• Please review our updated Terms and Rules here

Replica PCjr

K

k2x4b524[

Veteran Member
Joined
Jun 13, 2009
Messages
1,521
Location
Tacoma, Washington, USA. Zip code 98404
I was reading in the S-100 forum about the Altair 8800 micro dealy-bopper, and was wondering if such an undertaking could be done with the PCjr? Same idea, keeping the functionality of the JR, but say, add some isa slots, a dma, and such? would that work? Hell, i'd even be willing to donate my pcjr to the cause of researching it :)
 
I was reading in the S-100 forum about the Altair 8800 micro dealy-bopper, and was wondering if such an undertaking could be done with the PCjr? Same idea, keeping the functionality of the JR, but say, add some isa slots, a dma, and such? would that work? Hell, i'd even be willing to donate my pcjr to the cause of researching it
Click to expand...

Hi there,

Well if Tandy could do an 'improved PCjr' 25 years ago (with the Tandy 1000), then surely it can be done now! ;)

I have been working on a somewhat similar project (refer signature line), but still has some ways to go yet before it becomes better than 'PCjr' spec (or perhaps less than PCjr in the 3-voice sound or expansion bus departments, for now). Is this something along the lines of what you were referring?

Regards,
Valentin
 
i've been reading your other thread aswell, but i mean something that could look like a PCjr, has the cartridges, maybe sidecar support, and a couple isa slots, keeping the sound and the graphics, as well as the simplicity that the pcjr had. I like your idea with the SOC thing however, but a replica pcjr or a homebrew clone would do just as well, especially if you could give it all the dongles, the 3 voice, the 16 color special mode, that sort of thing, i think i would support it :)
 
I can see the attraction for (if I understand correctly), a 'new-build' machine, especially if you already have hardware that is specific to it..

While recovering from the flu, I have been thinking of a two-board (system mainboard / ISA Bus expander board) combination to move things forward.. As for the 'dongles' (by those I think you mean the cartridge slots and sidecar etc.), those are features are completely exclusive to the PCjr and weren't on the radar for my project since I have been focusing almost totally on the Tandy 1000 architecture recently..

I suppose (myself or others) could design something like a PCjr-specific expander board (which would include the sidecar, cartridge slots etc.) which would work with the mainboard..
 
Probably a better route would be to create a video/sound card with the PCjr video and sound capabilities that could plug into an IBM PC.
 
If someone does what geoffm3 says, I will buy one pretty quick.

As for the original ideas, I'd recommend skipping the sidecar bus, as with a full ISA bus there's not much point IMHO. The cartridge slots, on the other hand, are useful. Unless someone can think of something that ONLY was available in a sidecar and not as an ISA card.
 
geoffm3 said:
Probably a better route would be to create a video/sound card with the PCjr video and sound capabilities that could plug into an IBM PC.
Click to expand...

I've been thinking about something similar.

From what I gathered, it can be done for the 3-voice integrated sound (and I'm frankly surprised that apparently no company or hobbiest has done this yet), but making a compatible graphics card would apparently be more problematic, since the PCJr / Tandy 1000 graphics architecture is built around the notion of shared (conventional) memory.

Even if one were to create a compatible graphics card, it would not work in machines that are already equipped with the full conventional 640KB memory (since the card's on board memory could then not be mapped to the correct area within the 640KB, since there would already be RAM there)...

...Unless some kind of clever DMA-based workaround could be implemented where the card could periodically read the conventional memory at the correct address range, I guess... Of course the PCJr and Tandy 1000 don't have a DMA controller, but since such a card wouldn't make sense in those machines anyway, that doesn't matter. ;)

So would that be too crazy? Could anyone more knowledgeable than I provide some insight here?
 
digger said:
I've been thinking about something similar.

From what I gathered, it can be done for the 3-voice integrated sound (and I'm frankly surprised that apparently no company or hobbiest has done this yet), but making a compatible graphics card would apparently be more problematic, since the PCJr / Tandy 1000 graphics architecture is built around the notion of shared (conventional) memory.

Even if one were to create a compatible graphics card, it would not work in machines that are already equipped with the full conventional 640KB memory (since the card's on board memory could then not be mapped to the correct area within the 640KB, since there would already be RAM there)...

...Unless some kind of clever DMA-based workaround could be implemented where the card could periodically read the conventional memory at the correct address range, I guess... Of course the PCJr and Tandy 1000 don't have a DMA controller, but since such a card wouldn't make sense in those machines anyway, that doesn't matter. ;)

So would that be too crazy? Could anyone more knowledgeable than I provide some insight here?
Click to expand...

