The Original Macintosh:    4 of 13 
Diagnostic Port
Author: Andy Hertzfeld
Date: July 1981
Characters: Steve Jobs, Jef Raskin, Burrell Smith, Brian Howard, Steve Wozniak, Rod Holt
Topics: Hardware Design, Management
Summary: Burrell tried to sneak in some hardware expandability

Expandability, or the lack thereof, was far and away the most controversial aspect of the original Macintosh hardware design. Apple co-founder Steve Wozniak was a strong believer in hardware expandability, and he endowed the Apple II with luxurious expandability in the form of seven built-in slots for peripheral cards, configured in a clever architecture that allowed each card to incorporate built-in software on its own ROM chip. This flexibility allowed the Apple II to be adapted to a wider range of applications, and quickly spawned a thriving third-party hardware industry.

But Jef Raskin had a very different point of view. He thought that slots were inherently complex, and were one of the obstacles holding back personal computers from reaching a wider audience. He thought that hardware expandability made it more difficult for third party software writers since they couldn't rely on the consistency of the underlying hardware. His Macintosh vision had Apple cranking out millions of identical, easy to use, low cost appliance computers and since hardware expandability would add significant cost and complexity it was therefore avoided.

Apple's other co-founder, Steve Jobs, didn't agree with Jef about many things, but they both felt the same way about hardware expandability: it was a bug instead of a feature. Steve was reportedly against having slots in the Apple II back in the days of yore, and felt even stronger about slots for the Mac. He decreed that the Macintosh would remain perpetually bereft of slots, enclosed in a tightly sealed case, with only the limited expandability of the two serial ports.

Mac hardware designer Burrell Smith and his assistant Brian Howard understood Steve's rationale, but they felt differently about the proper course of action. Burrell had already watched the Macintosh's hopelessly optimistic schedule start to slip indefinitely, and he was unable to predict when the Mac's pioneering software would be finished, if ever. He was afraid that Moore's Law would make his delayed hardware obsolete before it ever came to market. He thought it was prudent to build in as much flexibility as possible, as long as it didn't cost too much.

Burrell decided to add a single, simple slot to his Macintosh design, which made the processor's bus accessible to peripherals, that wouldn't cost very much, especially if it wasn't used. He worked out the details and proposed it at the weekly staff meeting, but Steve immediately nixed his proposal, stating that there was no way that the Mac would even have a single slot.

But Burrell was not that easily thwarted. He realized that the Mac was never going to have something called a slot, but perhaps the same functionality could be called something else. After talking it over with Brian, they decided to start calling it the "diagnostic port" instead of a slot, arguing that it would save money during manufacturing if testing devices could access the processor bus to diagnose manufacturing errors. They didn't mention that the same port would also provide the functionality of a slot.

This was received positively at first, but after a couple weeks, engineering manager Rod Holt caught on to what was happening, probably aided by occasional giggles when the diagnostic port was mentioned. "That things really a slot, right? You're trying to sneak in a slot!", Rod finally accused us at the next engineering meeting. "Well, that's not going to happen!"

Even though the diagnostic port was scuttled, it wasn't the last attempt at surreptitious hardware expandability. When the Mac digital board was redesigned for the last time in August 1982, the next generation of RAM chips was already on the horizon. The Mac used 16 64Kbit RAM chips, giving it 128K of memory. The next generation chip was 256Kbits, giving us 512K bytes instead, which made a huge difference.

Burrell was afraid the 128Kbyte Mac would seem inadequate soon after launch, and there were no slots for the user to add RAM. He realized that he could support 256Kbit RAM chips simply by routing a few extra lines on the PC board, allowing adventurous people who knew how to wield a soldering gun to replace their RAM chips with the newer generation. The extra lines would only cost pennies to add.

But once again, Steve Jobs objected, because he didn't like the idea of customers mucking with the innards of their computer. He would also rather have them buy a new 512K Mac instead of them buying more RAM from a third-party. But this time Burrell prevailed, because the change was so minimal. He just left it in there and no one bothered to mention it to Steve, much to the eventual benefit of customers, who didn't have to buy a whole new Mac to expand their memory.

