The Field Found Amiga 500 broke again and it was all my fault!
Time to continue working on the Field Found Amiga 500. In this video let's try to run some software on the computer to make sure I actually fixed it in part 1. Unfortunately, this work didn't go exactly to plan!
Part 1: • This Amiga 500 was lef...
Part 2: This video
Part 3: • From Death's doorstep ...
-- Links
The Field Found 64: (the video that started it all)
• Commodore 64 left outs...
Amiga Test Kit: (Disk based diagnostics)
github.com/keirf/amiga-stuff/...
Amiga Diagnostic ROM:
www.diagrom.com/
Amiga 500 Schematics:
www.amigawiki.org/dnl/schemat...
archive.org/details/system-sc...
Adrian's Digital Basement Merch store:
my-store-c82bd2-2.creator-spr...
Adrian's Digital Basement ][ (Second Channel)
/ @adriansdigitalbasement2
Support the channel on Patreon:
/ adriansdigitalbasement
-- Tools
Deoxit D5:
amzn.to/2VvOKy1
store.caig.com/s.nl/it.A/id.16...
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.com/products/digi...
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.com/item/33000...
TS100 Soldering Iron:
amzn.to/2K36dJ5
www.ebay.com/itm/TS100-65W-MI...
EEVBlog 121GW Multimeter:
www.eevblog.com/product/121gw/
DSLogic Basic Logic Analyzer:
amzn.to/2RDSDQw
www.ebay.com/itm/USB-Logic-DS...
Magnetic Screw Holder:
amzn.to/3b8LOhG
www.harborfreight.com/4-inch-...
Universal ZIP sockets: (clones, used on my ZIF-64 test machine)
www.ebay.com/itm/14-16-18-20-...
RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)
www.retrotink.com/
Plato (Clone) Side Cutters: (order five)
www.ebay.com/itm/1-2-5-10PCS-...
Heat Sinks:
www.aliexpress.com/item/32537...
Little squeezy bottles: (available elsewhere too)
amzn.to/3b8LOOI
--- Links
My GitHub repository:
github.com/misterblack1?tab=r...
Commodore Computer Club / Vancouver, WA - Portland, OR - PDX Commodore Users Group
www.commodorecomputerclub.com/
--- Instructional videos
My video on damage-free chip removal:
• How to remove chips wi...
--- Music
Intro music and other tracks by:
Nathan Divino
@itsnathandivino
Thanks so much for leaving in these types of troubleshooting issues. Breaking the social-media norm of only showing the prettiest parts of life -- reality is much more instructive!
Hear hear. And you're dedicated and also likeable Adrian, so we can bare with. Your expertise will only grow the more Amiga XP you gain. :)
I'm just starting this vid but one thing that never ceases to amaze me is just how durable integrated circuits are.
Yeah, especially when they fall on the floor, pins-up, then you step on them with bare feet. 😮
it's shocking for MOS chips considering those aren't particularly reliable
Fancy seeing you here. :)
@@TrolleyMC it's just the MOS-technique MOS-brand chips which tend to have reliability issues, MOS's CMOS ones were usually fine (I don't actually know if the Amiga has CMOS inside, but that's the pattern. Okay I just checked and Amiga used NMOS, which should also be a lot more reliable.)
Yep, I have plenty of 40+ year old ICs still running in one of my old fruit machines, despite it having spent 10 years in a damp unheated garage.
I love that you strive to get to the root of issues instead of just changing stuff, I love knowing why something wasn't working, even if it takes ages to fix, it's interesting to know and makes a great video IMHO : defo not a waste of time 😊 Very impressed that Amiga still works, gives hope for other dumpster found rusty miggys. Cheers!
Thanks! Yeah if I weren't making a video about fixing this, I might not go about it the same way -- and if I was trying to repair it quickly, I likely would have just put deoxit in every socket and/or replaced them and called it a day. But that doesn't teach anyone anything :-)
@@adriansdigitalbasement thanks for intentionally doing it differently to actually teach us your methodology.
