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Reverse Engineering the Macintosh SE PCB & Custom Chips for 1:1 reproduction

Kai Robinson

Well-known member
I checked the presence of the RESET and HALT signals - both are pulled HIGH, although i was measuring from the SWIM Chip socket, and although it was high, it looked like it was dipping for exactly 1 clock cycle to ground, then back to being pulled high. 

 

techknight

Well-known member
Are there Address/Data pull-ups somewhere on that board? you verify they have their proper VCC/GND connection? 

 

Phipli

Well-known member
Are the bus arbitration pins like /BR functioning? Does the CPU think a peripheral is using the bus?

 

Kai Robinson

Well-known member
/BR is pulled up by R43 to VCC

BGACK is pulled up by R45 to VCC

DTACK is pulled up by R46 to VCC

RESET and HALT are connected together - but also to the 6522, SWIM, 5380, SND & BBU...might be one of those causing the (potential) wierdness on those pins...?

 

Kai Robinson

Well-known member
Out of curiosity, to see if there were any buried traces, I sanded around U4E with 80 and 100 grit, to get down to the ground plane... If the area is anything to go by, the inner two layers are power and ground only, no traces missing. 

DSC_0397.JPG

 

quorten

Well-known member
I checked the presence of the RESET and HALT signals - both are pulled HIGH, although i was measuring from the SWIM Chip socket, and although it was high, it looked like it was dipping for exactly 1 clock cycle to ground, then back to being pulled high.
The problem could be that RESET is not held for long enough for the processor's internal state to stabilize.  According to the MC68000 User's Manual, page 10-12, note 4: "For power-up, the MC68000 must be held in the reset state for 100 ms to allow stabilization of on-chip circuitry. After the system is powered up, #56 refers to the minimum pulse width required to reset the processor."

Specification #56 indicates that during a normal operational state after power-on, a RESET only needs to be held for 10 clock cycles (1.25 microseconds).

Did you try pressing the programmer's RESET button after power-on?  If this isn't actually directly connected to the processor RESET line, you can trigger it yourself using jumper wires on the PDS slot connection to RESET.  BUT you would need to ensure this is low-pass filtered, so best just to use the programmer's RESET button as it already is filtered.  (N.B. HALT and RESET are wired together at the PDS connector to achieve a proper external reset condition for the MC68000.)

After ensuring that RESET is being properly asserted, check that *AS is asserted.  This at least ensures that the CPU is attempting to fetch the RESET interrupt vector, which is the very first memory operation performed after RESET.  Then if the BBU is responding with *DTACK and *ROMEN, and the ROM is responding...

 
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Phipli

Well-known member
Folks, look at what someone posted on the Facebook "Vintage Macintosh Restoration and Preservation" group. It is a 3rd party BBU that allows for a higher bus speed on the SE!

SlimSocial-12.jpg

SlimSocial-26.jpg

 

maceffects

Well-known member
@Phipli I forgot about the Brainstorm!  That would be a brilliant thing to reverse engineer!  A bit out of my domain, and obtaining one would be very difficult.  Thanks for sharing.

Update: I replied on the post to see if I can obtain that SE board so it can be reverse engineered.  I am already making the PLCC 84 breakout adapter anyway. 

 
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Well, i was a very lucky boy for Christmas and santa brought me a new DS1102 Oscilloscope! Along with a Yihua 959D Hot Air station - I've just bought a Meanwell RT65B which has all the right voltages with enough current to set this up as a bench PSU for the board. I can monitor the video out signal on the scope. I tried in a separate SE/30 chassis but still no dice. I'm not getting anything. I think @techknight is right - there's a clock signal missing or not quite right somewhere. 
Pretty sure I mentioned that it was a Clock Signal Issue in My Prior Post... JS...

 

maceffects

Well-known member
@maceffects

These folks seem to have one. Anyone live close? Do you think they'd let someone borrow it? :)

https://www.freegeektwincities.org/
Our very own @ScutBoy happens to volunteer there and if the other individual doesn't snatch up this machine I'll see if he can help us out.  I think with the PLCC 84 breakout board designed by @Kai Robinson I'm building, it should be able to be cloned.  I suspect the PDS card is also part of the system but that (should) be easy to do unless it has PAL in which case I'd have to send those out for to be reverse engineered. 

 

maceffects

Well-known member
Thanks @Kai Robinson I'm hopeful that with your breakout someone can reverse engineer that and I'd clone the PDS 16mhz upgrade board.  I'd assume it needs a logic probe but that is, I'm afraid, outside the scope of my knowledge. 

 

ScutBoy

Well-known member
Our very own @ScutBoy happens to volunteer there and if the other individual doesn't snatch up this machine I'll see if he can help us out.  I think with the PLCC 84 breakout board designed by @Kai Robinson I'm building, it should be able to be cloned.  I suspect the PDS card is also part of the system but that (should) be easy to do unless it has PAL in which case I'd have to send those out for to be reverse engineered. 


OK - I purchased the Brainstorm SE from FreeGeek. Had no idea it was something pertinent to this project otherwise I could have snared it before it went to the store, but as a volunteer and board member I don't mind giving them the money. :)

I should have the machine back in hand by the weekend; if it needs to then go somewhere else I would probably just ship the logic board, unless the whole machine is really needed.

 

ravuya

Active member
The other nice thing about having an aftermarket BBU that we know about is that it opens up another set of engineers that we could potentially track down and ask questions of.

AF had some drivers here, but the link seems to be broken now.

 
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