Mike's PCjr Page

I have a PCjr with Racore Drive II + Memory expansion and a JrIDE. Recently got patched MS DOS 5.0 and have been playing around with all the different utilities when I started having problems. Intermittent Error A on boot up and random freezes or reboots while running programs that typically didn't cause issues before. I yanked out the socketed RAM from the Racore Drive II expansion and the intermittent Error A went away. I found it interesting that it will only count up to 384k now but still has the full 736k after booting but after reading more about the JrIDE I get why that is. My problem is I can no longer run Edit or Qbasic! It just freezes up! So I guess I have 2 questions.

1) Did removing the ram from the Racore expansion do this? It must have ya? Worked fine before, not so much after.

2) Any advise on replacing the chips? Will any ol' 128k-bit in a 16 pin DIP package work or is there a recommended brand or vendor?

I would remove the memory card from the Racore as you can't have two different sets of memory competing with each other. Don't just pull the chips; remove the entire memory card, as it has no purpose with the jrIDE installed.

The jrIDE should be providing any memory that you need. Your machine should not have been working with both sets of memory active - I'm kind of amazed that it did.

Huh. I've never had any issues despite my setup having 304 KB from the Rapport expansion and an IBM Memory sidecar in addition to the jrIDE.

Well, aside from the PSU on my Rapport unit being borderline when it's having to power the jrIDE.

I'm not quite sure how the conflicting memory cards can ever work.

The short story - the cards are all on the same common bus. And the decode logic for the memory chips are also on the same bus. You have a case where multiple chips will be responding to read requests, and that's basically an accident waiting to happen.

You should only have one device supplying memory at a particular address. If you have conflicting devices and you are not seeing problems, you are getting lucky. (And also pulling more current from the power supply for no benefit.)


Mike

Well removing the card did the trick. Qbasic and Edit functionality now restored. Can also run Word Perfect 5.1 and a few other programs that wouldn't run before. Kind of weird when I first installed the JrIDE everything worked great as far as Edit and Qbasic. Then intermittent freezing issues then would boot with Error A. I yanked out 128k worth of chips (the socketed ones) from the memory card in the rapport drive and things started working again - except Edit and Qbasic. You think both memory banks were being used and the issues related 2 sets of chips driving the same bus? I probably blew a chip and caused the Error A having it set up that way right? Man I'm glad the bad memory was in the expansion and not the JrIDE but still, that sucks Can the chips be replaced with any 128k DRAM or is there a preferred brand, speed or whatever?

Anyway thank you, Mike. Rookie move on my part I suppose.

The general problem is timing issues, not "blowing" anything, so I doubt that you did any damage. You can try to run the machine with just the Racore memory and see what happens. (You have to remove the jrIDE.)

The DRAMs on any system like this are generally 64x1 at 150ns or better. (The technical reference has the exact part number; the important thing is that each chip has 64K address spaces that are each one bit wide. The other alternative is 256Kx1 chips, which is used on the memory expansion sidecars that can be modified from 128K to 512K. The Rapport and Racore expansion units don't do that so they should always be populated with 64Kx1 chips.

No dice on booting w/o the JrIDE. Still get error A. Yanked out the chips again and it booted but with half the memory. The good half is soldered in the board so good thing one of them didn't go belly up. Ah well, live and learn I don't see myself using the Jr much w/o the JrIDE so will put the memory card on the back burner with a ton of other repair jobs/projects I got going on. I have a relative with a PCjr who has 512k expansion inside the unit. I always assumed it plugged into the modem slot or something but now wonder if it is a card like this with an IDE connecter?

A memory expansion can not plug into the modem slot. It doesn't have the correct wiring.

If a machine has 512K without a sidecar it probably has a daughterboard under the floppy drive that plugs into the CPU socket. There were two or three cards that worked this way.