128K Vs. 128k/512K Logicboard

[Mac128]

Sometimes you just know when a question is targeted to one person ... So, Tom, if you have any input on this I'd be grateful.

The 128K board looks virtually identical to the second generation 128K/512K hybrid. The notable exception being the resistor and I/C network near the RAM array. Presumably this is the multiplexer built-into the digital board vs. being a seperate mini-mux board added to the original 128K when updating RAM to 512K.

An aside: I always wondered why Apple took so long to offer the 512K board when the modifications to the original 128K board were so simple to do. I suppose it makes sense and is far more "elegant" to build the circuitry into the board rather than tack-on a separate circuit board.

Either way, the main difference between the 128K & 512K configuration is several resistors and an IC chip necessary to decode the extra RAM. However, present on the 128K board is a jumper only, which is removed on the 512K board. Any idea what this jumper does? Does it perhaps bypass the 512K decoding multiplexer circuitry? If so, then it would seem this hybrid board would be optimal for my dip-switch modified board to switch back & forth between RAM configurations.

I am want to just re-connect the jumper without understanding what it does at the risk of shorting something.

As long as I'm asking about decoding RAM ... REGARDING THE MAC PLUS:

I have been unable to locate a 64K 150ns RAM SIMM. Assuming I could find one or construct one, would the Mac Plus be able to use it? I know the Mac Plus can use a mere 512K RAM with just two 256K RAM SIMMs, but if you put only 128K RAM in two 64K RAM SIMMs would the Mac Plus use it without issue? I just don't want to keep pursuing this idea if it's not possible.

 

[tomlee59]

Yes, you're absolutely right: That added circuitry is the mux needed to support the added RAM. As to why this wasn't done from the beginning, we have once again design by fiat. Jobs was fixated on the notion of the Mac as a complete, finished consumer item, like a toaster. His philosophy of Mac-as-toaster precluded allowing upgrades. That's why Torx screws were chosen, and no upgrade port was included. It drove his engineering team nuts -- they knew that computers weren't toasters, and that upgrades of computers would forever be the norm. They tried all sorts of sneaky things, like putting in a "diagnostic port" that just happened to connect to the CPU bus pins in a way that would enable upgrades. Jobs saw through this trick, and nixed it. He only permitted larger ram when forced to by the software vendors. And even then, he insisted that the 512K Mac not be expandable beyond that capacity.

I do have a schematic somewhere showing exactly how that jumper is hooked up. If you don't mind a potentially unbounded amount of time for me to dig it up, I'd happily send you a copy. The hybrid 128/512 board would indeed be an ideal candidate for you to perform the dip-switch experiment.

As to 64K simms, I've never seen one. I suspect that they do not exist. However, were you to kludge one, my guess is that, with possible tweaking to the address decoder, you could probably make the Plus use them (although not every Plus function would necessarily work), given how little the Plus fundamentally differs from its ancestors. You might have to change the ROMs too (then again, maybe not). It would be a fun experiment for you to try for us.

 

[Mac128]

OK, so now I'm determined to give this a try. The plan is to take the 512K hybrid board which is arrayed with 16 256 RAM chips.

If what Pina says is accurate, I should be able to turn the 512K board into a 128K board by simply removing the decoder chip, 3 resistors and one cap required to recognize the additional 384K RAM. In addition there is a jumper mentioned above needing to be added in, which I should be able to fabricate out of a paperclip. Much less soldering than removing 16 RAM chips.

One question, after I remove the components, do I need to fill the pin holes with solder?

According to Pina, even though 512K worth of RAM is installed on the board, only 128K will be recognized without the decoder array installed. Now, he only mentions this for the original board onto which a makeshift decoder board must be inserted. However, it seems like this principle should apply to the hybrid board as well.

One thing that concerns me is this jumper for the 128K. Pina makes a specific point that when upgrading to 512K, that the jumper must be cut so that it cannot possibly reconnect. Now this could simply be so that the computer works properly, or could it be that the jumper could damage something with the decoder array installed? One would think Pina would have mentioned that as a trouble shooting step in the event one forgot that step should 512K not be present after power-up, though he does occasionally assume more than the novice reader and skip such details.

