My PCW Project

[greatwolf1283]

I recently was given my late father's old Amstrad PCW 8256.  After a little bit of playing around (and maybe one or two steps back at times!), I've managed to get it back to good working condition.  It's obviously looks it's age but reads all of the disks fine and boots as if it was new.

With that in mind, and knowing that the more basic machines are easier to build expansions for, I've decided to have a go at building some of the diy mods that are out there.  With that in mind (and having looked at the old articles from the Amstrad PCW Magazine), I've started by designing a bus board that will enable the easy add-on of expansions that I can build, obviously with a view to building the boards to just slot in like an old ISA or AppleBus card.  So I thought I'd see what people thought about my design:

[attach=3]

As you can see, I went with four card-edge connectors for the expansion bus, with a straight forward 50pin IDC Cable-to-Board connector that will take a 50 strand ribbon cable (going to the back of the PCW itself).  I've run the lines straight down in a double-sided arrangement essentially mimicking the arrangement on the expansion bus on the back of the PCW.  I've also added the pin-out in the correct orientation for the card-edge connectors so if anyone wanted to use the design it would then become relatively easy for them to design their own cards.

I've done a quick calculation, and the +5V, +12V and GND tracks should be able to handle around 1.9A each.  Obviously, with the number of contacts for +5V and GND, I should be able to run more current along those lines.  One of the worries I do have is if the PCW's own internal power supply is able to handle the load, or should I look to an external power option (and if there's any easy ways of achieving it that would be fantastic!) Obviously, it'd be nice to hear if anybody has any comments or suggestions before I take the plunge and order it through the fabricator.

It may seem a bit of an extravagance for something like this, but I do feel it will give me a solid foundation onto which I can put add-on cards in the coming months.

I've attached the Gerber Files for you're perusal.

[Bryce]

Definitely look at adding an additional power connector. The tracks may be able to handle 1.9A, but the flat cable and edge connector certainly won't and you'll probably have a considerable voltage drop at the cards.

Otherwise, nice design, are you sure that the ports are far enough apart for expansions with high parts (socketed ICs, Electrolytic Caps and switches etc)? I'd use a standard PC drive connector like the one on the disk drives. Easy to get, cheap and does both 5V and 12V at once.

Bryce.

[MacDeath]

So basically it is a PCW MotherX4 ?

[TFM]

Hi and Welcome to the forum!    Great to see something happen for the good old PcW. 


Such an expansion card is very neat. Did you take a look at the MX4 for CPC? That could be of interest.
CPC Boards - Cent Pour Cent


Personally I don't like edge connectors that much, they always oxidize and make bad contacts IMHO. You could use MX4 connectors like TotO did, or go for Centronics. Something more reliable. But that's just my thought. Good luck with the project! 

[greatwolf1283]

Some good ideas there.  I was worried about the potential power supply requirements, especially if running a full compliment of boards, so I've started having a play with improving the power supply options.  Been at it for a few days but got a good draft of a circuit that can supply a 12V and regulated 5V using a switch-mode circuit.


[attach=2]


This should make life nice and simple as the board would then run off a single 12V supply, which can be picked up quite cheaply, and it doesn't add too much cost to the design of the board.  Obviously, with something like this I'd be ordering the parts and then testing it before taking the plunge and ordering the board.

[Bryce]

That's a switchmode circuit?? What's the TO-220 component?

You'll have several issues using a switchmode circuit for the PCW. It will be clashing with the regulated 5V in the PCW and one of them will potentially loose the fight and die. It will also introduce switching noise directly beside the expansions, so they may become unstable.

Bryce.

[TFM]

Well, looking at shared experiences in this forum a lot of people hat serious problems with switch(bitch)mode PSUs. So it's really better to keep the fingers off IMHO. 

[greatwolf1283]

Just a very quick reply to let you know that I've not given up on this just yet  .  Currently very encouraged with the simulations I'm running with just a need to tweak a few extra bits of my new design.  It seems to be working how I'd want it admittedly it's just in the simulations at the moment), with the circuit intelligently acting to project the power supply of the PCW.  So fingers crossed going forward!


PS. This is taking slightly longer than I originally thought, although it is almost 20 years since I did any kind of serious circuit design so I'm a little rusty!