It's so satisfying to see a genuine 'field found to working' video, with so much fuss generally (not necessarily retro computing) about fake restorations. With you we know it's real and it's always so educational too.
Not only your videos are informative, I love your relaxed and yet humorous approach to things. You also have a clear speaking voice, I live at the UK and I'm hard of hearing and always have subtitles on, but in your case that wasn't needed. Keep up the good work as always.
second that!
That extension memory card did not test all well. You can actually see the diagnostic program reporting an error in the video. Thanks for the video and for leaving the bodge wire problem in, very educational!
“Deoxit in the socket.” That goes together perfectly. 😀
Those who do nothing do nothing wrong. Love watching retro repairs.
Your videos are, in a single word, captivating. I love that you show your train of thought when troubleshooting the older machines and leave in faults like your bodge wire mishap.
"My fix can't have caused that problem" I have heard that before, I _may_ have even said it myself... Thanks for being honest!
Take a look at 47:21 while you were testing the expansion memory. It said "Bad Block start at $00C135F4". I know absolutely nothing about Amigas, so that may not mean anything, but it seems suspect to me.
Heh unfortunately it's not helpful, just a memory address. We're looking to find out which bit is bad at a particular address.
@@adriansdigitalbasement I just mentioned it because right after that you put up a card saying the memory tested good. Then you couldn't get it to boot with the expansion card plugged in.
I dive into all of the trouble shooting of the RAM card issue in part 3 :-)
@@adriansdigitalbasement I figured as much. Looking forward to it.
A new movie is out: "Murder on the Omiga Express" by Agnus Chipsies. Thanks for a great zip around the Amiga schematics... as a non-electronics person watching you decipher the clues, makes me appreciate that someone (a team) put all this together in the first place and made it work. Good stuff!
I did camera repair through the 90s at a small shop and my boss was an electrical engineer. He taught me everything I know about general troubleshooting. Now, I'm definitely jealous of yours and his understanding of what the circuit is actually doing but you fell into an easy trap. If you change something that hurts the circuit, the first thing you do is remove/fix whatever you did! My boss would have told me to remove the bodge and reconnect the cut traces, swap all the chips with known good ones and see if it returns to the earlier state. It would have saved you a LOT of time.
Yep. Returning to the last working state is a common troubleshooting step.
This is the kind of repair where you can use dead chips from the scrap box to use as "socket cleaners". Though you should get an actual chip pulling tool that grabs a chip from both sides to use for this kind of repair. So using dioxit with these types of corroded sockets and the dead chips would allow you to more easily diagnose and repair your boards without having to solder in new sockets. Just make sure to re-tin the chip legs to clean the corroded parts. Also a good idea would be to get a plastic pan large enough to submerse a large motherboard so you can clean it up with Isopropyl alcohol and a clean soft bristle paint brush.
I literally just finished watching the first part - the tension was real hahaha - I should make popcorn for this one haha
💯 I'm making some snacks now and praying the old baby remains healthy and working.
That’s your best video in ages Adrian! These kind of troubleshooting ones are my absolute faves 👍👍 nicely done 😎
Thanks for being so transparent, Adrian. I don’t know how many times I’ve changed something in a circuit or (more frequently) my RTL code and broken something else I thought was unrelated. This is a great video and I am thoroughly enjoying the content you create!
Excellent teaching video. I like watching you talking your way through your logic process as you work your way through the schematic. I found it very helpful.
