PDA

View Full Version : Replica PCjr



k2x4b524[
April 23rd, 2010, 03:32 PM
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 :)

NeXT
April 23rd, 2010, 04:15 PM
If you added DMA it wouldn't be a PCjr. ;)

basman74
April 23rd, 2010, 04:15 PM
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

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

basman74
April 23rd, 2010, 04:29 PM
Sorry, forgot to mention that I started a thread about my little project elsewhere on this forum:

http://www.vintage-computer.com/vcforum/showthread.php?19443-Tiny-XT-class-machine

It originally started out as an PC-XT with (ultimately) VGA, but I decided to take the PC/Jr Tandy 1000 (slightly easier) path instead..

Regards,
Valentin

k2x4b524[
April 23rd, 2010, 05:08 PM
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 :)

basman74
April 23rd, 2010, 05:53 PM
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..

geoffm3
April 26th, 2010, 11:20 AM
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.

Raven
April 26th, 2010, 12:33 PM
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.

digger
April 27th, 2010, 01:24 PM
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.

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?

k2x4b524[
April 27th, 2010, 01:50 PM
but see, the PCjr could have dma through an add-on, as could the tandy 1000.. so many ideas.. so little PCjrs left in the world...

geoffm3
April 27th, 2010, 05:44 PM
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?

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.

NsMn
April 28th, 2010, 03:35 AM
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.

geoffm3
April 28th, 2010, 04:38 AM
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.

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?

mbbrutman
April 28th, 2010, 04:52 AM
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 ...

geoffm3
April 28th, 2010, 05:35 AM
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 ...

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

mbbrutman
April 28th, 2010, 05:58 AM
I suppose that's a fair question... but then, why would anyone do any number of things with vintage computers. :)

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.

basman74
April 28th, 2010, 06:23 AM
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.
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.


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?
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

basman74
April 28th, 2010, 07:08 AM
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 ...

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-Personal-Computer-Original-Advertisement-/290428769059?cmd=ViewItem&pt=LH_DefaultDomain_15&hash=item439ee75323

Regards,
valentin

NsMn
April 28th, 2010, 07:22 AM
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!

geoffm3
April 28th, 2010, 07:31 AM
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.

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.

k2x4b524[
April 28th, 2010, 12:03 PM
To answer your question brutman. Why not? I think it would be fun to do if it's possible, plus we could come up with a PCjr board that would fit in a standard AT style case, or ATX, admittedly that would be far off, but it is JUST an idea.

mbbrutman
April 28th, 2010, 12:13 PM
Rainbow ponies for everyone!

No seriously, I'm really confused here. It makes no sense to recreate something that is already in plentiful supply. There are so many other things to be working on that pass the sniff test:


An IDE controller that the Jr can boot from
A memory expansion card using an SRAM chip or two instead of DRAMs. This is faster, lower power, and gets rid of the DRAM controller.
An Ethernet card based on whatever the C64 and Apple ][ people are using. (Better, stronger, and faster than parallel port attach ones.)
Another run of PCjr bus to ISA adapters so people can experiment with ISA cards on their Jrs.
Updated BIOS or a BIOS ROM extension for 3.5" 720K floppy support.
An upgrade to a 16450 UART instead of the built in 8250 UART
A serial port card that goes in the modem slot
An XT or AT keyboard adapter



Jrs are unwanted, low dollar machines - go adopt one today. Save the recreation work for 50 years from now.

hargle
April 28th, 2010, 01:18 PM
An XT or AT keyboard adapter


This is where I'd like to start. (ok, I want to do #'s 1,2,4,5 too, but this one is pretty low hanging fruit I think)
I would use my jr a lot more if I could use a normal keyboard attached to my KVM switch. We've already got the AT->XT keyboard converter project, all that needs to be done (I think) would be an rj-45 adapter instead of 5 pin DIN and some different decoding logic in the uC.

k2x4b524[
April 28th, 2010, 03:33 PM
you'd still have to get it from the adapter to the jr, unless this is where the rj-45 comes in, run a keyboard to the adapter, then run that via the rj45 to the pcjr?

hargle
April 29th, 2010, 05:35 AM
yes, you'd still use your existing PCjr keyboard cable.
The adapter would have a PS/2 connector on the AT side and an RJ45 socket on the other:

AT keyboard------>[ps/2 socket]:Adapter:[RJ 45 socket]----->pcjr cable----->PCjr

I could probably even rip apart an old telephone to get RJ45 socket and see if it can be bolted onto the existing at2xt keyboard PCB somehow (with a little duct tape I'm sure)
The real trickery is learning the code in the microcontroller, but I am pretty sure that's within my realm of understanding, since I already do x86 assembly for a living.

edit: oops. it's rj-11.(telephone connector) Too much time spent playing with ethernet.

Raven
April 29th, 2010, 06:20 AM
Rainbow ponies for everyone!

