Internal 512k expansion?

[jmetal88]

Very nice! Write it up nice, and we'll get it posted. I'd love to do a technical review of it ..

The SRAM is a nice solution - no DRAM controller needed. I'm glad it worked.

Mike

I'll probably draw up some illustrations before writing up a description, both so that it's more clear what I'm talking about and so I don't have to write so much. Right now I'm too exhausted from all that soldering.

For a thorough testing of the new chip, I just played two games of Wheel of Fortune and one game of Jeopardy on the PCjr with my family, using the livingroom TV as a monitor. That wireless keyboard really works a treat! Found out the games exit to 80-column mode though, which is damn hard to read on a TV, haha.

[Brutman]

One thing I noticed that you did differently is that you modified the motherboard quite a bit, and either co-opted (or hijacked) the original address decode logic. This is a different approach from boards like the jrHotshot, who did it all on a daughtercard before passing the signals to the motherboard.

Without seeing the details, I'm guessing that you left the first 128K for the motherboard, but redirected anything above 128K from the bus to your new circuit. Is this right?


Mike

[jmetal88]

One thing I noticed that you did differently is that you modified the motherboard quite a bit, and either co-opted (or hijacked) the original address decode logic. This is a different approach from boards like the jrHotshot, who did it all on a daughtercard before passing the signals to the motherboard.

Without seeing the details, I'm guessing that you left the first 128K for the motherboard, but redirected anything above 128K from the bus to your new circuit. Is this right?


Mike

Yup, that's exactly what I did. I actually hooked up my address decode circuit in parallel to the PCjr's. The 74154 4-bit decoder does most of the work, converting the top four address lines to 16 mutually exclusive TTL signals. The 128k on the PCjr motherboard and expansion slot sits at addresses 00000 to 1FFFF. Since each hex digit corresponds to four digits in binary, this made things pretty easy for me. Using the 4-bit decoder on the top four address lines, when the PCjr tries to access addresses, the active output of the decoder actually corresponds directly to the top hex address digit. I connected outputs 2-9 (9FFFF is the top of 640k) of the 74154 to two 4-input NAND circuits in a 7420, and I in turn connected those outputs to a 2-input NOR in a 7402. I also connected the ALE signal from the 8088 to the enable lines of the 74154 to make sure the SRAM chip was off during address transitions. The output from the 7402 becomes Chip Enable for the SRAM.

I'm quite pleased with myself for having figured all this out, considering how little experience I have with such things. My reason for hijacking the PCjr's address latches was so that I wouldn't have to duplicate them on my board (limited space), and also so I'd be able to stick with a 'breadboard' type setup. It's more signals to pull of the motherboard this way, but it's a lot less traces I'd have to set up on the board itself (I wouldn't feel comfortable using my own address latches without having the board professionally fabricated, which is currently out of my budget).

[jmetal88]

The other thing I did was I made sure all the signals I tapped on the motherboard were directly accessible through the sidecar connector. This way if someone wanted to, they could make a PCB layout of my circuit with a sidecar connector on it, and use it externally.

[jmetal88]

Sorry, I still haven't started putting together an easily readable description of the modification yet, and I'm already thinking about my next project, haha.

I'm trying to think if there's some way I might be able to add another 96k, inside my parallel printer sidecar, to get to the upper 736k limit. I'm thinking a programmable logic device might be the way to go there, as I think I'd have to sample 5 of the address lines to get as specific as 32k sections of memory.

[MattCarp]

Wicked. Very cool.

If there's a super simple way to plug this into the motherboard (CPU socket?) I think you've got something.

It's nice not to have a sidecar for this. Then you'd just have a sidecar for a parallel port.

I'd plug in the PCjr Sprint (7MHz mod) with a V20, a Racore second serial port in the modem slot, and you've got a sweet system!

[jmetal88]

Wicked. Very cool.

If there's a super simple way to plug this into the motherboard (CPU socket?) I think you've got something.

It's nice not to have a sidecar for this. Then you'd just have a sidecar for a parallel port.

I'd plug in the PCjr Sprint (7MHz mod) with a V20, a Racore second serial port in the modem slot, and you've got a sweet system!

Don't know if I'd be able to come across the Racore card (at least at a price I could afford), but I've already got a V20 in there that I stole from a Tandy 1400FD with dead video hardware. Sometime I'll do the 7mhz mod, but probably not for a while as I'll start attending classes again on Monday.

As for plugging the mod into the CPU socket, yes, that is what I originally wanted to do, but I'd need to put a second set of address latches on the board in that case, and I'd probably want to get a board printed up professionally so I could get all the components in the limited space. My idea for this was just something people could hack together themselves with perfboard and wire - it's too much work for me to actually construct these for other people, haha.

I wouldn't mind working on a schematic and PCB layout for the CPU socket though, if someone wanted to print up a batch.

[jmetal88]

Or if enough people are interested, I could even investigate getting boards made up for self-assembly of these units. I actually found a place once that did small production runs (I mean down to the individual board level) for unbelievably low prices, but I think I lost the link, so I'd have to find it again.

EDIT: I can't find it now, but it looks like batchpcb.com does the same thing as the other site, at similar prices, and they have a nicer website to boot.

EDIT 2: Actually, now that I look at it, batchpcb is probably the way to go. I'll try to get a PCB layout ready next weekend so I can order a board to test this with myself. If it ends up working pretty well, I'll post my design up in their store.

[jmetal88]

Alright guys, I just finished a board layout in DipTrace. I uploaded it to BatchPCB just to make sure I exported the files correctly, and it looks like I did. I'll be printing it out to scale soon so I can make sure it would fit in the PCjr without getting in the way of the floppy drive posts or anything.

This one'll work by plugging it directly into the CPU socket, since it seems people are more comfortable with that than doing the whole piggyback socket and ribbon cable thing I did with my original version. I just need to figure out what kind of 40-pin socket would be suitable for through hole mounting in the board and also seating in the original CPU socket. So far my two options are one with slightly longer pins made for 4-layer PCBs rather than 2-layer PCBs and one made for wire wrap, although I don't know if the pins on the second one would be thin enough to fit into another socket, and I'd likely have to cut them down as well.

[jmetal88]

Had to make the board about an inch shorter and make it kind of L-shaped to get the fit right, but it might be usable now. I'll have to do the print and test fit again next weekend though, I've run out of time here and need to be getting back to the dorms in an hour or so. Unfortunately, it'll cost me $45 to get a prototype shipped from BatchPCB, and $30 a board thereafter.

Oh, wait! I finally found the site I was looking at before, it was futurlec.com! It'll cost me $38.74 to get a prototype shipped to me from there. That's due to the exclusion of silkscreen and solder mask though, it costs about twice as much from them with it. It's only a savings of $6 that way, but if I ordered two it would only cost me $40.48, which is a very significant savings, considering BatchPCB would cost $75.25 for two boards.