the shop and asked if I could tell them what was wrong with their computer over the phone without having seen it, and of course, I couldn't, had to explain to them that without having the computer in front of me to diagnose any faults with it that I couldn't tell them what was wrong as it could be anything from a bad OS install to failed hardware which I couldn't guide them through over the phone...

[@mikebarushok5361]

The one I remember most was a phone call that began with "I can't print!" It was caused by a full hard drive.

[@kaitlyn__L]

@@mikebarushok5361 for me it was a complaint of a key not working. After fighting the keyboard case and plate, eventually locating the pins, I find it was fine! Turned out to be a locale issue in the OS.

[@IlBiggo]

It looks a lot like the debugger you invoke by pressing the Programmer's Key. I never knew you could reach it from the serial port. Nice find, Adrian!

[@Electronics-Rocks]

Brilliant! I knew it existed but did not have an idea how it worked & did not have access to a Mac for long so it was board swapping only!

[@delmonti]

.....better than an episode of Columbo!......to be continued....cant wait!

[@johnwells558]

I always take a little bit of you on all my repairs , thanks John (uk)

[@svenax]

That was some awesome investigations. Kudos! Really looking forward to then next episode.

[@flashjazzcat]

Fascinating video. Once you get your teeth into a problem like this, you don't let go.

[@nysaea]

I'm supe rexcited about this and I don't even have a compatible machine lol The whole community is going to have a field day with this!

[@lynyarwood7977]

One of your better Videos I was spel bound to the end.

[@infamousacidrain]

Adrian, destroying the secondhand market for techstep. Big Techstep will not be pleased! /s Excellent work sir.

[@JibunnoKage-cj2kz]

Inside Macintosh IS ONLY a programmers reference. It illustrated the applicable Macintosh OS APIs to support software development on the early generations of the Macintosh. Once Apple moved to N.E.X.T object oriented OS as the foundation of the newer Macintosh OS, Inside Macintosh was effectively obsolete. And you are correct, you had to send Macintosh hardware back to Apple, and they destroyed what they could not repair. Worked for Apple for a few years in the mid to late 80s at the high tide of Apple's hardware inventory control methods.

[@aplund]

Yay for the great diagnostic mode. Boo for the "to be continued...". Won't be able to sleep for a while.

[@EpsilonsReviews]

Wait a sec, those commands are similar, if not identical, to the diagnostic screen you have within the Mac’s operating system when pressing (Command+Power?) and receiving that white command prompt! Or, if you have the diagnostic init installed using the NMI/Combo!!!!

[@lindoran]

Absolutely bonkers!! Just amazing

[@Keith-g2b]

i shall call you Sherlock! that's some good investigation work.

[@AnthonyZenrick]

Two hours is fine. I'd love two hours. After you release the second part, wanna drop the Long Play? Two hours would be perfect! I'll take long plays of all of the series!

[@lightmagick]

When the video gets over an hour long I'm totally fine with a cliffhanger.

[@krandall5285]

Best ADB in a while, bravo

[@me0262]

13:40 It's called a core charge. With a core charge, Apple expects the bad logic board back. While the logic board is only $565 and you have to give the old board back (even if it had liquid or physical damage), Apple charged $1165 if you wanted to get a new logic board. It was a deterrent so you couldn't build a frankenmac. 16:45 I'm surprised that Wayback was accessible this quick after it was DDoS'ed and hacked.

[@oliverw.douglas285]

I like the concept of keeping the repairs relatively simply with off the shelf equipment. That being said, I would also be interested in advance repairs, using reverse engineered test setups or 'test jigs' that were originally used by Apple. I've found that manufacturers have deployed some interesting methods for troubleshooting, that can be used to our advantage, when servicing legacy equipment. Motorola had Lab Tools, while challenging to get, was very handy when servicing some of their radio equipment.

[@zrodger2296]

"Unlike a 6502 which doesn't really know if the RAM is there or not." I found this oddly funny! 😂

[@desiv1170]

"and on that cliffhanger..." NOOOOOOOOOOOOOOOoooooooooooooooooooooooooo!!!! ;-) This is an awesome vid and I love the serial diagnostic discovery/work! This is super interesting!!!

[@ToxicwasteProductions]

Funny sidenote when you say star-r that reminds me of the boss in first resident evil that goes STARS. Also gives flashbacks from airplanes in a real airplane they have a delay. So it says Stall-All kind of like a echo effect. If you pull fuse 17 or 19 I think in the fuse box of the 747 it doesn't chime several warnings and stall sounds normal. Not like standard stall-all Loving your videos. Your brilliant.

[@der.Schtefan]

More thrilling than a true crime story

[@HalfMonty11]

1:00:40 I like the transition to Adrian the Grey, full diagnostic wizard at the end

[@MonochromeWench]

Being able to send and execute Machine code changes everything. Sure Machine code is not user friendly but it opens up the entire system and gives the ability to execute your own tests. I could see mac enthusiasts creating a small library of Machine code tests for this Null modem cables... we used those for Doom and C&C Multiplayer before we got Ethernet cards. Re people suggesting looking at other videos. Video not being initialized on a system not booting almost never indicates any specific failure on any system old or new unless the post steps before video is initialized are very limited (I see people all the time ask how to fix a black screen on a non working computer when the problem could be anything that causes post to fail ). If a system boots but without video that is different and is probably bad video circuitry.

