16 bit connections
The Megaprocessor is a 16 bit processor so we have 16 bit data
buses connecting registers, multiplexors, adders etc. There's
quite a lot of 16 bit connections. Having 16 individual wires for
each would have been very messy and confusing. Having to solder
each of these each would have been horrendously tedious and error
prone so the obvious answer was to go for ribbon cables and IDC
(Insulation displacement connections) connectors. Add a few
grounds and I decided to go for 20way cables and connectors.
This was sensible enough. It was certainly not a bad decision.
It's just that it wasn't a brilliant solution either. I still
don't know what the right answer is.
The only actual error is that I didn' t properly anticipate what
the address bus would look like. The address itself is 16 bits
wide, just like the data. The trouble is that there is also an
associated direction (read/write) signal and an enable signal. To
the extent that I thought about them at the start my vague thought
was that they would be handled as essentially independent point to
point wires. That wasn't a bad guess but when it came to
actually building the system I (too late) eventually realised that
I didn't want individual signal wires going between modules and
frames, just wide connectorised buses). I therefore had to define
a second bus "standard" with 16 bit data and a couple of control
signals and get a second family of interconnection boards made for
handling address buses. It wasn't a disaster, I didn't have to
redo anything. But it was more money and time that a few minutes
thought at the start might have saved.
The next issue relates to clarity. The ribbon cables are
relatively wide (one inch, 2.5 cm) and so take up quite a bit of
real estate on the front of a module. In quite a few situations
they obscure the schematic showing control signals which is a bit
of a shame. I did spend a bit of time contemplating how I might
take them through the board and route them round the back of the
module. The trouble is that the hole would have to be big enough
to take the connector and I couldn't think of a way of
drilling/cutting such a hole in a neat fashion. Or at least not
without it taking an infeasible length of time. Whilst I like the
way the ribbons show data moving round, the clutter they introduce
is one of the things I'm particularly disappointed by.
And then there is the expense. I've spent far more money on these
than the other components (transistors, LEDs & resistors). I
do wonder whether if I'd done things differently I could have
taken advantage of some cheap standard cables. They would also
have saved me the work of making up the cables (I've had to make
up nearly 400).
My current thoughts as to what I should have done is standardised
on an 8 bit interconnection using RJ45 connectors and ethernet
cables. Ethernet cables have 4 twisted pairs which would give us 8
wires. The data bus would use two connections and the address bus
would be done as three with one of the connections only carrying 2
- It would have been cheaper.
- Two ethernet cables have a smaller width than the 20way
ribbon so would have obscured less of the board. Or
alternatively as you don't need a huge hole to take an RG45
perhaps I could have taken the cable through the board and run
it round the back like the single control signals. (Actually
you probably would want to take the cables round the back as
the cables would come in fixed standard lengths so there'll
always be some slack you'd want tidied out of sight.)
- As an extra bonus there are a few places where I have to
split out/join together the LSB and MSB of a 16 bit bus, and
with 8 bit buses you'd get that for free.
- The only downside I can think of at the moment is that I
think the footprint of two RJ45s is greater than that of the
connector I did use, but there's not much in it.
- Too late now.
1 bit connections
I'm not sure I went down the right route here either. Soldering
directly to the board seemed like a bad idea. I wanted
something I could connect/disconnect to easily for testing and
then fit into main system. So I went for these clippy things. I
used the brass pins for input signals and the coloured ones for
output signals and power. It worked out ok. But they are
surprisingly expensive, and my soldering connections were
sometimes a little messy.
I should probably have investigated some other options in more
depth. In particular some crimp connections. But at the start when
I was wondering what to put on my first board I didn't know what
lay ahead and the thought of making up crimp terminals did not
appeal. I'd only ever made them with pliers and it's not been a
lot of fun. If I'd known just how big the project was going to get
then I'd have invested in the right crimping tools and maybe made
a different choice. Once I'd had the first few boards made it
seemed to late to go back and start over with a different choice.