4 Mar
2014
4 Mar
'14
4:48 p.m.
opinion...
4 layer????
This is absurd. PDP-8 omnibus was and is very tolerant and also by
todays speeds SLOW.
This machine is from the era of 50 mil traces (back then .025" was very
fine lines!).
The logic used was mostly straight TTL which back then was slow and not
terribly noise
immune. I've wire wrapped boards for my 8f (a ram card using 1uS 2102s
all 48 of them).
The board will be fine at two layers if done reasonably. Also it doe
not need to be full height
which saves both copper and cost.
However properly plated fingers is a must. That's Nickel over Copper
then Gold. Why?
My first Altair had a poisoned bus some of the early MITS cards were tin
plate, some
Gold over Nickel and the nasty ones were Gold over Copper. Ever see
gold turn green?
That's what happens you get copper migration and the connectors
especially gold plated
ones get contaminated by that... then its a remove and reinsert before
using forever
or crash. The fix for the Altair was replace the backplane. Not
likely to hppen on a
PDP-8.
Allison
On 03/04/2014 05:37 PM, Marc Howard wrote:
|On 3/4/14, 3:05 PM, Marc Howard wrote:
I just don't want to see someone doing a good
deed get burned. Another
suggestion is that as long as he's doing this it should probably be a 4
layer board. Doesn't increase the price that much and does wonders for
signal integrity.
|4 layer boards at that size are very expensive.
Depends. About 2x which is fair given the you've double the # of layers.
A $20 board would now cost $40. The signal integrity of boards with
integral ground planes is much better and so is routing.
I never look at that end of the segment where I work. Most of our boards
are about 500mm x 600mm, have 22-28 layers, buried vias, buried caps, etc.
The 1-3 piece price will buy you a nice car.
Marc
On Tue, Mar 4, 2014 at 1:49 PM, Ryan Brooks <ryan at hack.net> wrote:
> On 3/4/14, 3:05 PM, Marc Howard wrote:
>
>> I just don't want to see someone doing a good deed get burned. Another
>> suggestion is that as long as he's doing this it should probably be a 4
>> layer board. Doesn't increase the price that much and does wonders for
>> signal integrity.
>>
> 4 layer boards at that size are very expensive.
>
> I'd like to complement him on the writeup on his site. Really a great
> read and excellent web content, diagrams, etc.
>
> -Ryan
>
>