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| Need capture and signal help for homebrew accelerator project |
Posted by: MacPlus2300 on 2026-07-27 17:13:47
Looking for a logic analyzer capture on a Macintosh Portable (or M5126 backlit) motherboard, PDS connector side. I’m scoping the bus timing for a homebrew accelerator project.
Signals needed, all captured simultaneously, PDS pins in parens:
• /AS (c8)
• /DTACK (b7)
• /DELAY.CS (a5)
• /EXT.DTACK (b30) probably idle/high in stock config, but capture it anyway
• 16M reference clock (b31), just as a timebase
Trigger on falling edge of /AS. Want to see: (1) how many 16M clock cycles between /AS falling and /DTACK falling on a normal RAM access, (2) whether /DELAY.CS ever asserts during a normal cycle or only under specific conditions (e.g. slow ROM/I-O access), and (3) confirmation that /EXT.DTACK is truly unused in stock config. A few captures; one during a plain RAM read, one during a ROM read, one during any I/O access (SCC/VIA) if you can trigger on those that would be huge. 8-16 channel analyzer is plenty, all this data would be extremely helpful and valuable to me if anyone has the capability to do this. |
Posted by: Froggy814 on 2026-07-27 17:59:33
I only have a 4 channel logic analyzer. Would I be able to do it with that? And I don't quite know how to trigger ram rom or scc/via read If you could tell me how those would be triggered. I would be happy to help. I can help with a lot if you need. |
Posted by: MacPlus2300 on 2026-07-28 11:42:43
Four channels should be fine, if you do a few passes.
Pass 1 (most useful): /AS (c8), /DTACK (b7), 16M clock (b31), /VPA (c5). Trigger on falling edge of /AS. /VPA tells us for free whether a given cycle was RAM/ROM vs SCC/VIA
Pass 2: swap /VPA for /SYS.RST (c30), same other three. Start capture right at power-on to catch boot-ROM timing specifically
Pass 3: /AS, /DELAY.CS (a5), /EXT.DTACK (b30), clock. Just want to know if these two ever actually assert during normal use.
For RAM vs ROM without decoding the address bus you can look for dominant activity in each such as power on capture for ROM heavy and desktop operations capturing RAM activity. If your software can trigger on /VPA falling instead of /AS, that’d isolate an SCC/VIA cycle directly. Even getting pass 1 I wrote here would be a huge help and a step in the right direction for my project! |
Posted by: Froggy814 on 2026-07-28 11:57:14
Cool! I'll get a couple of videos for you made up when I can today.
EDIT:
I'll read three separate things.
Boot up: meaning when spacebar is pressed and the computer successfully boots into mac os.
Playing Oregon trail: you know what that means! 😛
Inserting Floppy and formatting it or other. (not certain if my floppy drive works... I have cleaned it but it doesn't seem to be working correctly...) |
Posted by: MacPlus2300 on 2026-07-28 12:29:23
Cool! I'll get a couple of videos for you made up when I can today.
EDIT:
I'll read three separate things.
Boot up: meaning when spacebar is pressed and the computer successfully boots into mac os.
Playing Oregon trail: you know what that means! 😛
Inserting Floppy and formatting it or other. (not certain if my floppy drive works... I have cleaned it but it doesn't seem to be working correctly...) That will help. Does your analyzer also export as a format? Like csv or another kind of file |
Posted by: Froggy814 on 2026-07-28 12:58:25
It exports as a .csv, .wfm, and .bin! I hope i can figure out how it all works! I bought it very recently and am still learning the quircks! |
Posted by: MacPlus2300 on 2026-07-28 12:59:56
That would be great to get either one of those files. I can send you the pinout of the PDS if you need for the passes. |
Posted by: Froggy814 on 2026-07-28 13:03:32
Oh good! I have the pinout on hand... I printed it! |
Posted by: Froggy814 on 2026-07-28 15:27:54
Sent PM for folder... If anyone else needs it I can send the link to them if I think they are trustworthy enough)
There you go... Sorry about the phone, I can't figure out how to capture to my computer. At about a little under the mouse click is when it booted.
This is just Test 1. Trying to figure out a better way before going hailmary! 😛 |
Posted by: Froggy814 on 2026-07-28 15:39:17
Sorry also I didn't tell you which was which! I forgot the bottom 2 but the top is 16M and the one below is /VPA. Very still. The others are /Dtack and /AS. Don't remember which is which but will hopefully get a better read soon. |
Posted by: MacPlus2300 on 2026-07-28 18:37:35
Thank you very much for this, I’ll dive in to these first few tests you took. |
Posted by: Froggy814 on 2026-07-28 19:06:35
It's just one test, bootup. Shutdown and then starts up and enters mac os, only pass 1 as well. If you need more and you can use this format, I can get 'em for you. |
Posted by: Froggy814 on 2026-08-08 15:58:56
Sent you again. What do you think? I saved it as all I could to a USB stick. |
Posted by: Froggy814 on 2026-08-08 15:59:26
Remove the .zip for the real file. This is just flashing floppy, working on the other ones right now. |
Posted by: Froggy814 on 2026-08-08 16:45:14
This is the insertation of floppy. dont know if my timing is down. But here is the second |
Posted by: Froggy814 on 2026-08-08 16:46:34
These are pass 1 BTW |
Posted by: Froggy814 on 2026-08-08 16:47:47
Alright. This is IDLE I havent pressed a button so it goes into a kind of sleepish state. Where only protector shark is eating! |
Posted by: Froggy814 on 2026-08-08 16:51:07
So this is I moved the mouse and theen it does this in protector shark mode. I cant run any applications becuase my memory is so low, and I cant insert a ram card into my PDS slot 😛 |
Posted by: Froggy814 on 2026-08-08 16:54:57
Alright. finished with pass 1.
onto pass 2.
if your wondering which station is which.
from top to bottom
s1 = Dtack
s2 = as
s3 = 16m
s4 = vpa |
Posted by: Froggy814 on 2026-08-08 16:59:41
Alright pass 2 pinout as above except s4 is now /sys.rst
this is captured as soon as it turned on, like as sooN! |
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