128k/512k boot sequence

[Cinan]

I am trying to diagnose a problem with vertical bars on a 512k and want to know if anyone has seen a documented boot sequence for a 128k/512k Mac so I can work through the sequence. I don't have any visual issues on the logicboard so I am down to trying to work out which chips may have problems.  The fuzzy schematics I have been able to find aren't that helpful - or readable.  What I am after is the triggering mechanism from when the power is turned on to get to a "bong",  somewhere in there I have a problem.

Details can be found over here - unfortunately it seems to have been taken over by discussions re image formats on modern Apple's. :-(

 

[Dog Cow]

Try this article: Macintosh Startup Revealed: From Boot Blocks to Finder

See also, Inside Macintosh, volume III, page 42

As to your particular problem, the ROM clears the screen to black very early in the boot process, even before the boot chime. So if you're seeing those bars, it means the video buffer in RAM is not being cleared. Likely it's not being cleared because the code in ROM is not being executed.

Therefore, I would suspect the problem most likely lays between the Motorola CPU and the two ROM chips. Since you indicated in your earlier thread that you believe that the two ROM chips are OK, I would look into the CPU.

 

[Cinan]

Thanks this is really helpful,  I will study over the weekend.  brings back many memories of Assembler coding at university !

Any easy way to test the CPU without a scope ?

 

[Dog Cow]

Any easy way to test the CPU without a scope ?

I don't know. And when I say CPU, I also mean with its supporting components like the oscillator.

 

[Cinan]

So some progress but only accidental.

I have checked all the connections coming off the CPU and traced them to other chips and performed some general continuity testing across the Logicboard today from what I can read on the very fuzzy Schematic of the 128/512k board.  (Note : I actually found an error in the schematic on pins 45 & 46 on the 8530 chip,  they read D14 & D12 but should be D15 & D13 !)

I had all but given up and thought I would try the interrupt switch at the vertical bars,  nothing.  Then I tried the reset switch and I got a bong and a Mac screen with flashing question mark disk,  what ?  This is able to be repeated,  but,  when I insert a disk I get wurring but a disk with a cross,  if I put an external drive on I get wurring with a disk and then X disk and then a sad Mac.  If I use a FloppyEMU in the external drive port it doesn't initiate and can't be used.

Voltages are good 4.99v on the 5v rail.

I have posted a video of the sequence.

This leave me to think that the CPU isn't initiating properly with the power on switch and needs a reset "Kick" to get it going.  But then it doesn't receive a good signal or can't interpret the signal from the IWM ?  Does this lead me to think I have a dodgy 68000 chip that I need to replace ?



View attachment IMG_7370.MOV

 

[cheesestraws]

This leave me to think that the CPU isn't initiating properly with the power on switch and needs a reset "Kick" to get it going

This much is normal on a lot of CPUs—it's certainly true of the 68000—but there's normally a circuit on the board that generates that kick automatically.  (see http://www.users.cloud9.net/~stark/txhmt4.pdf)

So it may either be that the reset circuit that automatically generates this has gone kaput, or the power isn't sufficiently stable when it happens.  You might be able to test the former with a decent multimeter with min hold, since to reset the system, both the RESET and HALT lines are pulled low, but I'm not sure I'd want to rely on that method myself.

(edit: that chapter I linked to tests their reset circuit just using an LED, which might be feasible; the human eye is pretty good at picking up brief pulses of light.  But I don't know how long the pulse on the Mac is, so take this with a pinch of salt).

I don't know exactly how this is implemented on the Mac, nor how this interacts with your other disc-related problems.

 

[Cinan]

Thanks this makes sense.  Just need to find now what pulls RESET and HALT low creating the RST circuit.  On the schematic it looks like the circuit above the IWM chip on the Schematic but it is too fuzzy to read.  Does anyone have any clarity on this ?

 

[Cinan]

Assume it's the LM311 chip.

 

[cheesestraws]

I don't have a schematic for the 512, where did you get yours from?  I've only got one for the Plus

 

[cheesestraws]

OK, so both the Plus schematic and the Guide to the Macintosh Family Hardware agree that the initial RESET is generated by, of all things, the sound chip:

 

[Dog Cow]

OK, so both the Plus schematic and the Guide to the Macintosh Family Hardware agree that the initial RESET is generated by, of all things, the sound chip

I don't believe that the original Mac 128K and 512K motherboard has a sound chip. Inside Macintosh volume III shows how sound is generated on these models.

 

[cheesestraws]

I don't believe that the original Mac 128K and 512K motherboard has a sound chip. Inside Macintosh volume III shows how sound is generated on these models.

Yup.  But I don't have a 512k schematic, so that should have a 'for what it is worth' rider.

 

[Cinan]

Found it here - http://www.digibarn.com/collections/diagrams/mac-512klogicboard/index.html

 

[Cinan]

And yes Sound is working ok to give a bong.  I will do testing on the LM311 today to see what it is doing at startup.

 

[cheesestraws]

Yup, based on that schematic it looks like looking at the output of the LM311 might be interesting.

 

[Dog Cow]

I don't believe that the original Mac 128K and 512K motherboard has a sound chip. Inside Macintosh volume III shows how sound is generated on these models.

Yup.  But I don't have a 512k schematic, so that should have a 'for what it is worth' rider.

Here's the followup from some research I did, because I'm not an expert on all things, and I am still learning. The Sony Sound chip was new with the Macintosh Plus and Macintosh SE. The original Macintosh logic board has an Analog Sound Generator chip. From its pinouts found in this schematic, it does not appear to interface with the 68000's reset line.

 

[Cinan]

Agree.  I have ordered a new LM311 chip so lets see what happens.  I did some more testing last night and it seems that after resetting once and then using a multimeter on pin 7 to see what was being sent,  the machine then started working normally after a few times of on/off's but then resorted to not starting.  I also got the FloppyEMU working when using System 2.0 as the system disk.

Some strange behaviour but at least it is showing signs of life.

Does anyone have a spec sheet of the smaller components - Especially the Diodes on the 512k board as I may need to replace some of these ?

 

[cheesestraws]

Here's the followup from some research I did, because I'm not an expert on all things, and I am still learning. The Sony Sound chip was new with the Macintosh Plus and Macintosh SE. The original Macintosh logic board has an Analog Sound Generator chip. From its pinouts found in this schematic, it does not appear to interface with the 68000's reset line.

You're right, I was mislead by some wording in the hardware reference book (or, rather, I misread it).  Sorry for the dodgy information.  From the schematic linked above, it's pretty clear that the LM311 is generating the reset.

 

[Cinan]

So I replaced LM311 without any change.  I am now thinking it might be C9 as per the image above.  In my understanding if C9 isn't working properly then the -ve won't be presented to pin 3 and therefore LM311 won't create the reset signal.  But if the reset switch is pressed (SW2) then it will complete the circuit and the negative presented to P3 and hence LM311 creates the reset signal when I press SW2 and the CPU reboots ??  is that logic right ?

My eyes aren't the best but C9 looks to be a non-polarised 4.7uf 35v on the schematic but on my board C9 looks more like a diode than a cap (image attached).  If it's a cap then what type is it ?  If it is non polarised (two parallel bars) then why does it also indicate a + symbol on the top bar ?