[@BigBadBench]

This is really useful information, thank you!

[@christianbauer6510]

This “test mode” is called the Serial Test Manager (STM), and the source code to a later version of it is included in the SuperMario sources (OS/StartMgr/UST*.a). So it should be possible to figure out exactly how all these tests work. 😉

[@JonathanSwiftUK]

Gripping, looking forward to the next video.

[@ArchangelTirael]

I can't even imagine that this video might not have existed at all and we can be stuck probably indefinitely with these broken motherboards if you tried to found that initial Diagnostic Mode protocol research a few days later then you done, after the Internet Archive got hacked and down.

[@HaveYouTriedGuillotines]

The moment you were leading up to showing reads and writes working, not even after but as you were heading in that direction, I sighed and said "The memory writes work, and it's something to do with the GLUE chip, isn't it?" It's a pretty obvious conclusion that if your RAM is working but you're getting RAM errors, either the MMU or the GLUE chip is somehow at fault. And you disabled the MMU, so... That only really leaves the GLUE chip. It's a good thing that it's just a trace somewhere, and not the GLUE chip itself, because the only way to get a replacement GLUE chip is to cannibalize another board.

[@RETROMachines]

I love mac videos like this. That's because I didn't have any mac...

[@richardkelsch3640]

If the Mac doesn't echo commands, then set the terminal to half duplex.

[@itstheterranaut]

Wow, I forgot all about Laplink....where did my cable go? Long lost. Also, interesting to see that the computer designed with a GUI in mind had a CLI of sorts underneath it.

[@adriansdigitalbasement]

I have to wonder if there are sellers still for these insanely useful cables?

[@itstheterranaut]

​@@adriansdigitalbasement there are things that claim to be, but aren't. Interestingly, though, I see a boxed copy with cable on the Bay right now for £25. Ooh.

[@rommix0]

@@adriansdigitalbasement I'm sure. I recently bought an old school null modem cable off of eBay to use with an old BrailleNote speech synthesizer.

[@mahtin]

CLIFFHANGER!!! But I love the journey.

[@Godzilla941]

...and memory writes over the serial port work since those are all 8-bit transfers... if anything of mine ever breaks this way, I'd NEVER figure it out... (A couple months ago, the only way I was able to get files from a "modern" computer onto a 386 was Microsoft's terminal program du jour for XP (in a VM) and Win 3.11 using Kermit at a whopping 19.2k baud.

[@RetroHQ]

Interesting. I’d be keen to see what the BERR input is to the 68K. Normally if you have a bus error you’d get an exception on the CPU and it would halt execution, and it’s obviously executing code ok with the screen animation. I’m wondering if the ‘bus error’ is actually being reported by the glue somehow?

[@fnjesusfreak]

"Simasimac" - probably comes from Japanese "shima-shima" meaning "striped".

[@Plarndude]

I love Google Docs, Sheets, etc. I think this is awesome.

[@logipilot]

Lots of usb equipment on PCs uses serial Protocols. I often have to snoop the starting sequence and commands with 2 teensys and the arduino serial monitor.( I m talking about controllers for stepper motors or spectrum analyzers and stuff.)

[@schonja9]

A thriller! Really high level stuff right there. (pun intended?)

[@TheDefpom]

Great work Adrian

[@Eyetrauma]

Local man so irritated by motherboard faults he starts reading and writing arbitrary data to ram and updating the screen himself, says “fine if the OS doesn’t run I’ll do it myself!” 😆 For serious though awesome analysis Adrian. Makes me wonder if a ROM dump exists of that diagnostic station. That combined with an emulator might make figuring out what all the options do easier.

[@adriansdigitalbasement]

Haha! Believe it or not it would be possible to bootstrap a machine over serial even, like boot a RAM disk. Would be cool for people who have no way to make floppy disks.

[@smcic]

This is awesome thanks Adrian!

[@Mr.MinerAL]

Japanese phrase "Sima sima" means "striped." So Simasimac (properly SEE-muh SEEmac, but often see-MAZ-zee-MAC) is the old Mac-mod community label for a (originally specific, but now seemingly any) striped pattern on a compact mac screen at a failed bootup.

[@stub1116]

This video is really awesome.

[@DanielRodriguez-ff5cs]

Thanks!

[@jasonwhite1957]

This is extraordinary!

[@kaitlyn__L]

Oh! I know where “Simasimac” comes from. It came from the Japanese Mac community, and “shima-shima” means stripey. So when you have a broken Mac showing that screen, you’ve got yourself a “shima-shi-Mac”! (A stripey Mac!)

[@rimmersbryggeri]

Could you make a dignostic apparatus from somethign like a pi pico that could do this.

[@jeromewink557]

The null modem cable could also be used to play Duke nukem 3d on two computers together without NICS and without using a phone line.