Adrian posting an Amiga video, complete with a 16 bit dance party! Life is good! 🤩
binging this channel. can't pretend to understand a lot of what you're talking about but just watching your process and listening to your presentation is really engaging
I've done console mods for th Wii and Xbox 360 and you should always make sure that the exposed wire is no longer than necessary. Even try to trim after soldering if there is excess exposed to prevent the type of short you had. I know all too well the rabbit hole you can get into when you think an issue has nothing to do with something that seems completely unrelated. When I went to a trade school to learn more about repairing computers, during my midterm exam I was having issues installing Windows NT where it couldn't copy and expand a particular setup file. So I tried the usual fixes like using a different installer CD, changing the CD-ROM drive,etc. The setup still kept failing on this particular file. Turns out the issue was the RAM location the installer was trying to copy and expand the file was defective and I guess the installer was hard coded to use this specific RAM location for this file and not try to use the next available free memory address. Everything's connected even it doesn't initially appear so. Many years ago I was working on 2 PC's and I only had 1 power cord so I unplugged the one computer and plugged it into the other and the second I turned on the second computer the motherboard from the first blew smoke even though nothing was connected to it. It could have just been a coincidence that it happened when I flipped the power switch on the other computer and there was some residual energy stored in a capacitor and something failed at that moment but I will never know. Keep bringing us these videos. Love it.
That's very interesting! I at first instinctively did that (trimming every leg and wire right up to the solder ball) in electrical engineering class, but the teacher admonished me that it makes it too hard to make changes later and I shouldn't be making any designs where that's necessary anyway. Clearly many actual manufacturers are "sloppier" than my teacher would like! I know thin trace widths are kind of unavoidable with higher than 8-bit systems, so it makes sense. But if I routed anything that close to a pad in the PCB-making part of the course, he'd always tell me to move it!
I honestly don't have a direct interest in retro computers, however, it is so much fun watching you problem solve. You have good resolve and are very passionate.
Awesome video. I really enjoy watching your process and progress, especially the way that you deduce what could be going on when something is not working. Great work! :)
So entertaining. I’ve watched pt 1 and 2 in full. Loved the tech knowledge and your passion. I have a Vic 20 c64 ,and Amiga 500 in my attack. Looking forward to part 3. 🎉😂❤
Love watching this channel. I have an Amiga 600 and 1000 and watching for techniques to repair them I never had an Amiga, but I have a working TI994a, apple iic, c64 and atari 600xl. All machines that I used or wished I had when I was young. Appreciate the care of these old machines
It is everytime s pleasure to follow your diagnosis and see you repairing. Even if you caused a fault rhe diagnosis is very valueable. 😊
That's what i like the most about your videos - honest testing, learning failure by failure and finally (most of the time) success. Feels awesome, thanks for creating this style of content for us.
Wow good sir I am loving your approach to this and am learning a lot in the process, you are confirmed brilliant. Very very impressive work and thanks for sharing it!
That OVL line sounds like it performs a similar function to what the Z80 processor in the C128 does whenever a C64 cartridge is inserted/Commodore key is held on boot, it asserts itself before the 6802 has a chance to do anything, it sets everything up to switch from native C128 mode, then it jumps to the reset vector, thus allowing the computer to boot "normally" into the C64 mode, something it couldn't do on its own. Always fun watching you troubleshoot issues like this Adrian, good work!
I bought my first Amiga 500 in 88. I so loved that machine and honestly it’s the only one I miss to this day. Wish I could get another but fixed income can’t afford it. Least I can watch you.
I understand very little of what you do in these videos but I am absolutely blown away with your determination and perseverance. I have similar traits with some things and it is sometimes a burden. I watched this and part 1 of the Amiga 500 videos purely because I used to own one back in 1988-1991. It was a great machine and I passed many an enjoyable hour sitting in front of it. I am looking forward to seeing part 3 and beyond, and I will subscribe and watch more of your content. Thanks for sharing.
It's amazing how resilient it all is. Get rid of the inevitable corrosion and it works. Now I am regretting throwing away my first computer (not an Amiga) which suffered some corrosion on its main board. I never would have thought that it might have worked with a bit of care.
Congrats on getting that basket case recovered. Impressive diagnostic technique. Loving this field find series!
