XT-IDE on PCjr

Hardware questions and modifications

Re: XT-IDE on PCjr

Postby jmetal88 » Fri Apr 15, 2011 10:42 pm

If we're thinking about ISA, how about one of these as an enclosure?

http://www.arstech.com/cat-Enclosures-enclosures.html

I'm not sure it's an exact fit for what we want, but something could be done with these.
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Re: XT-IDE on PCjr

Postby Brutman » Sat Apr 16, 2011 7:10 am

The ARSTech boxes look interesting. I'd like to get a few options to consider, so keep looking! (Which I dig out from this job change madness I'll be more active.)

Matt - the memory has to be visible to the PCjr. DOS uses the FDPT and the status bytes. So we get stuck doing extra address decode. This is an enhancement to the XT-IDE design - the base card doesn't do this because the XT and AT memory maps work well for them, while the PCjr memory map is kind of screwy.

Before we pick and layout any Ethernet chipset we should first know if we are doing a sidecar or an expansion box. If we do an expansion box then Ethernet can be thrown out - we'll have some reasonable ISA cards to choose from, all of which have existing drivers. If we think we are manly enough to do it ourselves on a sidecar, the first thing to do should be to prototype a sample to prove that it can work.

Another option to keep in mind - the modem slot. Addressing is a bit limited, but we've got at least 8 I/O ports that are not normally used. (Unless somebody wants to design a separate serial card for that.)


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Re: XT-IDE on PCjr

Postby jmetal88 » Sat Apr 16, 2011 8:45 am

Ethernet would require some investigation, but IDE looks like it uses eight 16-bit addresses, which would be taken as 16 eight-bit addresses by the XT-IDE. I'm not exactly sure how we'd get that working in the modem slot.
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Re: XT-IDE on PCjr

Postby Brutman » Sat Apr 16, 2011 9:20 am

I wasn't suggesting XT-IDE in the modem slot .. but Ethernet might be a possibility.

Believe me, as somebody involved at the beginning, testing, and having modified it for the Jr, I am all too painfully aware of it's requirements ...
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Re: XT-IDE on PCjr

Postby jmetal88 » Sat Apr 16, 2011 9:39 am

Now if only we can find an ethernet chip that only uses eight eight-bit ports...

Even the CS8900A has to use 16 of them, so I'm not sure how far we'll get with this.
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Re: XT-IDE on PCjr

Postby jmetal88 » Sat Apr 16, 2011 10:09 am

Hmm, actually, we might be able to base something off this guy's work:

http://www.chiark.greenend.org.uk/~theo ... index.html

It looks like he's externally converted the 8-bit interface to a 16-bit interface, which would allow us to have an interrupt and use the chip in 16-bit mode. Unfortunately, there are a decent amount of external components involved in this.
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Re: XT-IDE on PCjr

Postby jmetal88 » Sat Apr 16, 2011 10:36 am

Also, I think it would be technically possible to interface one of these chips across an 8255 PPI.

For example, with the CS8900A, you could have ports A and B on the PPI used for the 16-bit data bus and port C used to generate addresses. For the 8-bit chip I listed, port A could be data, and port B and C could select from the massive list of registers on that particular chip.
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Re: XT-IDE on PCjr

Postby jmetal88 » Sat Apr 16, 2011 12:48 pm

So Mike, I take it your idea for an expansion chassis would have the IDE controller and RAM on board, with ISA slots in addition? Just wondering.
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Re: XT-IDE on PCjr

Postby MattCarp » Sat Apr 16, 2011 4:11 pm

I was pretty excited for the possibility for a combination disk+mem+net sidecar, addressing the major add-on requirements for the PCjr. My thinking was that one or two sidecars keeps the jr from becoming a mess of attachments. I'd never fully get away with a few hanging attachments - a disk drive would have to be mounted somewhere - but a memory+disk+net attachment would take it long way. However, given the challenges with the two candidate ethernet controllers, my hope is fading for this approach.

For ethernet I thought the CS8900 was the solution, but the lack of interrupt support is a real problem. Once you talk about adding a bunch of logic to adapt 16-bits to 8-bits, I think you're right, this would definitely need to be prototyped. Even then, I don't think we'd be able to fit it on a sidecar after looking at the current board layout.

The Silicon Labs CP2200 controller may work, but there are a couple unknowns: driver support and power. Determining driver support should be a matter of googling for 15-20 minutes, but on the power side it looks like it gobbles up to 150 mA and looks like it would require 3.3V - 5V level conversions. The memory chip takes 60 mA, so with the couple other chips in that module, memory may be up in the 90 mA territory. That would leave only about 150 mA for the IDE controller. I'm not sure of the power requirements there, but we may be over, especially if we need to add things for the disk controller.

Unless there is another ethernet controller out there that would work out better for us, we'd have to jettison the ethernet controller from a sidecar design. I still think ethernet is important, so then you have to choose for either an ISA bus slot + existing ethernet card or a Xircom Pocket Ethernet. I know it works, but a Xircom adapter takes me back to a mess of attachments, plus I'd have to connect it through a parallel port, which is clunky to me. So now I'm back to an ISA bus.

If I'm at an ISA bus, I might as well slap another slot on there for my IDE disk controller. Then we just have 512k of memory on a sidecar, which, I already have and therefore don't need a new project to provide that. That leaves a PCjr-ISA bus adapter, which I already have. The only thing new in this project would be the possibility of an enclosure.

I suppose if I were at that point, I would have to think about what enclosure would work best.

I personally always thought that Racore and Legacy did it best with their case that was a top layer on the jr. It wouldn't be worth it to replicate their case, but to buy a Racore setup and start there. Then you would already have memory provided by Racore (not to mention a second floppy), so now you're just talking about building an IDE and ethernet enhancement to upgrade the Racore setup. If you've able to do add ide and ethernet to a Racore, would you still need a generic ISA slot?

Where my head is at is that I don't really need ISA capability if I've got memory, ide, and ethernet in a sidecar. But, if I can't get ethernet in the sidecar, it's a lot less compelling, because I more or less have the memory (512k IBM sidecar, AST sidecar, and there are many others) and disk controller (ISA adapter + either XT-IDE or IBM FDA (yikes!)).

So, I think we need to have some consensus on the enclosure preference, then if it's a sidecar, I'd want to rule in or out the available ethernet controllers starting with the CP2200.
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Re: XT-IDE on PCjr

Postby jmetal88 » Sat Apr 16, 2011 4:33 pm

Well, just for the heck of it, I'm doing a 10cmx10cm board design with three ISA slots and 512k SRAM on it (SRAM is indeed faster than DRAM on the PCjr, and I have firsthand experience that the difference is observable).

I still like the 3-in-1 sidecar approach the best. Actually, the board design I already posted has a full 640k on board, for the 736k upgrade. I could remove the 128k SRAM and the extra address decode logic for that chip, and we'd have more room to add ethernet components. I don't know if it would be enough room, yet, but we could give it a shot.
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