Gregory's Serverto get the Amiga to work with high density disks, the Commodore engineers took a little short cut and just spin the drive at half the normal RPM (150 instead of 300). this keeps the data rate the same, allowing the custom chip to remain unchanged.
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92 comments
Tube🌱Time
as is typical in the Amiga community, the mods have been done in such a way to make them hard to reverse engineer. in this case, the rework wire is hair-thin magnet wire covered in silicone. try to remove the silicone, and it shreds the wire.
Tube🌱Time
my guess is that it taps into the BA6986FS spindle motor driver and allows the PAL to slow it down when a high density disk is detected.
Tube🌱Time
the schematic is pretty simple. looks like it does the pin swizzle for the disk change signal, creates the READY signal the amiga needs, but then it does some clever stuff with the index pulse. additionally, it uses the two reserved pins and the unused drive select line presumably to plumb the rework wires.
Tube🌱Time
so presumably the way this works is that the two bodge wires hook into the clock signal from the controller chip to the spindle motor controller (pin 7 on this example). the motor controller derives the RPM from this signal, so the external PAL sneaks in an additional divide by 2 in order to half the RPM. not too shabby!
ShutterBugged replied to Tube🌱Time
@tubetime Doesn't this model of PAL (https://www.alldatasheet.com/datasheet-pdf/pdf/70181/LATTICE/GAL16V8.html) have latches on the outputs, some of which can feed back into the inputs of other latches' combinatorial logic? 
✧✦✶Catherine✶✦✧ replied to Tube🌱Time
@tubetime you're incredibly good at this I could probably do this, but nowhere near as quickly
Tube🌱Time replied to ✧✦✶Catherine✶✦✧
@whitequark thanks! i got a lot of practice with the Quadlink project which had ~10 GALs, some registered. 
Simon Frankau replied to Tube🌱Time
@tubetime How can I make galette less awful? It's currently extremely galasm-like, but if there are things I can do to make it nicer to use, I'm all ears. (The deficiencies are probably obvious, but I'm too used to galasm to notice.)
Tube🌱Time replied to Simon
@sgf ah thanks, I really appreciate your openness! I guess there are a couple of things: - better error messages. I had some underscores in term names and the error I got said "invalid character in line n" but didn't say what the invalid character actually was. - better docs. it's always a pain to write this stuff but it's super helpful for folks. - equation reduction is probably out of scope since palasm was meant to be a thin wrapper around the raw fuse map, but that would be cool. 😃
Simon Frankau replied to Tube🌱Time
@tubetime I worked on some error messages last week, for unrelated reasons - PTAL at Github latest & I'll fix anything missing. Docs: I love writing docs, but don't know what people need, so let me know. Equation reduction is tempting. I was thinking of writing something like a cupl2pld stage so that the pld format remains WYSIWYG. It's lower priority than just making it more usable in its basic form, though. Thanks for the feedback. If GitHub issues work better for you, that's also good.
Tube🌱Time replied to Tube🌱Time
first attempt at PAL equations in galette--umm--did not match the real thing at all. i'm swapping parts into the DuPAL to compare how they perform for the same inputs. UNFORTUNATELY i've discovered that DuPAL cannot differentiate between a tri-stated output and a low output, even though the hardware is capable of doing that. 😩 
Alan Martello replied to Tube🌱Time
RealGene ☣️ replied to Tube🌱Time
@tubetime
scrottie (he/him/they) replied to Tube🌱Time
@tubetime I'm sorry to @ you like this, and I'm sorry if I already did and forgot, but I have some DEC 3100s and Sun 3/60s, and I'm terrible with electronics. I keep trying to do minor repairs or kits and I just ruin things. I'd really especially like to get PLAs from those dumped if possible since that's kind of a ticking timebomb. Do you know of anyone who does stuff like that for randos like me? And regardless, thank you for sharing the hacking that you do do. 
Matteꙮ Italia replied to Tube🌱Time
@tubetime I read that name, did some digging and _of course_ it's from an Italian guy ("du' pal"/"du' bal" is slang for "due palle" i.e. "two balls", an expression commonly used to express frustration for a boring/undesirable task 😬) 
Otte Homan
@tubetime ... ah, a 20 feet GAL chip, one of those "I have no idea what this does but its probably quite clever" devices.
indrora, boot of journalism
@tubetime Voultar (of the console modding scene) has called this the hallmark of a shitty modder for years.   
Delta Wye
@tubetime Ugh. I was watching this video on this guy in Chicago who does fine art restoration (I think Baumgartner Art). All the materials he uses are fully reversible - the idea is that a couple hundred years from now a future art restorer can easily remove the stuff he used when better things are available to use.  |
this means you can't just use a standard PC floppy drive and expect it to work, even with a little PAL on the back. this drive has been modified.