IBM 5150 - Early Version

In the supply lifetime of the IBM 5150, IBM made numerous changes to the design of the machine (case, power supply, motherboard, and other components).

This web page covers some (not all) of the features of the early versions of the 5150.

Some people refer to the early versions of the 5150 as the 'model A', presumably because later versions have a large 'B' (within a circle) stamped on the rear.
I don't recall IBM ever using the terms 'model A' and 'model B'. They used model numbers, e.g. 003, 104, 164, ...

Some AST documentation refers to the early 5150s as 'PC-1' and to the later ones as 'PC-2'.
Some Tecmar documentation refers to the early 5150s as 'PC' and to the later ones as 'PC2'.

Case

From the rear, an early 5150 can be identified as follows:

The particular type of barcode shown on the case above cannot be used as an indicator of an early version 5150.
It was also used on some of the later version cases (example).

As shown on the left, the case of a very early 5150 is slightly different to above.

(photo source: Lorne at the VCF)

Power Supply

Early 5150s have a black (or blackish) coloured power supply.
Later 5150s have a silver coloured power supply (which is noisier than the black one).

	From Framer's very early 5150, which has first revision BIOS. (photo source: Framer at the VCF)

	Compared to the above power supply: 1. The fan nuts are now recessed; 2. Different finish. (photo source: Framer at the VCF)

	Yellow sticker, and fan nuts moved to inside of case. ( Note: 220 VAC )

Motherboard

The motherboard is the '16KB-64KB' one. Later 5150s have a '64KB-256KB' motherboard.

As you can see in the photo below, the motherboard is labelled '16KB-64KB CPU'. If you are viewing a poor quality photo and can not make out the label, then the easiest way to identify the motherboard is to count the capacitors next to each parity RAM chip. In the photo, those capacitors are the round orange things at the top-left. '16KB-64KB' motherboards have two (for +12V and -5V) per chip, whereas '64KB-256KB' motherboards have only one (for +5V) per chip.

Click on photo for larger view

The BIOS is in chip U33. As supplied from IBM, the '16KB-64KB' motherboard has either the first or second BIOS revision:
1. Dated 04/24/81. Chip U33 has "5700051" printed on it.
2. Dated 10/19/81. Chip U33 has "5700671" printed on it.

Both BIOS revisions have two significant limitations:
* No more than 544 KB of RAM is recognised (because, by design, only the first 4 switches on switch block SW2 are used).
* The BIOS expansion ROM in cards that have one is not recognised. Examples: EGA/VGA cards, hard disk controllers.

Later, IBM made available a BIOS upgrade kit to upgrade the BIOS chip to the third revision one (dated 10/27/82, "1501476" printed on it).
Use of that kit removes the two limitations.
The kit can only be used if all four banks of RAM are populated. More info.
After use of the kit, the switch settings on switch block SW2 will in most cases need to be altered. More info.

Card Bracket

Early cards have a bracket that is black or blackish in colour, and wider than later brackets (silver).

Early Cards - Monochrome Display and Printer Adapter (MDA)

Below is a photo of an early version of the MDA.

Click on photo for larger view

Compared visually to the later versions, the main difference is in the top-right corner. Note the Berg connector.
Also, on this early version, pins 3/4/5 of the DE9 connector are driven low by U64. On later versions, pins 3/4/5 are not connected.

Early Cards - Color/Graphics Monitor Adapter (CGA)

Below is a photo of an early version of the CGA.
Click here to see a visual comparison of this early card and a later version of the card.

Click on photo for larger view

Early Cards - 5.25" Diskette Drive Adapter

Below is a photo of the early version of the 5.25" Diskette Drive Adapter.

Compared to the later versions, an obvious visual difference is that the early version has three grouped metal covered chips (known as MST).
Electrically, one difference is that the early version uses minus 5 volts, whereas the later versions do not.

Click on photo for larger view

Early Cards - 32KB Memory Expansion

I have no photo of this card.

Known to have "32KB MEM. CD." printed on it.

The circuit diagram of this card is in Appendix D of the AUG81 edition of the IBM 5150 Technical Reference manual. (Link to manual here.)

Early Cards - 64KB Memory Expansion

As can be seen in the photo below, this card has "64KB MEM. CD." printed on it.

The card in the photo below has D4132 and TMS4133 chips on it, chips that are stacked (piggybacked).

The circuit diagram of this card is in Appendix D of the AUG81 edition of the IBM 5150 Technical Reference manual. (Link to manual here.)