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Mac XL downgrade back to 2/10
Posted by: pinto_guy on 2021-07-26 17:26:25 I'm not having too much luck posting my issue on an old thread in this forum, so I'll start a new one. I'm trying to convert my Mac XL back to a 2/10 model. To start with, I took the CPU board from the XL, swapped the two ROMs and the VSROM chip from my good 2/10, inserted this converted XL CPU board in my 2/10, and confirmed that it worked OK. So you'd say I just need to copy the two H ROM's using an EPROM programmer, and burn into two blanks EPROM's and that should work, right ? Well, it did not work. The copies I made (341-075-A and 76-A) are bit-by-bit the same as the ones that can be downloaded from bitsavers.org (using the DOS FC command), but I cannot boot or even see a good video screen out of the new EPROMs. If I replace with the original H-ROM chips, everything works, even though they're supposed to be bit-by-bit the same. I am clearly missing something here. Could it be something in the hardware itself ? Read voltage perhaps. I need to point out that my original EPROM's are Hitachi HN482764, and the new ones are AM2764DC (AMD) chips. They're both type 27, 64k, 28pins chips, and should be somewhat equivalent.... or not ? Would anyone have any idea what's happening ?
Posted by: mg.man on 2021-07-26 22:34:31 Not sure if it helps, but the "DC" AMD part appears to need 21V VPP... - https://forums.arcade-museum.com/threads/trying-to-burn-am2764dc-eproms-with-a-gq-4x.238603/
Posted by: cheesestraws on 2021-07-27 02:04:53 Can I suggest you post this on lisalist2.com? There are people there who are not here who have much more expertise than, say, I do—and you might get someone who has seen this before... (FWIW, I would probably reach the same conclusion as you, that the EPROMs are somehow not compatible; it sounds to me like your plan is good, so I would think it's probably the new EPROMs...)
Posted by: pinto_guy on 2021-07-27 07:58:10 Not sure if it helps, but the "DC" AMD part appears to need 21V VPP... - https://forums.arcade-museum.com/threads/trying-to-burn-am2764dc-eproms-with-a-gq-4x.238603/ Yes 21V. My programmer only had 18V and it took a bit of trial-and-error to write. But it did eventually burn, and verified OK. But yeah, perhaps the programmer could read OK, but my Lisa ran into some SNR issues. Thanks for the reply
Posted by: pinto_guy on 2021-07-27 07:59:10 Can I suggest you post this on lisalist2.com? There are people there who are not here who have much more expertise than, say, I do—and you might get someone who has seen this before... (FWIW, I would probably reach the same conclusion as you, that the EPROMs are somehow not compatible; it sounds to me like your plan is good, so I would think it's probably the new EPROMs...) Thanks @cheesestraws. I bit the bullet and ordered Hitachi ones. Will report back.
Posted by: dan.dem on 2021-08-10 02:14:05 A very basic question, sorry: How are the differences in video handled? I think the Lisa uses off-square pixels but Macs have the same screen resolution horizontally and vertically. And: Wasn't there a software only tool for making Lisas boot Mac system software?
Posted by: cheesestraws on 2021-08-10 03:17:12 I think the Lisa uses off-square pixels but Macs have the same screen resolution horizontally and vertically The MacXL "kit" came with a replacement video ROM (and I think a different filter choke?) to alter the video timings. This means it doesn't work with Lisa O/S any more, only the answer to your second question: And: Wasn't there a software only tool for making Lisas boot Mac system software? Yes, it's called MacWorks and there are several versions of it. You can run MacWorks on a non-square-pixels Lisa, and it will work, but the image will look a bit weird. The Mac XL is a combination of the screen mod + the software.
Posted by: dan.dem on 2021-08-10 06:55:30 Thank you, @cheesestraws. Unfortunately a Lisa is out of reach of my hobby budget.
Posted by: pinto_guy on 2021-08-14 10:36:49 So I received the Hitachi HN482764 EPROMs. I tried to burn them with my programmer, but the max voltage it could provide was 18V, and these chips require 21V. The AMD chips could be written at 18V, using several write cycles, but these Hitachi ones would not budge. So I used two small power supplies of 18V and 5V in series, and I fed the resulting 23V (specs said 26V max) to pin 1 that I bent up out of the programmer's socket (see pic 1). I also connected the two grounds on pin 14. I made sure that I unchecked the "check pin" option in the programmer, and this worked like magic. Once written with the H-ROMs, these two EPROM's were placed on my CPU bord. I installed the 341-0229-A Video ROM, and bypassed the CRT square-pixel adapter (Pic 2) on the video analog board. I reformatted the Widget drive using NeoWidex (thanks again @stepleton ), and installed Lisa System using de-serialized floppies. And voila: a Mac XL that is now a 2/10. See pic 3 (before), and pic 4 (after). I guess I still do not understand why the AMD EPROM did not work. I know EPROMs have serial numbers written on them. Could this be the issue ? Would the Lisa only work with known EPROM's ? Pics 1 and 2 Pic 3 and 4: Mac XL before (running Mac OS with square pixels) and after downgrade (running Lisa OS, and with rectangular pixels)
Posted by: MOS8_030 on 2021-08-14 10:53:48 That's pretty slick! Nice work.
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