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IBM 5150  -  Early Versions


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 (example).
IBM's explanation of the B stamp is at here.
I do not see the computer industry using 'model A' and 'model B'.  Instead, I see them using variations of 'PC-1' and 'PC-2'.
- Some AST documentation refers to the early 5150s as 'PC-1' and to the later ones as 'PC-2'.
- Some Intel 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'.
- Some Everex documentation refers to the early 5150s as 'PC 1' and to the later ones as 'PC 2'.
- Some Diamond Flower documentation refers to the early 5150s as 'PC-1' and to the later ones as 'PC-2'.




Case

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

5150_early_case_rear.jpg

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).


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

( photo source: Lorne at the VCFF )



Power Supply

Early 5150s have a black (or blackish) coloured power supply.
Later 5150s have a silver coloured power supply.


5150_early_psu_1.jpg From Framer's very early 5150,
which has first revision BIOS.


( photo source: Framer at the VCFF )
   
5150_early_psu_2.jpg Compared to the above power supply:
1.  The fan nuts are now recessed;
2.  Different finish.


( photo source: Framer at the VCFF )
   
5150_early_psu_3.jpg

Yellow sticker,
and fan nuts moved to inside of case.

( note: This is a 220 VAC version )

( photo source: modem7 at the VCFF )



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.

5150_early_motherboard_800x582.jpg Click on photo for larger view

The particular motherboard pictured above, has an 8088 CPU containing an interrupt bug.  More information on that is at here.

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 inspected by the early BIOS revisions).
* The BIOS expansion ROM in cards that have one is not recognised.  Examples: EGA/VGA cards, hard disk controllers.

If your motherboard has either of those two BIOS revisions, and there is less than 64 KB of RAM fitted/selected, then you may need an early version of DOS.  See here for more information.

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).

5150_early_bracket.jpg



Floppy Drive


Side count

The floppy drive in very early 5150s was single-sided.
Single-sided double-density (1S2D, or SSDD) floppies were used in those drives.
DOS 1.0 and 1.1 would format the single-sided floppies to 160 KB  (1 side x 40 tracks x 8 sectors per track x 512 bytes per sector).
DOS 2.0 and later would format the single-sided floppies to 180 KB (1 side x 40 tracks x 9 sectors per track x 512 bytes per sector).

Click here to see a method of determining whether your TM-100 drive is single-sided or double-sided.  Also, if double-sided, you can expect to see 'DSR' under the front latch per here.

DOS 1.1 introduced support for double-sided floppy drives, and so IBM may have moved to double-sided floppy drives about the time that DOS 1.1 was released (May 1982).


Logo

The floppy drive in very early 5150s did not have an IBM logo on the front.  It is not known as to when IBM had the logo added.



Early Cards - Monochrome Display and Printer Adapter (MDA)

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

5150_early_mda.jpg 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 (J2, which is for a light pen).
Also, on this early version, pins 3/4/5 (red/green/blue) of the DE9 connector are driven low by U64.  On later versions, pins 3/4/5 are not connected.

5150_early_mda_top_right.jpg



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.

5150_early_cga.jpg 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 IBM Monolithic System Technology).
Electrically, one difference is that the early version uses minus 5 volts, whereas the later versions do not.

5150_early_diskette_adapter.jpg Click on photo for larger view



Early Cards - Asynchronous Communications Adapter

The very early version of this card has no U15 shunt block (J9 and J10 controlled the port address).
A photo of this early version is at here.
A circuit diagram of the very early version of the card is in appendix D of the AUG81 edition of the IBM 5150 Technical Reference manual.  (Link to manual here.)

In a later version of the card, the U15 shunt block was added.
And even later, when the IBM 5160 was released, J13 (for operation in slot 8) was added.



Early Cards - 32KB Memory Expansion

I have no photo of this card.

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

Additional information on this card is at 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 µPD4132 and TMS4133 chips on it, chips that are stacked (piggybacked).

Additional information on this card is at here.


5150_early_64kb_memory_expansion.jpg



Speaker Holder

Very early 5150s have a speaker holder made of metal.

5150_metal_speaker_holder.jpg From Framer's very early 5150,
which has first revision BIOS.


( photo source: Framer at the VCFF )



Keyboard


Types 1 and 2

The first edition of the technical reference for the IBM 5150 (edition AUG81) shows a keyboard that has different circuitry to the circuitry shown in later editions of the 5150 technical reference.
The circuitry of the early 5150 keyboard required an external RESET signal (from the 5150 motherboard) to reset it at power-on of the 5150.  (Diagram)
In later 5150 keyboards, the keyboard resets itself at power-on of the 5150.

The APR83 edition of the IBM 5160 technical reference contains a lot of circuit diagrams, and there, it describes the two types of keyboard as 'Type 1' and 'Type 2'.

My early 5150 was supplied with a type 2 keyboard.


Physical

Some people who have acquired very early 5150s have observed that the accompanying keyboard has all all-metal connector.
Although possible, and arguably even probable, it should not be assumed that all type 1 keyboards have the all-metal connector.
To firmly establish that connection, a lot (a statistically significant sample size) of type 1 and type 2 (as determined by circuit board) would need to be examined.

5150_early_keyboard_connector.jpg From one of Shadow Lord's early 5150s


( photo source: Shadow Lord at the VCFF )


An online discussion of the early keyboards is here.