Cracking Open the IBM PC Jr.

Cracking Open the IBM PC Jr.

Summary: The PC Jr. was IBM's attempt to tap into the budding home personal computer market of 1983. While the PC Jr. had many positives with regard to the general home user, it also had several limitations that doomed it in the marketplace. I bought my PC Jr. in 1985 from my older brother who never quite figured out what to do with it. Feeling nostalgic, I decided to Crack Open the IBM PC Jr. to see what was actually in the case.

TOPICS: IBM, Hardware

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  • The red arrows point to the expansion slots and expose the fact that our PC Jr. is old. The connections, once bright and shiny, show definite signs of oxidation.

  • The other prominent chips on the motherboard are the three ROM chips marked in yellow.

  • These two IBM chips marked in yellow provide the PC Jr. with BASIC. BASIC is sort of the default operating system in the PC Jr.

Topics: IBM, Hardware

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  • Jumper Wires

    Having worked in the Austin IBM plant where motherboards where produced way back when, those jumpers were very often used to fix a defect in the motherboard.
  • IBM PC Jr.

    The IBM PC Jr. was the machine that introduced a lot of people to computers at a time when most people didn't have a clue as to how to turn on a computer, much less use one. The Jr. was my first computer. There were a number of cool software packages available at the time. In addition, you had to teach yourself Basic programing so you could write simple programs for your personal or business use.

    I don't know why, but I would love to get a PC Jr. case and put a new monster mother board and processor into it. Finding a power supply that would fit into the case might be a problem. The original Jr. power supply was actually built into the 110 volt electrical cord.

    And I might have to also upgrade the 16 color monitor. It would be cool to be able to put a new technology video system into a PC Jr monitor case.

    No, I'm not really crazy. I am just retired and bored.
    • Here is a modern solution

      Simply gut the JR and put an Intel Mac mini inside it.
      You will then have a modern computer that can run the
      superior MacOS X or the malware sucking Windows.
      • RE: Jump the connection revisited (Cracking Open the IBM PC Jr.)


        Windows IS malware.
    • RE: Jump the connection revisited (Cracking Open the IBM PC Jr.)

      @david602@... Dave, I have an IBM PC Jr. It's been boxed up for years as it was also my first computer, complete with a blinding-fast 300 baud modem! (I ate supper while It downloaded a 128 KB program from a local BBS in Indianapolis.) I ;ive in Daytona Beach Shores FL now.
      • RE: Jump the connection revisited (Cracking Open the IBM PC Jr.)

        Hi: I am Jud McCarthy, retired IBM engineer and manager. I am the president of the IBMSFQCCAA association. This is IBM?s 100th anniversary and we, some IBM retiree volunteers, are looking at restoring some old IBM systems, including PC jr?s. Although we a not in a position to buy any units, we would be very interested in aquiring your PCjr to add to the local Boca Raton Historical Society. You can dake this donation off of your income tax. Let me know your thoughts. Best regards ----- Jud

        Justin (Jud) McCarthy
        251 SW 9th Ave
        Boca Raton, FL 33486
        Home (561)391-1422 Cell: (561)504-7048
  • RE: Jump the connection revisited (Cracking Open the IBM PC Jr.)

    It's a resister pack! It consists of about 8 4.7k resisters, probably used for floating logic levels.
  • RE: Jump the connection revisited (Cracking Open the IBM PC Jr.)

    BASIC as the "operating system"'ve got to be kidding!!!??!!....ever hear of IBM-DOS? Basic, a programming language, was built-in on the IBM-PC and IBM-PCJr. Where did thjis author learn about PC's?????
    • IBM PC Jr.

      You are correct. Basis was not an operating system. The PC Jr. ran off of a 5 1/4 inch disk containing the DOS 1.0 operating system. There was no hard drive in the Jr.

      The problem was that very little DOS based software was available at that time. If you wanted a program that wasn't available on the open market for the PC Jr., about the only simple way to get one was to write a program in Basic and run it on the Jr. I wrote a number of business programs in Basic on my PC Jr. system and they worked very well. A instruction book on writing Basic Programs came with the PC Jr.
    • BASIC built-in.. anyone remember S.I.M.P.L.E ?

      On many IBM clones of the period (and perhaps
      even on the IBM's, I cant remember), if you did
      not put a bootable floppy disk in, the PC would
      default after a short period, and run BASIC from
      the built-in ROMS.

      So whilst (I agree) it's not the operating system
      I can see why someone might get confused about

      S.I.M.P.L.E - was a joke out about the same time.

      Single Input Multiple Programming *something* Environment

      3 commands available... Start, Stop and Halt.


    • I would actually agree with the Author.

      DOS - Disk Operating System
      BIOS - Basic Input Output System

      In the PC Jr, like most home systems of the time, you could
      boot straight into BASIC and it provided access to all of
      your features from graphics, timers, interrupts (through
      sys calls and such), I/O (through peeks and pokes),
      memory management with automatic garbage collection
      and other features of the hardware/system.

      Since the systems did not include a monolithic all encompassing Operating Systems, calling the ROM based
      BASIC interpreters the OS is as correct as anything.
  • RE: Jump the connection revisited (Cracking Open the IBM PC Jr.)

    The white chip is a multi-resistor pack. You can tell by the 4.7K marking on it.

  • RE: Jump the connection revisited (Cracking Open the IBM PC Jr.)

