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IBM 5150  -  Known Problems/Issues


Limitations of early BIOS revisions

The first two revisions of BIOS for the 5150 have limitations.  Click here for details.


Bugs in final BIOS revision

Two of the bugs in the third BIOS revision (10/27/82) for the 5150 mean that problems will be encountered if your 5150 motherboard does not have all four banks of RAM populated, and SW1 set accordingly.  For example, the POST in the BIOS thinking that there is more motherboard RAM than actually fited.  Click here for bug details.

Another bug: Hard disk drive controllers for PC-class and XT-class computers have a BIOS expansion ROM that typically sits at address C8000.  If that ROM (at C8000) becomes corrupted (except for the first two bytes), then what is supposed to happen is that the 5150's POST displays "C800 ROM".  But a software bug causes the 5150 to instead beep the error pattern of 1 long beep then 2 short beeps, leading you to believe that a video problem exists.


Short circuited tantalum capacitors

Tantalum capacitors are located on the motherboard, expansion cards, and drives.  Sometimes, one will go short-circuit, stopping the power supply from working (although the power supply fan might still turn).
It will appear to you that the motherboard is 'dead', although to be noted is that that symptom has many causes.
If you suspect that your 5150 may have a short-circuit tantalum capacitor, then try the diagnostic procedure here.


Minimum RAM requirement for DOS

If you try to boot DOS, and there is not enough conventional memory for that particular version of DOS, do not assume that DOS will always present a friendly message informing you of the situation.  See here for some examples.


Hard disk drive support

See here.


Floppy disk drive problems

Very early 5150s were fitted with a single sided floppy drive.  In those, you use single sided floppies (to be specific, single sided, double density, soft sectored).

Later 5150s were fitted with a double sided floppy drive, which was typically the Tandon TM100-2 (or TM100-2A), or the Micropolis equivalent.
Click here for issues associated with the TM100-2 (or TM100-2A), although most of the content also applies to the Micropolis.


POST cards do not work

The power-on self test (POST) of an IBM 5150 motherboard does not output POST codes.
Any numbers that you may see displayed by a POST card are not POST codes; they will be the result of something else.
More information is here.


16-bit ISA cards

Some 16-bit ISA cards advertised as being 8-bit slot compatible, may not work in the IBM 5150.
Here are some possible causes:

• When the maker wrote "8-bit slot", they meant "8-bit slot in an AT-class computer".  This kind of requirement is usually found in the card's user manual.  
   Click here to see an example.

• Some 16-bit cards need to be reconfigured for 8-bit slot operation.  That configuration is sometimes done by switches or jumpers.  On at least one VGA card, it is done via the card's configuration software.


Interrupt bug in early 8088's

See here for more information on this subject.


V20 CPU

Swapping out the 8088 CPU for a V20 CPU can sometimes cause problems.  See here for more information on this subject.


COM2 setting on serial card does not work

Reported by AST Research.  See here.


Modern power supplies

For whatever reason, someone may decide to power their 5150 motherboard using a modern power supply, with adapter.
Modern power supplies (depending on age) do not produce minus 5 volts.  The implication of a lack of minus 5 volts is:

Motherboard	Symptom
16KB-64KB	It will not work at all, appearing 'dead'.  This is because the RAM chips (type 4116) require minus 5 volts to function.  As a result, the POST test of the first 16 KB of RAM will fail, then the POST will simply halt (providing no audible nor visual indication).
64KB-256KB	At power-on, the POST will unsuccessfully test the cassette circuitry, resulting in the POST displaying a 131 error on-screen.  This happens because chip U1, which is part of the cassette circuitry, requires minus 5 volts to function.

Minus 5 volts is also used by some of IBM's early expansion cards.  See the 'Known Use of the -5V by IBM' section of here.

RAM failure in motherboard bank 0

Vintage RAM chips have a relatively high failure rate.
The failure of any chip in the 5150's first bank of RAM (bank 0) results in what appears to be a 'dead' motherboard.

Click here to see a diagram that shows the four RAM banks.

In the 5150, RAM failure in bank 0 is not easy to diagnose because the bank 0 chips are soldered in.

One way to diagnose this particular problem is to use a suitable SuperSoft/Landmark diagnostic ROM in socket U33 (replacing the existing BIOS ROM).  
The diagnostic ROM will identify any faulty RAM chips in bank 0.

The piggybacking technique will sometimes diagnose this particular problem, but because of the hit-and-miss nature of the piggybacking technique, use of the SuperSoft/Landmark diagnostic ROM is very much preferred.