Mike's PCjr Page

Plus side: My speedup mod functions with the Booster now.

I'm a little confused. Are you doing this off the side car slot or a inline daughter board between the 8088 and the socket? You're schematic a few pages back seemed to indicate the later, but you're using the minimum mode pin assignments. Or does the PCJR actually not have an 8288 and use minimum mode instead? (maybe the reason no DMA?)

alanh wrote:I'm a little confused. Are you doing this off the side car slot or a inline daughter board between the 8088 and the socket? You're schematic a few pages back seemed to indicate the later, but you're using the minimum mode pin assignments. Or does the PCJR actually not have an 8288 and use minimum mode instead? (maybe the reason no DMA?)

8288? What?

Umm... No, I sure don't see an 8288 on the PCjr board. There's an 8088, 8284, 8255, 8259, 8253, and... well, that looks to be all there is, as far as Intel chips go.

Anyway, yes, this was supposed to go between the 8088 and its socket. It didn't work. Hooking up the second 74ls245 to the CPU caused the machine not to power on, and I was unable to maintain a reliable connection to the data bus when I strung wires across the machine to the modem slot.

Thanks for looking. 8288 bus controller de-multiplexes the bus control strobes from S0..2 when the 8088 is wired for maximum mode. I'm not sure way it never occurred to me that the Jr's CPU is probably in minimum mode since there isn't support for a 8237A DMA controller or 8087 math co-processor (the 2 main reasons for maximum mode).

Were you able to find a socket that allows stacking of the 8088 directly on top of the original socket? The best I can come up with is a normal 40 pin 600 mil dip plus a double up/down pin 600 mil socket offset 100 mils to the side.

Thought about using a wire wrap socket, but the tail pins would be too long.

alanh wrote:Were you able to find a socket that allows stacking of the 8088 directly on top of the original socket? The best I can come up with is a normal 40 pin 600 mil dip plus a double up/down pin 600 mil socket offset 100 mils to the side.

Thought about using a wire wrap socket, but the tail pins would be too long.

Yeah, that's the best I was able to come up with, too.

For my prototype, I ended up using a wire wrap socket with the pins cut to half length and filed down to fit in the socket.

Alright, I've concocted a crazy scheme where I replace the 74ls245 with two 74ls244s and a 74ls155. I am extremely reluctant to actually give that a try, though.

Eh, just consider this post another addition to the scratch pad. Since I have the RAM upgrade working inside my Booster sidecar, this thread is pretty much all theoretical now.

Speaking of theoretical, after thinking of this thread a while, I wondered if it would be possible to design a board that literally fit into the footprint of an 8088? For systems (not just the PCjr) that were ether space constrained around the processor socket or had limited or no ISA expansion slots for RAM cards (eg. most early Tandy 1000 systems and IBM 5150s). So I tried it. I can say it's theoretically possible, though to actually build it would require about $500 just for PCBs (qty 1 to 50+ doesn't matter due to NRE) since it's 4 layer, has an abnormally high via density, 6/6 tech, and small to the point you lose a lot of efficiency on panels.

It's basically the exact footprint of a 40-pin DIP socket (2.2" x .7") plus a couple millimeters overhang on the front edge for three 2mm jumpers to allow changing of the RAM map after assembly/programming. At build time, the code can be configured for a minimum mode system (eg. PCJr, Tandy's, etc) or a maximum mode system (most PC/XT clones). And the jumpers provide 8 possible memory fill maps each with 32k resolution. Basically they feed into a lookup table along with A15-A19 with the RAM chip selects as outputs. They then get gated with the control strobes and routed to pads. The 8 memory maps could be customized on a order/build basis by changing the lookup tables before programming the PLD. If not, I can think of a few generic ones that would be useful.

It has 2x 512KB SRAMs on the top side of the board that sit in the reliefs provided by this wire wrap socket. On the bottom side, there are a couple latches, a buffer, and a 22V10 PLD. Everything is TSOP so the wire wrap pins could be cut down a bit so it adds less than a half inch to the processor height. Though with all the chips stacked, heat might be an issue - even though everything is HCMOS.

I put together a rough BOM to try and price things out if there were enough interest in a production run. It's all SMD/SMT so it would not be user assembly friendly Though it would be an open source hardware project - so you could free-build. The total parts cost including PCBs at a 25 unit run with some NRE factored in (Kapton stencils, solder paste, TSOP programming adapter, etc), is ~$45.00. I don't have a lot of time to program PLDs, assemble, clean and package boards, so I would tack on $25 (not including shipping) to help mitigate some of that; which is ~2/3 cheaper than most web assembly quotes I found and those only cover assembly. Not to mention fronting $1200. So if there is interest... and I can't imagine there would be much.. post here and maybe we can get enough people together to give me a warm and fuzzy about selling at least 20 of the boards. Otherwise, PM me for the Eagle files and PLD code if you are interested. It was more of a mind exercise than a serious idea anyway.

PCB Layout
Schematic
Bill of Materials

-Alan

That looks really cool. I wonder if the PCjr would be able to boot with the extra 74HCT245 attached to the CPU when it wouldn't with the LS version of the chip, though? Maybe I should try to acquire one or more and put one in my hand-wired prototype board.

I just had a groovy idea for getting the computer to boot with the second '245 attached. I ORed the Data Enable signal to the '245 with the Chip Select line for my RAM using a couple of diodes. The computer actually did boot this way, but my RAM was still not recognized. I have a feeling my address latches might have gone bad during my first mishap, but I guess I need to get a logic probe or other testing equipment before I know for sure what the problem is.