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There was a row of 7 holes in the PCB near the RAM array which brought together most of the signals needed to decode extra addresses to expand RAM. A remnant of the diagnostic port? Phat Ho worked out an ingenious circuit which added another bank of 512K, and I helped him design a piggyback card at Beck-Tech which allowed a megabyte of memory. I knew our design was right when it was copied by a dozen others within 2 months. In fact, I upgraded your Mac...
I don't have time to write out the details in full, but this account has massive errors. The "Bus Diagnostic Port" was something I wanted; it was to work like the expansion slot on the Poly 88s that we used to use in the Pubs department. I wanted to be able to get at the bus and expand the system, and I have no idea where the story originated that I did not. As usual, I wrote up the proposal in the Book of Macintosh, and readers who want facts and dates can go look it up for themselves on the Stanford site. One of the reasons I joined Apple was in admiriation of Woz's electronics, for example, the pre-decoded bus that made cards for the Apple II so much smaller and simpler than S-100 cards. I suspect that Andy does not know that long before coming to Apple, I had designed computers from the ground up and built them. Later, I built many kits beginning with the Altair. I was nowhere in the same league as a hardware designer as Woz or Burrell, but I did understand the issues and details. I still design or build electronics when I need something, and I solder a mean connection.
The schematic for the final MAC (Dated Feb '83 DGK) is on
Wow, I always wondered about that. I mean, for the solder-on 512k upgrades, you had to put in an extra chip, but it was amazing to me how minimal the upgrade was. We (Mac outsiders) always assumed that the reason for the minimalistic changes necessary had to do with trying to cut down manufacturing changes when the eventual 512K Mac came out. It's awesome that Burrell did that for us! I was still in college when I upgraded my first Mac; the fact that it cost $400 in parts vs. $999 from Apple meant that it was actually financially possible. In fact, since my dad worked at HP, we were eventually able to get HP's prices on RAM chips. By spring 1985, the parts cost was closer to $40, and I was making money on the side upgrading people's Macs!
I bought one the TurboMax boards for my MacPlus (I think that was what it was for), and our repair engineer fitted it. He was a genius at desoldering chips off a multilayer board. I watched him as he installed this in his spare time one hot summer afternoon - amazingly it worked first time and was the envy of my colleagues for several months, it also had a Davong 10Mb hard drive underneath it which meant that floppies were only required for programme install - do you remember the 28 disks that were required for Microsoft Word. I do and the first batch had a faulty disk at about No 17 Grrrr...
An early Macintosh design document written by Jef Raskin seems to outline his feelings about expandability: "You might think that any number of computers have been designed with these criteria in mind, but not so. Any system which requires a user to ever see the interior, for any reason, does not meet these specifications. There must not be additional ROMS, RAMS, boards or accessories except those that can be understood by the PITS as a separate appliance. For example, an auxiliary printer can be sold, but a parallel interface cannot. As a rule of thumb, if an item does not stand on a table by itself, and if it does not have its own case, or if it does not look like a complete consumer item in [and] of itself, then it is taboo. If the computer must be opened for any reason other than repair (for which our prospective user must be assumed incompetent) even at the dealer's, then it does not meet our requirements. Seeing the guts is taboo. Things in sockets is taboo (unless to make servicing cheaper without imposing too large an initial cost)." From:
Burrell did a great job! We (as students) used to do the RAM upgrades for Mac owners who didn't have the money to buy a 512k. I upgraded my own Mac first and with sweat in the hand, the Weller soldering iron in the other we operated on the 128k board with utmost care. A small blub of solder got us scared when turning it on and a sad mac appeared, but a close inspection brought out the short and there is was: A whopping 512k! I got the schematic to do the upgrade from a Unix administrator. It was a small file in MacPaint, send in ASCII and BINHEX/HEXBIN was used to decode it. What times.
It is indeed very interresting that the Rev A iMac sported the "Mezzanine" expansion port which was described as a "Diagnostic port" before 3dfx cards started showing up for it :) History repeats itself. for the raskin quote (and many things), it all depends on context. when i read that segment i was thinking it meant if those things were required of the user. im certain they didnt want anything the end-user had to do, but i cannot be certain he didnt think it was alrite for the adventurous end-user. are you?
that "anthrophilic.html" essay is great! say what you will about raskin, but some of his ideas were very ahead of his time and spot on. note the idea that users should be more concerned w/ case color than internal upgrades -- and in 1999, w/ the fruity imacs thats exactly what happened. (tho i wish we also had the $500 pricetag!)
I remember being shown a Mac Plus in the Beck-Tech lab, the winter of 1985, before it was released in January 1986. It was shown to me with the pronouncement, "This is what's going to put your memory upgrade business out of business!" At the time, my company had been doing RAM upgrades on 128K Macs for several months, based on the famous Dr. Dobbs article, clipping out sixteen 64K RAM chips per logic board (256 pins/holes each!), and soldering in 256K RAM chips in their place, and also soldering in a little decoder board at that row of 7 holes that Henry describes above, on the first revision of the 128K logic board; the second revision of the 128K logic board was similar or identical to the 512K logic board (Apple kept making both the 128K and the 512K Mac sizes for another year), and so it had a location on the logic board where the decoder chip and a couple resistors could be directly soldered to the logic board. The upgrade was a tedious task, so when I saw the Mac Plus and its clip-in RAM board upgrade, I was both alarmed and relieved. It took another year or two for the demand for 128K to 512K upgrades to dry up, since this manual approach to upgrading cost less than buying a Mac Plus or even a 512K Mac, but when we stopped doing these manual upgrades, I was glad not to have to breathe the fumes from the solder and circuit board cleaner every day (not entirely removed by the big fans we had in the ceilings that pulled the fumes outside). The market for even bigger upgrades had a lifespan of several more years--two-meg and four-meg Monster Macs from Levco, Beck-Tech upgrades, Dove upgrades, etc. Too many of these had mechanical problems in remaining connected to the logic board, requiring many hours of frustrating repair and replacement, so when the market for these dried up, I was glad to see them go too.
FYI, Raskin's "Design Considerations for an Anthropophilic Computer" URL has changed. the new URL: -- it's still a great read. his description is even closer to apple's ipad than it was to the mac: - This is an outline for a computer designed for the Person In The Street (or, to abbreviate: the PITS); one that will be truly pleasant to use, that will require the user to do nothing that will threaten his or her perverse delight in being able to say: "I don't know the first thing about computers" - The computer must be in one lump. - There must not be additional ROMS, RAMS, boards or accessories except those that can be understood by the PITS as a separate appliance - Seeing the guts is taboo. Things in sockets is taboo - There must not be a plethora of configurations. It is better to offer a variety of case colors than to have variable amounts of memory. - And you get ten points if you can eliminate the power cord. - It would be best if it were to have a battery that could keep it running for at least two hours when fully charged. - The system must not have modes or levels. The user always knows where he or she is because there is only one place to be. ...Raskin even got the ipad's price right: - The end-user cost for this machine should be $500 or less