Hi All,
I’ve uploaded new a schematic and
PCB layout based on the results of my reviews. There are many hidden
tricks in this schematic. You really have to watch the pin numbers
closely since it moves pins and signals around a lot. Each part footprint
is unique and some are rather unusual. Be sure to verify the proper pin
number for each signal connection to a device!
Unfortunately the chip the board uses for
the complementary clock driver (74265) is obsolete and is now
unobtainium. I’ve replace it with a 74F74 to get a minimum skew
using its Q and /Q outputs. The penalty is that the flip flop is actually
a frequency divider so we’ll need twice the frequency TTL oscillator to
get the equivalent CPU and system clock. In other words, to get a 6MHz
clock, we will need a 12 MHz TTL oscillator. I think that is easier to
fix than finding a 74265 chip.
Also to get sufficient drive for the clock
circuit, I’ve ganged the 74F74 flip flops together. The CLK68 and
SYSCLK signals don’t have much fan out but still need sufficient drive to
power the circuit. If only one flip flop is needed to drive the circuits
we can just bend the legs of the other flip flop out of the socket or clip them
if that is needed or wanted.
I have a question on the schematic though.
Does anyone know what the black filled in triangles mean on the inputs to
certain gates? Sometimes they are pointed up and sometimes down. I am
not familiar with that notation and cannot find an explanation in the
text. For example see page 491, gate U39A, pin 1 has a solid black triangle
pointing up. These are not (I think) indicators for pull up resistors
since the schematic notation chapter said pull up resistors are either specifically
called out or indicated as a white filled upward pointing triangle like on page
495 gate U34A pin 13.
Thanks and have a nice day!
Andrew Lynch
From:
n8...@googlegroups.com [mailto:n8...@googlegroups.com] On Behalf Of Andrew Lynch
Sent: Sunday, September 26, 2010
6:18 PM
To: n8...@googlegroups.com
Subject: RE: [N8VEM: 7958] S-100
68K board schematic capture
Hi Sam! Thanks! That’s
great! I placed the current version of the schematics and PCB layout in
the N8VEM wiki here
http://n8vem-sbc.pbworks.com/browse/#view=ViewFolder¶m=S-100%252068K%2520CPU
The more people able to review this
schematic and PCB layout the better the final product will be. If you can
scan in the original schematics and PCB layout from the book that would be
helpful to although it would be quite difficult since the book is bound.
I’ve accomplished a first pass of
the schematics and drafted a PCB layout. The board is enormously packed
with ICs and is very complex. We will need as much review as possible to
eliminate as many errors as early as we can.
Thanks for your help! Have a nice
day!
Andrew Lynch
From: n8...@googlegroups.com
[mailto:n8...@googlegroups.com] On Behalf Of Sam
Ducksworth
Sent: Sunday, September 26, 2010
5:35 PM
To: n8...@googlegroups.com
Subject: Re: [N8VEM: 7957] S-100
68K board schematic capture
I just received my copy of the book. I should have some free time this week so
feel free to send me a copy of the KiCAD schematics.
I'm your men, I've a lot
of free time, I've a copy of “68000
Microcomputer Systems Design and Troubleshooting” and I've patience to
check schematics. The only think I haven't is KiCAD...but, I'm searching it on
net.
2010/9/26 Andrew Lynch <lyn...@yahoo.com>
Hi! I
have started the schematic capture for the Alan Wilcox S-100 68K board.
So far it is going OK but I’ve only entered one page out of six. I
fell into one major problem with the complementary clock circuit since it uses
a 74265. That part is now obsolete so I am substituting a ganged up
74F74.
What I need
is someone who is willing to do quality assurance on the board. The
volunteer should be interested in S-100 68K development and own the Alan Wilcox
book “68000 Microcomputer Systems Design and
Troubleshooting”. Basically I am entering the schematic and
double/triple checking as I go but due to the size of this mammoth schematic I
would like some independent quality checks to verify it is right. Not a
glamorous job to be sure but an essential one and very important if we have any
chance of getting this right. You’ll need a magnifying glass and a
*lot* of patience. I will
send you KiCAD schematics or PDF print outs as you prefer.
Are there any
volunteers? Thanks and have a nice day!
Andrew Lynch
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