IBM 5150 maximum memory?

Jules Richardson jules.richardson99 at
Sat Apr 26 12:34:28 CDT 2008

Chuck Guzis wrote:
> Date: Fri, 25 Apr 2008 22:39:29 +0100 (BST)
> From: (Tony Duell)
>> Did very early machines have AC fans? All the later ones certainly had 12V
>> DC ones that would be independant of mains frequency, but an AC fan would
>> run slower on 50Hz and could cause the machine to overheat. Was it really
>> that marginal (Acutally, given it's the IBM PC [1], I could beleive it
>> was). 
> Yes, they did, but the fans were rated for 50-60 Hz operation and 
> impedance-protected.   With the factory, what 62-watt?, PSU, I don't 
> think it was possible to stuff the thing full enough of drives and 
> cards to make it overheat

63W, I think - close! I did try and add a hard disk to my 5150 once, but the 
supply just didn't have enough guts to spin up any of the spare drives that I 
had. (which probably implies that maxing out the system would just result in 
the PSU shutting down rather than going into some kind of overheat condition)

> slots) to make it overheat.  Wasn't the (black) PSU on the original 
> PC fitted with red tamper-telltales stuck on along the edge of the 
> PSU clamshell?  I can't remember exactly, but I think it was.

Not sure; mine had the silver PSU and I don't remember any stickers on that 
one (but then it wouldn't surprise me if IBM UK had* different procedures to 

* or whoever was responsible for any changes in the UK market

> As this was an IBM product, I fully expected that there would have 
> been some provision for changing the AC input voltage.  Certainly 
> other personal computers of the time had the feature, and this wasn't 
> supposed to be a product from "Fred's Personal Computer and Aluminum 
> Storm Door Company".  One expected a high level of engineering from 
> IBM.

Trying (misguidedly) to control the market by saying which countries which 
machines could be used in seemed right up IBM's street, though (probably not 
the case these days; as a company they seem a lot more flexible than they used 
to be)

IBM seemed to be in a state of "maximum revenue, minimum effort" when it came 
to the PC, though.

> [1] When I got my first IBM PC-family machine (a 5160), I took it to 
> bits
> (what a suprise) and sat down with the TechRef. Every few minutes I'd
> exclaim 'They did WHAT???' as I found aother it of misdesign...
> I never understood the design that featured a complete lack of a 
> clear airpath between the plug-in-cards.  They must not have been 
> expecting very much expansion.

I don't get that, either. With 64KB max on the motherboard, how could they 
*not* think people would expand the machines? (Particularly given that it was 
safe to assume most people would also have a video card and FDC)

Maybe the assumption was that the amount of metal in the chassis and case 
shell would act like some form of big heatsink, and dissipate heat that way.

I'm sure that IBM, being who they are, *did* test airflow and heatsink 
characteristics, though (unlike what seemed to happen with the Apple /// :-)

> The MDA card was the most bewildering experience to work through-- 
> circuit traces headed off to nowhere, ICs that seemed to perform no 
> function.  Perhaps it was designed for graphics operation originally 
> and not completed and rushed to market.

What might have happened is that it was a board designed for something else 
entirely, and in a different context - and that someone did a bodge-job when 
it came to 'porting' it to the PC. I've seen it happen with other vendors when 
products evolve but still share some common ancestral circuitry.

(After what Guy said about the System/23 roots, maybe various bits were lifted 
from there?)

> Similarly, one suspects that the printer port must originally have 
> been intended as a full bidirectional design and then changed at the 
> last minute--on both the MDA and the printer adapter, all the 
> necessary circuitry was present for bidirectional operation.  Cutting 
> and jumpering a single trace was all that was needed.

That one's always seemed weird. I assume IBM didn't want the headache of 
people blowing ports by using them for non-printer things - plus it wouldn't 
surprise me if they had the idea to produce a proper lab or I/O interface for 
that kind of functionality (at high extra expense to the buyer, of course) and 
this just never materialised.

> Some aspects were pretty good--the original keyboard was very good; 
> the casework with rolled edges was also notable

Agreed. PCB construction / soldering all seemed to be high quality, too. It's 
just the design decisions when it came to the schematics and choice of 
components that seemed rather insane.

 >, although the case
> design itself was questionable (particularly in mounting/unmounting 
> disk drives).

Y'know, I've forgotten exactly what they were like, now. I do think that the 
half-height drives just weren't readily available when the 5150 came about, 
which would explain the lack of mounting options.

What really puzzles me is that it wasn't a backplane design - putting the CPU 
and (basic) memory logic on the motherboard seemed a little odd, given that a 
backplane would have given more card slots and more future upgrade 
flexibility. It must have been obvious that memory and CPU technology would 
improve, and that the number of expansion slots wasn't anything to write home 



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