A real PC

November 15, 2010

IBM 5150 with video monitor

I’ve been restoring a 5150 model A. It has the second BIOS, Intel CPU, and a full 64KB on the planar. After having put the bits together and cleaned it all up I think I’m through with the hardware and can put in the two case screws. However, I’m still looking for some cork feet since the originals seem to have gotten scraped off. I’m also looking for another 16K memory chip, to replace the one I stole off another board so that I could get this one to work. There’s always some peripheral fallout to every project.

For those not familiar with the 5150, it is the original IBM PC which came out in August of 1981. Until then IBM did not make small, or affordable, machines. That is unless you count the 55 pound portable 5100. That one cost $9000 for the 16KB model and $20000 for the 64KB version. Regardless of the price, it was made for the scientific community and was not really suitable for an office, and certainly not for home use.

The 5150 was a completely new design. It was aimed at the emerging popular market. The included operating system was PC-DOS v.1.0, and thus was started a whole new world of computers. Yes, there were others on the market at the time, IBM isn’t known for being leading edge, but this one became the reference point. In fact it is really the starting point for the ubiquitous world of PC compatible computers.

There were two distinct versions of the IBM PC. The first one (which is what I’m talking about here) had 16KB on the motherboard and was expandable to 64KB. The second one had 64KB minimum, and was expandable to 256KB. I think there were also some minor variations along the way. Certainly there were three BIOS versions and machines were put together to suit the costumer.

There is much to know about these models and the DOSes that came with them. Suffice to say that DOS 1.0 was not very good, and gave way to a much better 1.1 in May of ’82. The next step was DOS 2.0 which was a complete rewrite and works like the DOS we know today. That was released March 1983 at the same time as the newer, and much more functional, IBM 5160 – also known as the XT.

I’m fairly comfortable with early DOS and the associated hardware, but the 5150 represents the formative period and I’ve learnt a lot of history from doing this. The first couple of years is when the stardards were set. That is one reason I’ve enjoyed this project so much. So, in order to avoid what I call “vintage creep” I’ve decided to freeze this machine at late 1983.

Rather than try for a machine that is “new in box” factory perfect, or a super-duper “loaded” setup that no one could have afforded back then, I’ve taken the approach of setting up something that a not so well off, but dedicated, computerist might have had. Something which is historically entirely possible but not what the average vintage buff collects. In order to do that I made up a story. Here it is:

Mr. Brown originally purchased an IBM PC in the spring of 1982 because he wanted to get stock market reports directly to his office. Although he didn’t go all out, he couldn’t resist taking advantage of, what was described as, the IBM’s “powerful color graphics”. The system included:

1 double sided floppy drive
Colour graphics adapter
64KB installed RAM
Asynchronous Communications Adapter
PC DOS v.1.1
Colour CRT display monitor
External modem

Soon finding the system limiting without either a printer or a second drive, Mr. Brown decided to sell it the next year when the IBM XT came out. However, he decided to keep the expensive colour monitor and the external modem.

Tom Mulligan was a willing buyer. He had a keen interest in the new bulletin board systems and jumped at the chance to buy a used IBM. He also decided to splurge on a fancy 1200 baud Hayes Smartmodem. He had had his eye on the flashy aluminum external version, but when offered a deal on the internal, decided that it was just as good, and besides, the now extra serial port could perhaps be used for a daisy wheel printer some time in the future. He also upgraded the system to 256KB RAM with an IBM memory expansion option that had 192KB installed. Of course he also purchased a copy of the newly released PC DOS v.2.0.

Having now spent almost a grand in addition to the computer, an IBM color display was just too expensive, so he decided to go with a cheap video monitor which he could use with the Color Graphics Adapter. He also wished he could afford a second floppy drive, but decided to wait.

It is now 2010 and that computer is mine. I’ve put it back to the state it was in when it was Tom’s and it now has all original IBM cards and the 1983 Hayes modem. I will display the computer with PC DOS v2.0 and try to make the best of that. My intention is to have an historical experience, not just a mythical “best of 8 bit” using software from 10 years after the machine was first announced. This is my idea of vintage. I want to relive Tom’s experience.

I’ve been collecting DOS software from 1981-83 and now I’m in the process of putting together a small selection of disks for various purposes. The 1983 “Exploring The IBM Personal Computer” disk is hilarious, but so is the PC DOS v1.1 with it’s BASIC programs. However, since I’ve found some excellent communications software and have plenty of room to spare on a bootable DOS 2.0 360K floppy, I can actually use this machine for BBS dialins, though only occasionally because none are local and there aren’t many of them. However, it does make for a bit of fun and an excellent demonstration.

PC DOS 2.0 video test screen

The video monitor is a bit unusual but it is from 1983. It is an Arrow 12 model DM-210G. It has both video in and out, so one can easily attach more monitors. It also has the added sophistication of having both a 75 ohm and high impedance input. The above picture shows the IBM PC DOS version 2.0 test screen. It’s actually pretty linear. The apparent problem on the right is just that it goes off the screen a bit. I can adjust it so it goes off on the left but decided to leave it this way until I can figure out which parts to adjust or replace since there is no internal pot for that.

Arrow 12 logoArrow 12 input

The inside of the machine is noteworthy as well. I’ll elaborate, and put up some pictures of the cards in my next post.

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2 Responses to “A real PC”

  1. Paul L aka Snowhog said

    Nice write up. My father would find your hobby facinating.

  2. Interesting scenario.

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