IBM 5150 maximum memory?

[Tony Duell]

> 
> 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 

Sure, but the fan would turn slower on 50Hz mains. I doubt that would 
cause the machine to overheat, but it's the only logical (as opposed to 
marketing) explanaition I can think of for voiding the warranty.

> think it was possible to stuff the thing full enough of drives and 
> cards to make it overheat (remember too, that there were only 5 
> 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.

Later PSUs had tamperproof Torx screws on them, and the IBM service info 
implies the PSU should be replaced 'for safety reasons' if there's any 
evidence of tampering. Needless to say I've taken IBM PSUs apart and put 
them back together with the original screws...


> As this was an IBM product, I fully expected that there would have 
> been some provision for changing the AC input voltage.  Certainly 

I've only seen the UK-supplied units, but they have an internal soldered 
link that is fitted for 115V nad open for 230V. Why it wasn't a rear 
panel switch I don't know (it was on the PC/AT and the Portable [1] of 
course)

[1] Whcih make a lot of sence given that that machine was intended to be 
moved around. On the other hand, it's not as nice as the HP Integral 
supply which contians a little circuit, the purpose of which is to short 
out the mains (triac and low value resistor) to blow the fuse if the 
machineis set for 115V and plugged into a 230V supply...

The IBM PC and XT supplies I've seen are clarely Zeniths. They are 
unueual in that they have a mains-frequency transfortmer for the startup 
supply and unlike many small SMPSU cannot be run from a DC input.

> 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.

In the same way that I used to expect a high level of engineering from 
HP, but look what they're turning out now :-( :-(

> 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.

On the printer adapter (althohgh not the MDA card), there was actually a 
set of solkder pads for this cut-and-jumper mod. They are not documetned 
in the Techref, I have no idea why not

-tony