    Well, it kind of was -- in a way. If you had not cartridge installed in the cartridge slots and booted the computer without a disk in the drive, the system booted into BASIC. You could do most things there that you could with BASICA (Which actually required Cartridge BASIC), but no access to the disk because the Disk Operating System was not there. I believe you could access other I/O, though, and may have been able to load/save files in another way with other devices -- possibly.

    PEEKing and POKEing around in the memory addresses could probably have done some amazing things.
  • RE: Jumper Wires

    Having done electronics repairs on computers for years, those wires are old friends.

    They are often also used, besides repairing bad traces, as ECO (or Engineering Change Order) upgrades to correct design errors or errors that reveal themselves after the manufacturing of the circuit board is complete.

    Back then, they didn't have a whole lot in the way of computer aided layout and thus was done by hand, so signal interference and a host of other problems wouldn't manifest themselves until after the board was made. Also, they could have just forgotten a connection prior to manufacture since they didn't have circuit simulators either. Or, they could have just run out of room on the layers for a trace, since they couldn't make them with as many layers as they can now.

    Most of the logic chips shown on the controller, drive and motherboard are nothing more than basic and/or/not gates. You can google the numbers for their exact function if you have the whim.
  • RE: Jump the connection revisited (Cracking Open the IBM PC Jr.)

    IBM had to plow out their own way and refused to consider that there was a multitude of computers that had been around for years. With no lessons learned, we stand today with a technology that could have been so much better if IBM would have stayed out.
    • Or it they had bought their OS from Gary Kildall

      rather than The cobbled up Quick and Dirty Operating System (QDOS) that Bill Gates purchased from Seattle Softworks.
      Update victim
  • one of the pioneers of the home personal computer???

    "See what's inside a 1983 edition of one of the pioneers of the home personal computer market."

    Pioneers, Timex Sinclair, Adam, Heath Kit, Atari, Apple, and Commodore with both the Vic-20, and the C=64 out before the IBM PC-Jr. The PC, Jr was a tag along, Johnny-come-lately in this group.And considering its performance, it was grossly overpriced for the time.
    Update victim
    • Re: one of the pioneers of the home personal computer???

      As someone who was already using and selling computers well before the PC-JR came along (a living techno-fossil), I have to disagree with you.

      I had a VIC-20 and two C-64s and although the C-64 Executive came close to being classed as a "pc", the other two were designed as advanced "tv games" consoles, not personal computers. The other brands you mention weren't much better either.

      I'm pretty sure that Commodore's PC-10, which was their first serious attempt at a home computer system, came out in response to the PC-JR, not before it. I also owned a PC-10 (but not a JR thank god)! And I am also fairly sure that Apple had not yet launched the Mac at that time, so you'd have to be saying the IIe was a "pc", which I'd argue it was not.

      Of course it must be said that my experience was here in Australia, and it may have been different in the US of A.

      So, from the viewpoint of a walking part of PC history, the PC-JR was a bit of a landmark - almost as "must have" as an iPhone today - a fashion accessory for wannabe techno-geeks.

      They walked out the door under their own steam for the first few weeks - we couldn't keep up with demand. But they were also a piece of crap that my employer at the time, Computerland, sold off in huge warehouse sales for months after, trying to get rid of its JR stockpiles.

      So there...
      • I would disagree...

        As someone who also sold computers (family owned business) at the time, I think you are confusing terms.

        PC - Personal Computer and has almost alway referred to
        IBM compatibles. It simply did not apply to the host of
        other systems. Ever.

        Home computers.
        [i]I'm pretty sure that Commodore's PC-10, which was
        their first serious attempt at a home computer system,
        came out in response to the PC-JR, not before it.[/i]

        The VIC-20 and C-64 were both SERIOUS successes (not
        just attempts) of home computer systems. While the VIC-
        20 was little more than a game system, I know 100's of
        people that got their first taste of programming with them.
        The C-64 was used for a host of applications from games,
        word processing, accounting, spread-sheets and dozens of
        other "serious" applications. Before that, you had the CBM
        and PET systems (like the 4032/8032 [quite cool serious business machine) that predated the IBM-PC by a few

        Tandy had a serious system with the TRS-80. Apple with
        their II+. While these were NOT PC's they were home
        computers and had great success for their time.
    • performance vs price

      I know it was too expensive for me. About Aust.$8,000 or $4,000 I think?? At that time the Australian dollar was not floated and was worth US$1.12.

      It was basically an electronic typewriter if you did not know a lot about computers - and who did.

      I managed to borrow a friend's Amstrad that was just a solid (heavy lifting) block keyboard with a raised square block on the right that took a floppy disk housed in a cassette. It connected to a B&W small TV monitor I think. It was easy to pick up basic and do some some programming. Had DOS but mainly CPM .. something. Was writing a screenplay with 'Tasword'.

      Do you remember the Compaq keyboards made in Ireland? Solid metal meant to withstand being picked up and dumped by twisters- :-)

      I see that the IBM PCjr was made in America and although probably had an dipped etched circuit board, were all the resistors and capacitors hand soldered on, or by machines? There are a lot of parts ... manufacturing at the time probably couldn't get the cost down for widespread acceptance.

      Be good to put original retail price (US OK) for the 'cracking open section' plus manufacturing methods at the time - for future generation use.