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Serial / Parallel I/O via Z80 bus adapter

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Hello again,

Here's some more news.

One of the problems with having and using a PCW is the difficulty with transferring stuff onto it. Amstrad made an adapter called a CPS 8256 which incorporated an RS-232 and Centronics printer port, so I decided that because these are fairly hard to come by and generally expensive as a result, I'd design my own.

So here's the thing. It attaches to the Z-80 bus I introduced with uDE-8 and uses a modern MAX222 charge pump to generate the 12v required for true RS-232, but also has TTL level RS-232 for direct connection to a USB serial adapter.

Other than those points, it should be compatible with the SIO and parallel drivers that are built into CP/M Plus. You can hook it into your uIDE Z80 bus and be away immediately.

To transfer files, just use KERMIT.COM, which will be on the uIDE image I plan to share (you can use this to get the uIDE driver onto your PCW too, but more about that later). And, of course, the Centronics port lets you plug a standard parallel printer into the 8256 - step up to bubblejet!

For now, let's take a look at the new board.

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The 40 way connector on the left is the Z-80 bus adapter. On the right is a block connector for RS-232 and Centronics plugs, and behind the Centronics connector is the TTL RS-232 header with GND and +5v. There's a power light bottom left.

I may add TX/RX lights too because there's probably space (below the RS-232 header).

These boards will go into production if there is enough interest. I do not propose to offer them built though, because they are much more complex than the uIDE and the key components, the Z80-DART and i8253, are a bit expensive. And please note, you need one of the Z-80 bus adapters to use it (Z-80 shim or PCW Expansion Port adapter), but if you have a uIDE you can daisy chain it to the Z-80 bus connector on there.

Oh, by the way, it's not "universal" like uIDE. PCW only, because there is no space on the board for the additional jumpers and circuitry, as you can see. I don't think it will work with the 9512 either; I have a genuine CPS8256 and that isn't compatible with my 9512. I need to find out why, though I suspect a clash with its inbuilt Centronics port. Research is needed...

Still, what do you think of it? Interested?

Your PCW Pal

Well, I'm currently working on my own CPS8256 clone, although I've plumped for the MAX241, due to the fact that the circuit requires 4 receiver circuits and 3 transmission circuits for the rs232 transceiver.  Also, I've also discovered that there's also a couple of errors on the published schematics that I'm waiting to check before announcing them.  Of course that involves taking some of my CPS8256 apart and as I've got it working again I need to be carful about it.

One thing I've also done with mine is include the DE9M and DB25S connnectors which are the standard these days.  I'm also looking at replacing the 8253 chip which is no longer in production with an 82C54, which is still being produced by Intersil.  Again, whilst everything suggests this should be a drop in replacement, I still want to test it first.  Once I've also got the boards back for the 8256 version I'm going to be building one and looking and seeing if I can make a version that works with the 9512 and also has the connectors in a far more sensible orientation.

That'll plug into the expansion port, I expect. Nice to have some options..

Mine uses the Z80 bus (per uIDE) and there is a new backplane to carry it and other boards:

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The backplane is a really rough render and I expect it will be altered (needs mounting holes and the gap between connectors may need widening) but that's basically it..


--- Quote from: JonB ---That'll plug into the expansion port, I expect. Nice to have some options..
--- End quote ---

Indubitably, although at times using the backplane Expansion Connector is the most annoying thing to use.  Almost all the pinouts for it are for the drilled holes just inside the case rather than the actual card edge poking out the back.  I'd already laid it out once and had all of the signal lines done when I realised that the bloddy thing was mirrored!  Having to double check the different signal lines to make sure there all correct was not fun!

One thing I did find on mine was an extra 1K resistor between +5V and pins 1 & 4 of the 74LS74.  Now it could be that mines a earlier spin of the board, but my guess is there was a problem with that particular part of it and they potentially added it to limit the current going through those 2 pins for some reason.  I'll take a photo of mine today at some point.

Pins 1 and 4 of the 74LS74 just need to be held high. Technically you could connect them directly to the 5V rail, but it's better practice to use a pullup resistor to limit the current and it will reduce the overall current the device requires. Anything up to about 10K should be fine.



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