MPX-16 census

[Tony Duell]

> 
> a scholar and a gentleman. I even considered building
> my own, though not sure if MM made the artwork
> available. Was put off by the fact that the board has
> 7 layers according to the article. If anyone is
> knowledgeable enough of pcb production - is it
> possible to make a 7 layer board w/o special

I've never heard of anybody making more than double-sided PCBs at home (I 
would love to be proved wrong!). But just about any PCB manufacturing 
company could do it.

> equipment/materials? Why did it even need 7 layers?

The advantages of more layes are : 

You can route more tracks in the same area. A trivial example of this is 
that you can normally only route one track between 2 adjacent IC pins of 
a DIL pacakge on a particular layer. More layers -> more traces can be 
fitted in.

More importantly you can have particular layes for power and ground 
planes. That is, one layer will be almost all copper, with 'holes' round 
pins you don't want to be connected to ground, and connected to those 
pins you do wany grounded. Doing this will have a much lower impedance 
(it's the inducatance that really matters, not the resistance, BTW) than 
normal traces. You can also shield traced inside the board between ground 
planes. And you can design traces as striplines over the ground plane 
(and thus know their characteristic impedance) for high frequency work.

I doubt any 8088-based design needs you to go that far, but the dedicated 
power and ground planes would probably be almost essential.

> How many layers were did the IBM PC mobo have? I know

At a guess, it was a 6 layer board. Certainly 4 layers (signal traces on 
the outside, intenral power and ground planes), I think there are hidden 
signal traces too.

-tony