Ideally it would be fantastic to find a single pin in the 512K array that I could switch in and out with a dip switch to enable 512K or 128K. I think the only way to figure this out would be through experimentation, but is there any chance of damaging the RAM chips or other components by isolating pins one by one?

Could it be that merely adding or removing the jumper required for the 128K is enough to bypass the additional RAM decoding array?

The first thing I will do is remove all the expansion components. perhaps replacing them all with sockets, so at least a manual restore could be accomplished if no single dip switch option exists. This will also aid in testing.

Is there any risk to running 256 chips as a 128K?

Practical application to this? Easy, many 128K boards have been upgraded to 512K boards and there are any more 512K boards out there, not to mention cheaper, in general. This would be a simple way to restore a 128K board without the difficulty of replacing all the RAM chips (which are often hard to come by), and possibly damaging the boards. The original boards are a little more complicated since they require physically cutting traces on the boards, and removing the jury-rigged decoder boards can pull traces and create other problems depending on how they were installed.

 

[Mac128]

Here's the link to the additional discussion about switching between 128K & 512K.

In addition I've been looking at H3NRY's MacDraw schematics which shows the 128K/512K hybrid board. It appears that if the W1 jumper, which terminates to ground, is not cut, then it bypasses the 512K multiplexer circuitry. At a minimum, it appears that a single DIP switch could be installed to switch the W1 between the ground and R42, effectively enabling or disabling the 512K expansion circuitry. But I don't know nearly enough about this to be sure.

 

[Mac128]

I've posted below the 512K expansion connector (E3) from the original 128K logic board as drawn in the Bowmarc schematics.

H3NRY, I'm just curious why your MacDraw schematics don't also show this insertion point. Aren't the original 128K & 512K boards virtually identical except for the incorporation of the additional multiplexing circuitry?

The Bomarc schematics are inclusive of all three logic board models, the 820-0086-C, 820-0068-F and the 820-0141-A. However, they do not really show how the multiplexing circuitry is incorporated into the E3 access. In trying to map these to your schematics, I was able to map the CPU access (A18 & A17) from holes 3 & 6 respectively, pin 1 of the RAM array to hole 2, and hole 5 & 4 to pins 2 & 14 on your 512K circuitry. The one pin I can't reconcile is 3 & 4 on your schematic, as I am out of holes on the E3. It's also interesting to note that hole 1 at E3 must be severed from 2, just like the connection at W1.

Also, before I go any further with this little project, is this unfilled triangular symbol (△) supposed to be a ground or a pullup? Both yours and the Bowmarc seem to have both this and a traditional ground symbol used interchangeably. More confusing is that you show resistors and capacitors in the 512K expansion circuitry yet, you show very few resistors and capacitors elsewhere in the schematic, while the Bowmarc is a bit more thorough. Love to learn more about how they came about.

 

[Mac128]

Sunday. Too much time on my hands. Impatience. Project. Completed. Ramifications? (no pun intended)

SUCESS. So my deductions were correct. On any 512K board, by simply adding a jumper at W1, the 512K circuitry is bypassed and the Mac only recognizes 128K.

Pumped from the success of that experiment, I went to an original 128K board which had been upgraded to 512K, and using the same rational, placed a solder bridge across pin 1 & 2 at E3, which trace is otherwise required to be severed completely when upgrading to 512K with a mini-mux board (seen in the image above). This has the same result as installing the W1 jumper on the 512K board, and the Mac only recognizes 128K RAM, without removing any of the other modifications.

That's right, despite the presence of 256 chips and multiplexing circuitry to decode 512K RAM, with the jumpers in place, the Mac ignores all of that and functions only as a 128K.

Clearly, for those of us who want to be able to switch between 128K and 512K without maintaining two machines, or swapping logic boards, this makes a simple switch an effective solution. Open the circuit, 512K. Close the circuit, 128K. The switch could be easily run through a vent slot on the right side of the Mac.