Adrian, it is such a pleasure watching your methodical work style. I haven't seen anyone else do this like you do it. And it was such a joy to see your reaction when that music played. I love your videos
I really wished that "methodical work style" included a double check of your own work if something didn't work after a change.
It is so entertaining what you do. I sure do not understand everything but i try to follow. And the fun that you transfer is so pleasant. Thank you.
Oh boy. Been on such a rollercoaster myself on a c64c. Put in a socket for the SuperPLA, and 'lo, there been a solder bridge UNDER that socket. geez. Took me about two weeks of tinkering and thinking on and off about the problem to finally find the correct question and subsequently fixing this problem. Fixed it and the machine started working like a charm. But at first ... nooooo, I haven't messed up anything, no, not at all, this has to be summad else. Not my work, not at all. Nah. was my work, so, I feel ya, man! And GZ on finding it. :-)
Interesting restoration and learning process of the Amiga 500 :) I've got a feeling Amiga is pretty robust system, as your Amiga still works after being exposed to water and all the elements for who knows how long. Just clean some corrosion and it works, pretty amazing :) Also my own Amiga 500 still works, while some newer devices have needed some repairs, and even failed completely. Well, at least last time I used it couple of days ago. At this age, every time it may be the last time it works, haha :D
Thank you for troubleshooting out loud. Thank you for showing mistakes and how you work around them. This is one of the things you do very well.
Great video,shows how much fun you can have picking up thrown away stuff taking it apart and playing doctor.I think most of your viewers had the same feeling of joy,thanks Adrian
I recommend evaporust for the rusty components. Its like magic.
THIS. And unlike vinegar, it won't eat away at the zinc plating.
I really love your channel, great content. You're so genuine, keep up the great work. Thank you!
I have no idea what you are doing but am captivated by your troubleshooting 😊
As soon as I saw that the diagnostic was reading back the same thing I knew it was reading rom but I probably would have never noticed this issue and just assumed it was a board failure that I couldn't fix. Great work.
These are the best kind of videos. 1) a likely hopeless cause brought back to life. 2) You get to learn new tricks. 3) last and best of all, we get to learn new tricks!
loving the series. something very special about hearing that mod start playing
I think you are very smart. Love the steps you show. Things that make me think things thru. Nice work Adrian.
Learned a lot from this one. The ROM/RAM overlay methodology is fascinating. I had no idea.
Thanks Adrian for another nice and relaxing video. Nice job! Amiga machine are complicated to debug. If it can helps, I usually change almost every socket, electrolytic cap and 7805 (if present). Re-tin solder points of no-socket chips, can avoid cold solder point. Your skills are mind blowing!
Wow, it's amazing what you had to go thru to get this old computer to at least function !!
awesome job Adrian hey simple faults and mistakes happen , have a great thanksgiving coming up and amazing weekend.
Thank you for leaving in the mistakes and entire troubleshooting process!
The A500 was my first proper computer and what I learned to program on, AMOS and 68000, so nice to hear the chip names again :) I think I still have a copy of that blue book The Amigas Programmers guide in the loft hehe. Really, really enjoying this series of vids on the Amiga, many thanks
If you made Adrian's Digital Basement t-shirts carrying the slogan "It's freaking working", I would buy one.
I thought you were going to get up and dance to that mod Adrian!!! :) great fun!!!
Lovely. Again I learned a lot about the Amiga architecture. This will come in handy at some point for sure.
Always check your work properly if problems arise ^^ Amazing video still, thanks for making it
Hurrah!! How cool to see this come back to life, how awesome! Miss my 500 :)
Kudos for not swapping out those sockets. I'm not sure I would be able to resist!
There's no 'Kudos' involved, in this case it's all too obvious that the crusty sockets are causing problems and the use of Deoxit is a temporary patch, if that. Those sockets should have been replaced BEFORE Adrian started diagnosing the board issues. Even though Deoxit seems to have 'fixed' them it's only temporary.
