Mike's PCjr Page

I am interested!

monzamess wrote:I am interested!

I'll probably knock together two prototypes if I feel like I'm headed in the right direction with this. You're welcome to buy one off me if they come out working.

Actually I might be willing to let go of both prototypes if anyone else is interested, considering my first idea is still working well and my PCjr motherboard is already well-modified to accept it.

EDIT: Oh, and I'll sell them at cost, I think. I enjoy doing these things so I don't think I need to make any money off my prototypes.

I just put in a sample request for a 40-pin DIP carrier that I think I might be able to use to make this board pluggable. Hopefully they'll send 'em to me. I said I was a student doing research (which is technically true, although the research is independent of my studies).

I've had good luck with getting samples out of a few companies before (my favorite was when I got etchable flex circuit material that I used to fix the front light in my GP32).

Hmm, clearly I'll have to do some more thinking about this. When I re-sized the holes on my PCB design to accommodate the DIP carrier (which will have to sit lower than normal if it is to plug into the CPU socket), I no longer had room for some of the traces I had laid out, and the board is too small to make use of very many alternate paths. I guess I have two options, either make the board bigger or start using vias.

Argh!

I'd love to chip in, but you seem to have all of the free time that I'm missing!

Don't run too far ahead - I'm sure a few of us would like to help you double check the work. I'm still very impressed at how quickly you hacked the original modification in.


Mike

I'll try to clean up the schematic and make it a little more human-readable if anyone wants to check out the design before I get a prototype made.

EDIT: Eh, I tried to clean it up, but it's pretty much still unreadable. I only used it as a transition point between the design in my head and the PCB layout software anyway, though.

Think it'd be better if I uploaded a PCB design that shows it as, say, one-sided with jumpers?

EDIT 2: Well, the PCB design isn't really any more readable, so I guess if anyone has time to go over the schematic, send me a message or something and I'll try to get it to you. Otherwise, I'll just keep it here as for the casual onlooker, there would probably be more confusion than benefit.

I might look into condensing the latches and address decode circuits into a programmable logic device, as well. I'd be able to do a really small 3-chip board, then, but I'd have to figure out the best way to program the chip.

EDIT 3: Maybe 4-chip, I'm not sure I could implement the data transceiver in a programmable logic device. I mean, it's probably possible, but I wouldn't know enough to figure out how to do that anytime soon. As far as latching the addresses, they have a nice example of implementing a latch included with the programming software, so that wouldn't be all that hard for me to come to grips with.

EDIT 4: Now that I think about it, a programmable logic device with enough inputs and outputs is probably only available as a surface mount chip. I could still save three chips even if I only used it to do address decoding, though, and an address latch isn't strictly necessary for A8-A15 either, although I'd rather have one since all the addresses seem to be latched on the PCjr motherboard.

Yay, first post of September. Thanks for keeping us updated. Wish I could help, I only took a couple of EE classes (way back when) and just about everything you've done is way over my head.

Aw, they won't send me the sample of the DIP carrier.

They pointed me to Digikey instead, where the carriers are about $9 apiece.

Alright, I condensed all the address decode logic into a PLD, which saves me three chips (4 chips down to one).

The board can be as small as 3"x2.5" now, which makes it quite a bit cheaper to have prototyped than the 3"x4" board I was trying to work with before. I wish I could get it down to 3"x2" or even 2.5"x2.5", but it just doesn't seem possible. I thought about replacing the three 8-bit latches with two 10-bit latches, but if I placed them horizontally it'd make the board wider and if I placed them vertically, it'd make the board longer. I had also thought about replacing the latches with another PLD, but I think that would require more board space than leaving the latches as they are, as well.

This is probably too late for your design but you can compare it to what you did. It was designed by an EE
that used to belong to the old PCJR forum before Mike started this one as a replacement. He was coming
up with alot of good designs for the PCJR but decided he didn't want to do it anymore. I believe this design
was to go in the Racore double-deck system. At the time, I believe he said it was working.
Alot of his old circuits and programs were on a companion web site to the old forum as were alot of
other inputs from members but are now lost (is that right Mike?).
Hope this circuit is of some use to you. I had to reduce the size so it would fit in the 100K max size. Also I believe he has the module types for the
memory modules reversed
Bob