The shared memory could be a bit of a problem, unless... the later Tandy 1000's (SX/TX) have the option to install an additional 128k above the 640k limit to be used as video RAM. If that would be compatible with all the existing PCjr/Tandy 1000 programs then you could just treat it as memory mapped.

I agree with you about the sound though... implementing that would be stupid simple... definitely a surprise that clone manufacturers didn't latch onto that, as I'm sure the sound generator was pretty cheap even by the standards of the day.
 
Now, I'm sorry, and I'm no expert either, but from what I know it's not going to be that good an idea since:

1. The 16-colour CGA plus modes can be emulated by connecting an EGA card to a CGA monitor.

2. You'd probably have to alter the ROM chip ID of the PC - AFAIK, the sound will only work on a computer that is recognized as a Jr or T1000.
 
NsMn said:
Now, I'm sorry, and I'm no expert either, but from what I know it's not going to be that good an idea since:

1. The 16-colour CGA plus modes can be emulated by connecting an EGA card to a CGA monitor.

2. You'd probably have to alter the ROM chip ID of the PC - AFAIK, the sound will only work on a computer that is recognized as a Jr or T1000.
Click to expand...

1. That's true, but they wouldn't be truly compatible I don't think... i.e. the capabilities would be the same but not the programming model.

2. I guess that all depends on the detection logic in the games. Are they checking the BIOS, or are they probing for the actual hardware?
 
I'm confused here. Why would anybody want to replicate or recreate a PCjr when so many of them are still available, and they are so undervalued? Many of them that are on eBay at the moment have been there for months. A basic machine never sells for more than $50 ...
 
mbbrutman said:
I'm confused here. Why would anybody want to replicate or recreate a PCjr when so many of them are still available, and they are so undervalued? Many of them that are on eBay at the moment have been there for months. A basic machine never sells for more than $50 ...
Click to expand...

I suppose that's a fair question... but then, why would anyone do any number of things with vintage computers. :)
 
geoffm3 said:
I suppose that's a fair question... but then, why would anyone do any number of things with vintage computers. :)
Click to expand...

I put my energy into preserving what is already available. It's when things don't exist or can't be found anymore (XT-IDE) that you have a reason to recreate.

The eBay on Jr with the second floppy drive would be particularly interesting, because to add that second floppy drive you either had to modify the controller or buy something different. (The standard controller can not drive two floppy drives.) That's worth saving and documenting.
 
geoffm3 said:
1. That's true, but they wouldn't be truly compatible I don't think... i.e. the capabilities would be the same but not the programming model.
Click to expand...
Agree with the above comment - indeed the programming models are significantly different i.e. bitplane-based (EGA) vs scanline-based (CGA/PCjr) buffer memory for example.

geoffm3 said:
2. I guess that all depends on the detection logic in the games. Are they checking the BIOS, or are they probing for the actual hardware?
Click to expand...
Most PCjr/Tandy specific games generally interrogate the BIOS area first i.e. machine bytes, function return tests / data area checking etc. even checks for embedded vendor-specific strings(!). The remainder do both..

Regards,
Valentin
 
Last edited:
mbbrutman said:
I'm confused here. Why would anybody want to replicate or recreate a PCjr when so many of them are still available, and they are so undervalued? Many of them that are on eBay at the moment have been there for months. A basic machine never sells for more than $50 ...
Click to expand...

Actually, I wouldn't mind getting an IBM JX (Australasian version of the PCjr?).
However, the best epay can do for me at the moment is this :( :
http://cgi.ebay.com/1990s-IBM-JX-Pe...em&pt=LH_DefaultDomain_15&hash=item439ee75323

Regards,
valentin
 
Well, good luck. These things are fricking hard to get - after all, they sold pretty much worse than the PCjr as they weren't compatible with PC software as I've heard (and they also used 3.5" diskette drives with some unusual density). Still I'd like to meet someone here on the forum who has one.

But I digress!
 
mbbrutman said:
I put my energy into preserving what is already available. It's when things don't exist or can't be found anymore (XT-IDE) that you have a reason to recreate.

The eBay on Jr with the second floppy drive would be particularly interesting, because to add that second floppy drive you either had to modify the controller or buy something different. (The standard controller can not drive two floppy drives.) That's worth saving and documenting.
Click to expand...

Well, people can do things for any number of reasons I suppose. If you were looking for an interesting hardware project to build this could be a possibility. I can understand why someone might want to build their own XT replica motherboard from scratch, but it probably doesn't make much practical sense to do so since there's so many of them out there already. I would think that before anything like this were attempted a good survey of the available software would be necessary to see if you could make it reasonably compatible with PCjr/Tandy 1000 software.
 
You must log in or register to reply here.
Back
Top