I appreciate the fact that you share your mistakes almost as much as your Rocky Horror t-shirt
I like that you are literally keeping these old systems alive, I always feel its a tragedy when people throw them out to rot somewhere. I wish I could read schematics.
I'm so glad that you saved this piece of computer history ❤
I know people suggested this in your last video, and I do love seeing the tinkering around, it's fun and you learn something (hopefully).. BUT yeah maybe next time you find an old corroded Amiga or any other old computer FIRST you de-socket all the chips and clean them.. Love the videos.
Exactly. That'd be my immediate first step.
Another great video, you learn alot when you edit your videos. I like that you don't edit them out.
A bit of advice from an ' experienced bodger ' . When fitting bodge wires to a chip ( or resistor , or anything ' through hole ' ) I tend to feed the end of the ' bodge wire ' down into the hole beside the pin , this does two things 1. Allows for better contact with the pin and 2. there is no loose end sticking out to possibly cause a short ( as you had ) . Maybe you could pass this on to other people via your channel and help stop people having problems like this in the future !!
OMG, a bogded bogde wire! A bit of bad luck there. Glad it was sorted.
Agreed, the 1.3 workbench hand should come up after about 0.5 after the light grey screen, I noticed this long delay in the previous video and knew something was still amiss.
Very educational! I'm happy to see one more working A500 in the world.😃 Maybe you could get a copy of the Amiga A500/A2000 Technical Reference Manual.
Adrian, the long trouble shooting hunt was awesome. It was like a "who dunnit" movie. I hope you will always do these long form types of videos. Regards from South Africa
Yeah, I know this feeling - my first C64 repair required replacing a bunch of the 74 logic chips and I did that before i owned a heat gun - I lifted so many pads on those I had to add five or six bodge wires to the back of the board. I managed to get the machine running asgin - but it's never been particularly stable since. May that's not casued by my bad desoldering, but I wouldn't at all be surprised if it was.
With a lot of lifted traces it could as well be, unfortunately. Try bending the board just a little bit while running a diag test. It may be enough to find the problem. But also check if power is fine, and capacitors work, especially when i.e. sound is playing.
Sometimes the extra capacitance in all the bodge wires can juuust make the difference for TTL thresholds/timing coordination, as well.
@@kaitlyn__L On modern devices - absolutely. Be very careful with PCI, USB, etc. USB in particular doesn't like traces too far apart - learned it the hard way... On retro-stuff - in theory it could be a problem - but in practice I have never heard of it causing any so far (as long as the bodges are soldered properly and not with a huuuge wire). What it CAN cause is crosstalk like video signal degradation or audio noise unless you use shielded wire or carefully routed where it matters. This happens and a lot. Mod boards may or may not take that into account.
@@jwhite5008 the only time I’ve seen it come up for real has been some Ben Eater videos when he showed really long unshielded cables for a data bus, so yeah I was mostly thinking of the “huuuge wire” lol.
@@kaitlyn__L Fair enough but that could hardly be called a bodge wire unless you want to fix an issue with an system-bus-daughterboard in another room or something....
it even said BAD PAULA on the diag screen........... IT TOLD YOU....
Thanks for very interesting hour spent here!
Adrian,,,, your videos are amazing and your explaination are exzelent. 51:48 The music 😍😍😍😍😍😍😍😍
If it offers any comfort: I have done similar things as well. Even with being certain not having broken it by a modification, if things appear to break just right afterwards, it is worth a try to reverse the modification. Debugging is craftsmanship and sometimes art, but sometimes things just don't to turn out pretty.
"There's something going on with Gary" - sounds like the beginnings of a musical.
Very thorough insight into how the Amiga works. Though I bet you weren't pressing the "any" key...
