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There were personnel computers before the IBM 5150 but the 5150 is the computer that solidified what an IBM compatible PC would be. The 5150 featured an x86 processor, internal slots for various upgrade cards and was primarily intended to run off of PC-DOS / Microsoft MS-DOS operating system, all things we would learn to associate with the future IBM PC compatible market. The IBM 5150 was released in 1981 and was primarily intended as a machine for serious business tasks but was certainly capable of playing games and performing other non business functions.

The overall construction of the 5150 is extremely sturdy with a heavy metal case. The face of the case features no buttons or LEDs that we expect from later PCs and just features a stylish IBM PC badge, some vents and dual full height drive bays. The 5150 was sold in a few configurations including dual and single floppy drive variations as well as a version with no drives at all.

The dual full height bays commonly housed one or two 160k/360k (single/double sided) floppy disk drives. You can add two half height drives into these bays though this will require some means to secure the mounting as the bays are designed only to secure full height drives.

The power button is located on the left rear side of the 5150 case and uses a large on/off switch.

A locking mechanism was sold by IBM which one would place over the power switch which allowed the PC to only be turned on with the use of a key though I believe this also worked with XT and AT models as well.

Lets take a quick look at the rear of the case.

On the left side we have one female and one male power port. the male connector uses a standard three prong power cable with the female connector intended for a monitor to to plug into. To the right of the plugs we have a large round vent for the power supply followed by a keyboard and cassette connector. On my PC these ports are conveniently labeled.

The original keyboard for the IBM 5150 is the model F.

Model F keyboard

The model F is similar to the later model M in that it is very well built and does feature “clicky” keys. As you can see above the function keys are located to the left and there is no space between the num pad and the rest of the keys. This is a PC/XT class keyboard and will not work on later AT class machines.

The cassette port located next to the keyboard port was rarely used on the 5150 though I suppose if one had purchased a variation without any disk drives this would have been the expected method of data transfer. Finally we have five slots for adding expansion various cards.

Before opening this case up and looking inside lets take a quick look at the monitor.

The intended monitor for the 5150 was the IBM 5151 green monochrome monitor. Being mostly intended as a machine for office use where sharp text took priority over color, although IBM did release a color CGA and EGA monitor with very similar styling. The 5151 monitor gives a very nice and sharp monochrome image and is also surprisingly light compared to the general heft of the 5150. The two dials are for controlling brightness and contrast.

Unfortunately the two power cords on the 5151 are hard wired to the monitor itself. One cord is a 9-pin connector to connect to a monochrome video card with the other being the power connector intended for connecting to the female plug on the 5150’s power supply. There is no power button on the 5151 as the monitor was intended to power on with the 5150.

OK, now lets take a look at the inside of our 5150.

Take note that I have upgraded the power supply in these images to a 130w power supply in order to accommodate a possible hard drive. The stock power supply in the 5150 is a fairly anemic 63w supply and may not supply enough power if a hard drive is added.

There were two motherboard versions released for the 5150 depending on when it was manufactured. Earlier models used the 16kb-64kb boards which only were capable of supplying a maximum of 64kb of RAM on the board’s RAM sockets. My 5150 uses the later 64kb-256kb motherboard indicating a total of 256kb of RAM could be installed on board.

1 ) CPU – The CPU is the classic 8088 running at 4.77MHz. This was a cost saving measure as the 16-bit 8086 would have been faster but more expensive. Keep in mind that the NEC V20 which offered a slight speed boost over the 8088 was a fairly common upgrade on these machines.

1.5 ) math copro – Located alongside the CPU socket is the FPU socket for adding an 8087 math co-processor if desired.

2 ) RAM – My PC has been upgraded to the full 256kb of memory. Notice how the first row of memory is directly soldered to the motherboard while the following three rows are socketed.

3 ) PC Speaker – The 5150 uses a real PC speaker located on the far side of the case.

4) Switches – Two switch blocks located on the motherboard are used for setting things such as installed memory, type of video card installed ect… switch settings as well as a large amount of other information on the 5150 and other IBM PCs can be found here.

Lets take a look at the various expansion cards I have installed in my 5150.

Unlike later IBM PC compatibles which upped the standard expansion slot count to eight the 5150 only sports five 8-bit ISA slots. This can be a little limiting considering there is virtually nothing built in as far as video and I/O goes as well as the fact that your going to most likely be losing one ISA slot right from the start to a floppy drive controller card.

This is the standard IBM floppy controller. It features an external floppy port and when paired with the 5150 can support up to four floppy drives. Drives connect via the edge connector on the rear of the card. This card is currently running my dual 160k/360k drives but is also capable of supporting 720k 3 1/2 inch floppy drives.

Next up of my currently installed cards we have the monochrome video / printer card.

The IBM monochrome monitor card (MDA card) is designed to connect to the 5151 monochrome monitor and you will usually find this card installed in almost all 5150’s. The MDA card is not capable of displaying pixel graphics but only characters thus making it largely unsuitable for games but great for business which mainly benefited from having a sharp text display. There are games like Rogue which can be played on an MDA monitor since it only uses text and characters for graphics. Thankfully IBM understood the situation with limited expansion ports so the IBM MDA card also included a printer port.