No seriously, I'm really confused here. It makes no sense to recreate something that is already in plentiful supply. There are so many other things to be working on that pass the sniff test:


An IDE controller that the Jr can boot from
A memory expansion card using an SRAM chip or two instead of DRAMs. This is faster, lower power, and gets rid of the DRAM controller.
An Ethernet card based on whatever the C64 and Apple ][ people are using. (Better, stronger, and faster than parallel port attach ones.)
Another run of PCjr bus to ISA adapters so people can experiment with ISA cards on their Jrs.
Updated BIOS or a BIOS ROM extension for 3.5" 720K floppy support.
An upgrade to a 16450 UART instead of the built in 8250 UART
A serial port card that goes in the modem slot
An XT or AT keyboard adapter



Jrs are unwanted, low dollar machines - go adopt one today. Save the recreation work for 50 years from now.

If they're THAT plentiful feel free to ship me one, but I haven't seen for one go for too cheap. :P

mbbrutman
April 29th, 2010, 07:12 AM
<rant>

Gee, here is modified system on eBay right now: 250621775862


512K
Modified floppy controller and an external floppy drive
Monitor
Printer port
PC Compact printer (the relatively uncommon thermal printer designed for the Jr) with paper
Three cartridges and other software
Probably the original owner ...


Let's see, one day to go and only $20 yet. I've been watching the eBay sales for years, and I have never seen a system go over $150 yet. And most are under $50. (And for $150 you got the extremely rare SCSI controller and hard drive system from PC Enterprises.)

Look people, forgive me for being blunt here, but you've got to put up or shut up ... After years of reading here I'm am amazing at the people who will:


Whine about the travesty of others discarding old computers
Complain that they can't get what they want for just about free
Suggest that the 'community' do something without ever lifting a finger to do something themselves


So here's a challenge .. if this machine sells for under $75 and it doesn't go to somebody here, then nobody has a right to bitch about not getting one anymore.


Mike

</rant>

Raven
April 29th, 2010, 07:41 AM
$30 shipping, that's $50 and that's not so cheap. I wasn't trying to "bitch about not getting one", was just saying, they aren't as common as you're making them out to be - not any more common than a 5160 or what-have-you in my experience. I'd toy with one if I got my hands on one for $20-30 locally, but don't personally want one bad enough to pay shipping. If there were a tiny device capable of Tandy and PCjr functionality that would ship for $10 or less and take up much less space, using a standard monitor and such (a big problem for me, as I can't seem to get my hands on a working CGA monitor), I'd consider buying one.

My point is don't dismiss projects and discourage people just because you've got a pile of PCjrs and there are some for sale around - it doesn't mean someone wouldn't rather have a non-proprietary smaller version.

mbbrutman
April 29th, 2010, 07:51 AM
My point is don't dismiss projects and discourage people just because you've got a pile of PCjrs and there are some for sale around - it doesn't mean someone wouldn't rather have a non-proprietary smaller version.


Like I said, I've been watching prices on and off eBay for years. There are at least two people on this forum who have gotten their machines for free, and one of them didn't even pay for shipping. They are out there, and they are cheap - not just in my basement. There are machines on eBay that languish for days and weeks with no bites.

People can do whatever projects they want to do - I'm just pointing out the logical fallacy in recreating something that already exists, and is in plentiful supply. Sorry, but not willing to make a one time investment of $50 to $100 to acquire a machine for your hobby just seems terribly cheap and unrealistic. Wait until you figure out what a round of golf/flying/sailing/<insert hobby here> costs ... In the grand scheme of things $100 for something you want is close to nothing.



Mike

basman74
April 29th, 2010, 07:19 PM
If there were a tiny device capable of Tandy and PCjr functionality that would ship for $10 or less and take up much less space, using a standard monitor and such (a big problem for me, as I can't seem to get my hands on a working CGA monitor), I'd consider buying one.

My point is don't dismiss projects and discourage people just because you've got a pile of PCjrs and there are some for sale around - it doesn't mean someone wouldn't rather have a non-proprietary smaller version.

This is pretty much in line with my way of thinking: a tiny, PC-compatible machine with around a $10-$15 price tag.

The inclusion of classic expansion features like ISA or the jr sidecar buses would make it anything other than tiny (or cheap), unfortunately..

I will agree with Mike Brutman in that USD$100 is on the low side of things as far as hobbies go.




Rainbow ponies for everyone!

In that case I'll have the one with the shiny silver tail, if you please.. :D


Regards,
Valentin

digger
May 2nd, 2010, 11:45 AM
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 ...

I guess everybody has their own itch to scratch. I for one think it would be interesting and fun from a hobbiest perspective to replicate the unique features of a PcJr/Tandy 1000 in the form of either a complete reimplementation of a PCJr compatibe computer from the ground up, or in the form of add-on cards for common XT clones.

Don't take me wrong: preservation of original hardware is commendable and important, but there's nothing wrong with a little creative tinkering here and there. :)