Hi Adrian. I also live in Portland,OR I think were close to each other. Anyway. Your oscilloscope should be able to save traces lines, that is, check it with a known good, or a Gold board. In my experience it also good to have as much info as possible before I begin. It's good idea to eliminate any possible problems in the beginning. Since this was in so bad a condition I would have changed the connectors out first. Then you would have a known good quantity. And the via are test points to use an auto-tester on. Good documentation should have photos of important traces lines. Have a good one!
Another fine repair video. I've always wondered if your workbench is ESD safe somehow?
Not to add insult to injury, but those Rev 5 ROM adapters I gave you at VCF Midwest might have averted the bodge issue. 😁 j/k Enjoyed the video/process as always. Take care!
Adrian, you know you can always holler at me and I’ll help ya. Rule 1: if you changed something and it stops working blame your work before anything. I have fallen in this trap so many times. I am happy to see you fixed it, but technically you only half fixed the rom issue. You’re really supposed to add some resistors to pull the lines up or down…always forget.
Death by bodge wire. That feels good! says Adrian. Well done! Most enjoyable. One step forward, two steps back.
I love how listening to the audio test is your victory dance. Can totally relate!
Whenever I do repairs to anything, I always assume that I screwed something up :P :P and then double and triple check my own work, with soldering it is much more challenging to trace and see where you might have cross shorted a line, usually just charge one of your lines and see if it is jumping signal to anything else in the area.
Those rusty connectors deserve their own place on your display wall up stairs. Looking forward to part 3 on the field found Amiga!
This machine has a sense of humour and likes you! It was just messing with you with that short! 😆
This little Amiga is a survivor. Counted, but not knocked out. Insert Captain America "I can do this all day-meme" here....
A very honest person! Hard to see nowadays.
Little bugger of a thing, a tiny bit of solder mask missing right there next to the pin you soldered to. I bet you could do that same mod 10 more times and not get the short. So frustrating. Very cool to see that poor Amiga running again!
This is honestly the first time I've run into this particular issue with a bodge wire -- and I've probably soldered them onto boards thousands of times. So yeah stuff like this is a learning experience!
Not only that but I think Adrian was correct in that this one particular corroded machine has brittle solder mask so on every other machine it would probably not even be an issue
Keep doing the mistakes! Makes the channel so authentic! Thanks
Nice video but at 47:21 during the memory expansion test it say bad block at $00C135F4, it's actually a good thing ? doesn't it means there is some bad memory ?, maybe i'm wrong :), anyway always entertaining.
Was just going to point out the same thing. It's weird that it didn't increase the error count.
Very enjoyable, thank you!
Adrian fixes stuff like diffusing a bomb. I'd just be slaming chips and sockets in different Amigas until something worked, with no real idea what caused the faults. Great stuff Adrian. Fascinating watching Richard Dreyfuss play with old computers😂.
In this case it's all too obvious that the crusty sockets are causing problems and the use of Deoxit is a temporary patch, if that. Those sockets should have been replaced BEFORE Adrian started diagnosing the board issues.
I have several amiga 500 ... I even bought one with IC upside down (ebay...not working)... and not one IC ever... was killed... the amiga IC are impossible to break... they can outlive you... the amigas will be the only thing working in 1000 years time... Long live AMIGA!
It's really good to see your way of error tracking, other people would have given up on this beautiful babe a looong time ago... ❤❤❤❤❤❤❤❤
that was satisfying! those pesky solder failing solder masks and shorts!!!
As always, the troubleshooting was the BEST part 🙂
To the best of my recollection, all 68000 systems do the OVL trick in order to map the reset vector to ROM. Regarding that short… electrons gonna electron 😊
How interesting! It seems that some manufacturers (*COUGH* Apple *COUGH*) don't really have detailed information about how their systems work, etc... But it seems like, based on what I read, it's totally necessary to get the system bootstrapped.