BUT DOES THIS HARM THE MAC? Well, that's now the question. Obviously it works, and everything functions normally with the jumpers in place, but is this healthy for the Mac? Is the additional circuitry truly being bypassed from the electrical path, or is it still affecting voltages across the system? I didn't know what I was doing to begin with, and I have no idea what is actually happening now. Another question, is it authentic? In other words, will 256 chips without the additional decoding circuitry behave exactly like the 64 chips?

Nevertheless armed with too much confidence, I'm gonna move on to working on the 64K/128K ROM switch concept. As others have confirmed, it should be as simple as a single switch enabling one set of ROMs or the other. In theory, these mods should allow a 128K through 512K Plus (with SCSI board), to operate uniquely within the same box, including the fabled 128Ke. I'd like to find some killy clips for testing, to install the 128K right on top of the 64K ROMs. This allows the stacked ROMs to be used with a SCSI board. Unfortunately it might be too thick to fit in an unmodified chassis.

 

[H3NRY]

As you found, the switch disables the extra address line, making a 128K Mac. The switch should connect to +5V and the address decode output as shown at W1 (wire jumper #1). Running with the switch closed should be safe as long as the resistor is still present. Have fun with the ROMs. You could in theory make a 4-in-1 Mac!

 

[Mac128]

Thanks for confirming H3NRY. Not sure I understand the +5V resistor, where is it? It appears the W1 jumpers to nothing, or a ground layer to which numerous things attach all over the board.

Another thought occurred to me. The Mac Plus has the same RAM decoding multiplexer chips right? Would there be a way to configure the Plus in the minimum 512K SIMM configuration, then disable a multiplexer chip to recognize less of it, say even down to 128K? (I mean why have a 4-in-1 when I can have a 5-in-1?)

Is there a Mac Plus logic board schematic someplace?

 

[H3NRY]

Another thought occurred to me. The Mac Plus has the same RAM decoding multiplexer chips right? Would there be a way to configure the Plus in the minimum 512K SIMM configuration, then disable a multiplexer chip to recognize less of it, say even down to 128K?

Sure, it should work. I don't know of a Plus schematic published anywhere, so you'd have to trace the decoders and find the matching address line to a 128/512.

On my Mac 512K MoBo, the W1 jumper is between a pad connected to +5V (not ground) and an address line. The 47 Ohm resistor is between the output of the decoder chip and the jumper pad and should keep the decoder from burning out when its output is shorted. Grounding the pad will also disable it, but Apple tied it high.

Unless you're doing software compatibility checking, I can't see the point in disabling memory. You might as well block off memory with software, for instance create a RAM Disk that leaves 128K for the OS and apps. It's like taking 2 or 4 spark plugs out of a V8 so you can have a 6 or 4 cylinder car. It doesn't improve anything, it just cripples the car and makes it slow. 128K is too little RAM for a Mac. The original Mac which Steve unveiled at the stockholders' meeting had 512K. A 128K Mac couldn't run the demo. 128Ks were sold to the public because 512K was too expensive for the margins Scully insisted on. Unless you have a 128K museum piece, there's nothing special about that small RAM. I suppose running out of RAM is a novelty today, so it's fun to experience? :lol:

 

[Mac128]

Ha H3NRY, you're right, it is merely a trip down nostalgia lane. But then this entire site is a trip down nostalgia lane. For me, there was a period of about a year when the 128K Mac was the greatest thing I'd ever seen, and despite its limitations, obvious even then, I couldn't wait to play with it every chance I got. When I finally got my hands on a 512K, I never looked back until some time later as nostalgia got its rusty hooks into me. The focus on limiting RAM to 128K is to experience an authentic experience. Your idea about a RAM Disk is a good one. Unfortunately, it won't work with the earliest system software. Then again, I have no idea how the Plus hardware will respond to operating under a 64K ROM.

All good food for thought and tinkering.

Now, this has got me thinking about ultimate compatibility in one box. I'm wondering if a SWIM chip could be substituted for an IWM chip in a Mac Plus, if the SWIM ROM code were patched in via a system INIT, like the HD20 & HFS code is patched on the 64K ROM? On the other hand, if that were possible, I would think someone would have come up with this solution for the Plus, considering the volume of them sold and their continued viability into the mid-90s.