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Thoughts on a PCjr version of XTIDE

H

hargle

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minneapolis, MN
Mike Brutman and I had discussed this briefly; just thought I'd start a thread to gather up some more ideas on the subject.

In a nutshell, we want the ability to hook up IDE devices to the PCjr.
There aren't a lot of options though. The modem port would be ideal, since it would just be another plug-in card, but there doesn't appear to be enough IO available, and what about the BIOS?

Then the idea of a sidecar came up. Since we can't realistically do up a whole plastic shell sidecar, we thought that perhaps taking apart a commonly available LPT sidecar might do the trick. Replace the entire PCB from an LPT sidecar and wedge a new XTIDE PCB in there instead, possibly even keeping the LPT functionality.

Last night, I did exactly that, and here's what I found:

The entire LPT sidecar:
http://i870.photobucket.com/albums/ab264/hargle/pcjrLPT1.jpg

The 30x2 connector on the card is a big problem. where can we find those?
http://i870.photobucket.com/albums/ab264/hargle/pcjr_sidecarheader.jpg

Here's the pinout for it:
http://www.brutman.com/PCjr/pcjr_isa_adapter.html

It turns out we might not have to. Half of it comes apart with a couple screws:
http://i870.photobucket.com/albums/ab264/hargle/pcjr_headerRemoved.jpg

The part that comes off just happens to be the side that you'd push onto the side of the PCjr. That means that our sidecar would have to be the last car on the chain, as there would be no thru-connector for the next sidecar. Probably not that big of a deal.

then I noticed that the PCB is the exact same size as the IDE->CF adapters I've been using on my other 8088 machines:
http://i870.photobucket.com/albums/ab264/hargle/pcjr_CFIDE.jpg

All it would take would be to make a new XTIDE PCB with a few additional (large) holes drilled in it for mounting into the sidecar housing, and then change the ISA tooth connectors to a pin header. Route a very short IDE cable over to the CF adapter, and then figure out a way to mount the CF bracket to the back of the card. Some use of a dremell required.

Another option would be to use some more modern parts to re-create the LPT functionality in a smaller footprint, squish the entire thing down over to the back side of the card, and then make an entirely internal XTIDE on the other half of the PCB and mount a microdrive inside it. I think there would be room for that. No room or power for an actual 12v hard drive, but I think a 5v CF or microdrive should have enough juice through the sidecar connectors.


Just kicking ideas at the moment. This would be a very, very small run of cards, and you'd have to supply your own LPT sidecar. Cost should be under $100 easily.
 
I'd definitely be interested in one of these. Right now I use a parallel attach Zip drive with my PCjr but like the idea of being able to attach IDE devices to the jr especially if you ever implement the ability to attach CD-ROM drives to the XT-IDE.
 
Just a few thoughts ..

YES! YES! YES!

Onto the more serious discussion ..

We should not completely write off the modem port. It has power, four address lines, data, and some extras. If the address lines are not to our liking (A9, A2, A1, A0) then we can run jumper wires from other places on the motherboard to get what we need. Many of the PC Enterprises modifications did this.

If we go the sidecar route I'd like to throw a super I/O chip on the board to preserve the LPT port function.

Mechanically, I really don't care how it is attached. It might make more sense to consider doing a run of PCjr-to-ISA bus adapters and identifying a nice enclosure to use with them. That would give you two or three slots for the XT-IDE card, an Ethernet card, and anything else that you wanted to put on. It also makes the PCjr version of XT-IDE the same as XT-IDE, except for the BIOS of the card.

Lastly, software is a problem here. And this time I don't want you to do it alone - I want to learn the dark secrets!

The Jr software should have the following attributes:
  • No IRQs used.
  • Definitely no DMA
  • An 8088 version of the IN/OUT code and a NEC V20 version (The NEC is extremely popular on the PCjr)
  • Forget boot menus .. let's save some space and add code to do things like boot in 80 col mode ...

The Jr CRC algorithm for checking BIOS extensions is a little different, but in general I think everything is just about the same as it is on an XT.

I have a good example of a boot BIOS from SCSI for the PCjr. I can probably dig up an MFM boot BIOS, but that shouldn't be that much different for the parts that we are interested in.

As an aside, I know of many more PCjr owners over at http://www.brutman.com/forums ...


Mike
 
Let's consider the sidecar possibilities again .. Instead of parallel port sidecars, I'd sacrifice a memory sidecar. Before you accuse me of hitting the wacky weed, let's consider:
  • Every PCjr in use has at least one
  • It would be trivial to put address decoders and a 512KB SRAM chip on the board
  • The single 512KB SRAM chip requires no refresh controller, making for a faster system

Less parts, less heat, less power, better reliability and better performance .. And we get a sidecar shell to use.


Mike
 
And for the record, the sidecar connectors are Amphenol 532771-2 and 532773-2. The bad news is that they are obsolete, so a place like DigiKey isn't going to have them. The part number shows up on the broker websites, but g-d only knows if they actually have them or not.


Mike
 
Just a few random thoughts here - I know practically nothing about the PCjr.

What's the pinout of that modem slot? If you don't need DMA you can get away with the lower 8 ATA registers, so limited address lines may not be a problem.

Is the sidecar connector 0.1" pitch? If so, would an IDC connector with a short ribbon cable and/or some kind of spacer do the job?

Anyone think about a modem slot SD-card interface? A small PCB with little more than a microcontroller and an SD socket would probably do it hardware-wise, although there'd of course be little to reuse of the existing XT-IDE design.
 
An IDC connector with 0.1" pitch works, but the ribbon cable is problematic. If you want the sidecar look and feel, you need the original Amphenol connectors. If you can live with it being the last sidecar on the machine an IDC connector will work.
 
mbbrutman said:
Just a few thoughts ..
Mechanically, I really don't care how it is attached. It might make more sense to consider doing a run of PCjr-to-ISA bus adapters and identifying a nice enclosure to use with them. That would give you two or three slots for the XT-IDE card, an Ethernet card, and anything else that you wanted to put on. It also makes the PCjr version of XT-IDE the same as XT-IDE, except for the BIOS of the card.
Click to expand...

I'd actually go with this alternative... in fact, when I browsed your page a bit, I thought: why not make a "top" expansion like the Legacy ones - only with ISA slots and without a diskette drive. You'd have enough space to had hard drives, ISA cards and, well, a disk drive. The biggest problem would probably be finding a nice-looking case though...
 
problem with that would be we would need someone who is a plastics and or sheet metal worker to fabricate to dimensions, i think that lowes will bend and cut plexi sheets to dimensions, but we'd still have to install mounts and the hardware. it would be difficult, but do-able
 
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