@@adriansdigitalbasement Inside Macintosh Volume III does cover the use of OVL on the early model Macs (specifically the original 128K and 512K models). From the "System Startup" section on page III-42: "First, the processor is held in a wait state while a series of circuits gets the system ready for operation. The VIA and IWM are initialized, and the mapping of ROM and RAM are altered temporarily by setting the overlay bit in VIA data register A. This places the ROM starting at the normal ROM location $400000, and a duplicate image of the same ROM starting at address 0 (where RAM normally is), while RAM is placed starting at $600000. Under this mapping, the Macintosh software executes out of the normal ROM locations above $400000, but the MC68000 can obtain some critical low-memory vectors from the ROM image it finds at address 0. "Next, a memory test and several other system tests take place. After the system is fully tested and initialized, the software clears the VIA's overlay bit, mapping the system RAM back where it belongs, starting at address 0. Then the disk startup process begins." So one of the bits in VIA (yes, this is a 6522!) data register A (bit 4, according to the table on page III-39) is connected to OVL and causes the ROM that's normally mapped at $400000 to get mapped in at address zero. (It also appears to remap RAM to start at $600000, so the ROM can make use of RAM while in this state. I'm not sure why they bother with this, though, as you'd think they could just immediately to a location in ROM at the usual mapping and turn OVL off before doing much else. But for whatever reason they wait until after the RAM tests are finished before switching to the normal memory map.) I don't think they ever provided this level of hardware detail for most later Macs, as they quickly figured out when they started changing things in the SE and later that developers were hardcoding too many things to depend on the exact hardware implementation in the original Mac and causing software not to run on newer models. So later volumes of Inside Mac mostly only described the supported software environment, with hardware details only occasionally described in tech notes and developers were told not to depend on any of that. Of course they could (and should) have provided separate schematics and repair manuals, but by that point Apple was no longer even letting their dealers repair things so that never happened. 😢
@@adriansdigitalbasement The 68000 start to executing code at address 4. Then it expects some command interpretation error interrupt vectors below that. The Atari ST copies the first bites of the ROM permanently to 4. The GLUE pulls the ROM0 select line for address 0-4 low and the RAM select stays high. With this the first two bites of the RAM are inaccessible in the Atari ST. In the first bites of the ROM is a jump command to the ROMs itself.
On the A1200 (and I guess also on the A4000), Gayle replaces Gary and does not have an OVL input. Instead it will select the ROM for the first 512KB of address space at reset. When it sees any write to the odd CIA, the RAM will be mapped instead. If you search for 'S-100 68000 CPU Board' you will find a design which uses yet another way of handling this. They use a shift register clocked by the address strobe signal to count the number of AS cycles. Initially the ROM will be decoded at address 0 but after 4 AS cycles, that decoding will disappear and only the normal address will work. 4 AS cycles means 2 32bit words have been fetched, being the reset stack pointer and the reset instruction pointer. The second word simply contains the address of the boot code in ROM at its normal address. The first word can simply point to memory.
If they had the ROM in low address space and the RAM in high addresses this trick wouldn't be necessary. This would've been obvious to the designers so why did they insist on RAM in low space? I'm not a 68k expert so the answer might be super obvious to those familiar with it.
Thanks for keeping in the failures Adrian. We all make mistakes that end up costing us, but if we persevere we can undo some of those errors and you proved that here. Just having the humility to admit we are wrong gives us the ability to find it and fix it.
Thanks for great video, i really enjoy watching your troubleshooting, and those "minor" fuckup just makes things more funny to watch. You gotta admit that you for sure going to check more closely and not being that sure about your work. Been there done that! 🙂
Same expression I see on the will-it-run channel guys when they get their field found cars started again haha Yanno it might not be a half bad idea for you to collab with some of them, especially if they're fiddling with a late 80s early 90s car with primitive EFI/feedback carbs. The parallels between the computers you usually work on and the ECU in my 85 F150 are startling! And if you can fix this Amiga 500 surely you could coax life back into an old EEC-IV box.
Can't wait for part 3