An option even better then the IBM monochrome card would be a Hercules card or Hercules compatible card. These cards also worked with a monochrome monitor and featured a parallel port but also had the added benefit of being able to display graphics on a monochrome monitor for games which supported Hercules mode and with a small program even emulate CGA graphics in monochrome.

Thankfully you could also install a CGA card (as well as an EGA or VGA card) in the 5150.

There are a few revisions of this card from IBM with minor differences but I am using a later revision here. Although you can install a EGA and even some VGA cards in the 5150 I find the machine lacks enough power and RAM to properly run those sorts of games and is best suited for playing CGA titles. There are also a number of later more advanced CGA cards from companies like ATI which offer more display modes in a smaller form factor but I was trying to keep things more or less IBM and early 80s with this build.

Most commonly CGA offers games to be played with four colors from just a few palettes. Black, Cyan, magenta, white is the palette most associated with CGA though there are other palette options.

The IBM CGA card also offers a composite out for connecting to a standard TV. This produces a less sharp image then CGA on a computer monitor but techniques like dithering can be used to blend colors and create an image resembling EGA in some cases.

You also have the option, like I have done with my 5150, of installing dual video cards. I currently have both a MDA and CGA card installed. You can switch back and forth between MDA and CGA monitors with a DOS command MODE CO40 for CGA 40 column mode, MODE CO80 for CGA 80 column mode and MODE MONO for the monochrome display in 80 column mode.

The final card I have installed is an AST MegaPlus II card.

multi function cards like the MegaPlus II and the AST SixPack Plus were fairly common for the IBM 5150. The lack of expansion slots gave an opportunity for third parties to step in and create expansion cards which incorporated several useful features into one card. The MegaPlus II adds a real time clock, up to 512kb of memory to bring your 5150 up to 640kb, a serial port as well as multiple headers for daughterboard expansions.

Adding a hard drive / 720kb floppy drive

I kept my 5150 fairly stock and period correct but for some extra quality of life adding a hard drive and 720kb floppy drive is fairly simple.

The first issue you’re going to have with adding two half height drives is that the bays were only designed to securely mount a single full height drive as we can see in the image below.

Any upper drive would have no points to mount to. Thankfully there are some solutions as mounting plates were made to allow the use of dual half height drives. Theoretically I suppose if you had some sheet metal and tools you could create your own mounting plates. Below is a mounting plate I used, unfortunately it has no labeling so I’m not sure if it was homemade or part of some official kit.

If you do not have a mounting plate I found improvising works just as well though may it may not look as eloquent of a solution. I was able to position a mounting rail from a random PC diagonally and use it as a fairly sturdy mounting solution.

Once the mounting situation has been taken care of its time to well….mount the drives. Mounting the 3 1/2 720kb is fairly simple as even the stock IBM floppy controller will support it. You just either need a cable with the correct connectors or install a pin to edge connector adapter on your 720kb drive and it should run just fine. Newer 1.44mb drives will also work as 720kb drives without issue.

Installing a hard drive is a little trickier as the stock power supply is under powered for the task and the 5150 was not initially designed with a hard drive in mind as IBM never offered a hard drive as an option although later it did offer a 5161 expansion unit which did house a hard drive but which was the size of the entire 5150. Your first task is to upgrade the power supply with a beefier one. I upgraded mine to a 130w supply.

If you want to be more period correct you will want a MFM or RLL hard drive controller. I used a WD controller as well as an ST-225 20MB drive which worked without issue for me.

If your not trying to be period correct your best bet would probably be an XT-IDE controller with a CF card as these draw much less power then period drives and offer much faster speeds and reliability.

A friend once asked me why I would want a 5150 and proceeded to ask me if I “wanted to emulate being a 1980s office worker”. This comment is actually pretty fair since the 5150 is first and foremost a business machine. You certainly can play games on it and with a little work upgrade it well for this purpose but in truth there are many better options if you’re looking for an early 80’s 4.77MHz niche machine. The weak power supply and limit of five expansion slots does not do the 5150 any favors. If you want an early 80’s 8088 based machine find or build a nice machine with eight slots a turbo button and 640kb of memory built in.

On the other hand as a collector of vintage PC hardware the 5150 is a no brainer, it needs to be in your collection. The 5150 is iconic and set the stage for all IBM PC compatibles for decades to come.

One Comment

  1. The IBM PC 5150 was the first “real” PC I worked on in my student’s job as research assistant. I was young and inexperienced so I got the oldest computer there, an original IBM PC. It had the now iconic green-displaying monitor attached, since only the dedicated graphics PC (probably an AT) had a VGA card and IBM’s color monitor. What really drew my attention was the incredibly precise feeling of the model F keyboard. No wonder the IBM keyboards are so famous up to this day.

    The most important piece of software was the “IBM professional editor” which allowed a little comfort editing ASCII text files, be it some BASIC source code or texts prepared for printing. Comfort means for example that you didn’t have to use line numbers when editing text (as in edlin, the DOS line editor, or the BASIC editor). We also used IBM’s BASIC Compiler that produced object files needed to be linked in a separate utility to make it executable.

    I also remember the two full height 360k floppy drives. This full height drives are what constitutes the look of the original IBM PC for me. I understand a hard drive is convenient but I wouldn’t do the change to half height drives, especially not 3.5-inchers.


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