Well, guys, I've tried wiring my design up by hand three times now, with three different 74ls245s, and each time, the PCjr would not boot while a 74ls245 was in the circuit and would boot when I disconnected the 74ls245.
Apparently you cannot connect two 74ls245 chips to an 8088 in parallel and still have it function. I'd love to know how the jrHotshot and The One card used their programmable logic chips to get around this. At the moment, all I can do to still make this a pluggable upgrade, is get the data signals from my unused modem slot.
Internal 512k expansion?
Re: Internal 512k expansion?
Chips are limited in the amount of signal they can drive on a pin. Are you exceeding the fanout limits for the chips you are using?
The datasheets can tell you what they are spec'ed at. If you are exceeding the fanout limits you can move to a chip family that is more tolerant, or add buffering/redrive logic. (Which would not be pretty.)
Mike
The datasheets can tell you what they are spec'ed at. If you are exceeding the fanout limits you can move to a chip family that is more tolerant, or add buffering/redrive logic. (Which would not be pretty.)
Mike
Re: Internal 512k expansion?
I think it's what the 8088 can drive that's the problem. Apparently, it can't drive two data bus transceivers, at least not if they're both 74ls245s.
On the bright side, this experiment has revealed to me why my attempt to wire the SRAM into my Jr Booster in place of the internal DRAM failed - my 512k SRAM chip is fried! I guess the data lines are shorted, internally - the PCjr gave me a blank screen and a multi-tone buzz. I tried the upgrade board with my 128k SRAM chip in place instead, and while the PCjr did attempt to boot, it gave me a memory error, indicating that both my SRAM chips are bad, but in different ways.
I guess now the thing to do is go back to Jameco, get myself some new SRAM, and try wiring it up inside the Booster again ('cause really, I like how the Booster has the mouse port and the calendar built in, I just don't like that the DRAM won't work with an overclocked PCjr).
On the bright side, this experiment has revealed to me why my attempt to wire the SRAM into my Jr Booster in place of the internal DRAM failed - my 512k SRAM chip is fried! I guess the data lines are shorted, internally - the PCjr gave me a blank screen and a multi-tone buzz. I tried the upgrade board with my 128k SRAM chip in place instead, and while the PCjr did attempt to boot, it gave me a memory error, indicating that both my SRAM chips are bad, but in different ways.
I guess now the thing to do is go back to Jameco, get myself some new SRAM, and try wiring it up inside the Booster again ('cause really, I like how the Booster has the mouse port and the calendar built in, I just don't like that the DRAM won't work with an overclocked PCjr).
Re: Internal 512k expansion?
Hmm... Now I'm wondering what would happen if I just connected the data lines directly to the SRAM without a transceiver. I think I might just try that.
Re: Internal 512k expansion?
Alright, I tried it. The computer does try to boot, but instead of a memory error at 128kb, it just acts like the memory isn't there at all. I guess I'll need to find a new SRAM chip before I know for sure what the deal is.
Re: Internal 512k expansion?
Alright, guys, I put in an order for some new SRAM chips. We'll see soon if the internal upgrade is something I can make at all. If it isn't, I guess I'll be posting something to the effect of how to add SRAM to a Jr Booster.
Re: Internal 512k expansion?
I'm not doing any better with the new SRAM. Sorry guys, this internal ram expansion is a no-go. I'll concentrate on adding the SRAM to my Booster now.
Re: Internal 512k expansion?
Finally got 512k SRAM inside my Booster. I wouldn't recommend trying it yourself. 
It took me a couple of hours to get all the connections working. Unfortunately, I don't think there's room in there to add another 128k chip (I'd have to piggyback it, and I don't think I could squeeze the shell shut then), even though I ordered one with the 512k chip.
I wish I knew how the commercial internal RAM upgrades worked, I might be able to rig something up then.
It took me a couple of hours to get all the connections working. Unfortunately, I don't think there's room in there to add another 128k chip (I'd have to piggyback it, and I don't think I could squeeze the shell shut then), even though I ordered one with the 512k chip.
I wish I knew how the commercial internal RAM upgrades worked, I might be able to rig something up then.
Re: Internal 512k expansion?
Well, I decided to have a crack at adding the extra 128k anyway. I'm not sure if my soldering is to blame or it's the SRAM chip, but my machine now freezes as jrconfig.sys is loading. 
Ah well, I have finals to take the next few days, I'll get back to it later this week.
Ah well, I have finals to take the next few days, I'll get back to it later this week.
Re: Internal 512k expansion?
I guess I'll have to stick to 640k. The computer just won't boot with the full 736k enabled, and when I hard soldered all the connections, it just got worse. When I wire the chip as disabled, it boots to 640k just fine, though. I'll leave it in just so I don't have to do extra desoldering.