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SWTPC 6800 Arrives

falter

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At long last -- the coveted one is here! My CT1024s finally have something to talk to (maybe)!

Yes, at last, after years of searching I've finally snagged a SWTPC 6800!

It's amazing to see the thing you've lusted after and admired in youtube videos up close. One thing I will say, the videos and pictures don't convey how large it is. It's nearly the same size as my DG Z80. I expected it to be a fair size of course, but not quite this big.

It has the most wonderful 70s early computer feel to it. In fact, my CT1024 terminal almost manages to feel more substantial for some reason. Despite its size, the case actually feels kind of flexible/breakable, so I'm being really gentle with it.

I've taken some pics and pulled out some of the cards. I *hate* the SWTPC bus connector design. Those pins make it a savage job to pull cards safely.

Not sure what the floppy disk looking board is. I'm guessing it's a floppy controller of some sort. Looks like it says 'F&D Associates - MDI (or L?) -1.

No idea what the little breadboard does. Also it seems we have two serial cards -- MP-C and MP-S.

Not sure how to proceed here.. so far power supply looks okay. I'm tempted to strip it down to the bare essentials to get going, like we did with the DG, but there's a lot of wires, jumpers, etc to figure out. Really hoping to get it to work with the CT1024 it was intended for. That'd be a great youtube video -- a few people have posted 6800 videos but nobody but me has posted any of the CT1024, and nobody has posted any of the two devices together.

I don't know how early/late this machine is. I was told the earliest ones have a silver bar around the front of the case. The later ones are said to have the proper cutouts for DB25 connectors. However, mine has the black (later) case front, and the back is all circles (earlier). CPU is fifth week of 76, lots of chips from earlier around... there's a later 78 SWTBUG ROM that was likely added later. Thoughts?

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krebizfan

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1771 chip does suggest that the supposed floppy controller is a single density floppy controller with hookups for both 5.25" and 8" drives.
Ah, didn't see the FAD ASSOCIATES logo at first. Yup, floppy controller from 1979 or 1980.

The breadboard looks weird but I do see a Motorola 6820 for parallel port as an option from SWTPC. Probably equivalent but home brewed.

Enjoy that new acquisition.
 
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falter

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Well, I gave 'er the old college try.. brought the machine down to bare essentials -- CPU card, serial and first RAM board (I checked the jumper to make sure it was board '0'.

So with the baud rate and so on all known, what I get when I power up is a single random character.

According to the manuals, you have to check to make sure the 6810 RAM on the MP-A board isn't disabled (it would be to use FLEX). It looks intact though.

Wonder -- what would happen if I took the MP-A board and shoved it in my MSI? They're fully compatible, right? Might be a useful way to see if boards are alive or dead..
 

glitch

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I *hate* the SWTPC bus connector design. Those pins make it a savage job to pull cards safely.

Yup, welcome to the Molex KK-156 club! Ohio Scientific used the same series of connector on their backplane connections, but their boards are 8x10 inches. There's a *lot* of flex when inserting them.

Glad to hear this finally came through for you!
 

falter

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I literally grit my teeth every time I pull a board.. it's just awful. And the board itself has so much flex. There's just no good way to do it. I don't know what SWTPC was thinking.

I probably won't get very deep into this system -- I really want to get all this DG hardware I got from Marty dialed in. I'm so close on that 32 TVC board.

I did make some useful discoveries though. It's clear on examination of the CPU card and such this 6800 was set up for Flex use, and that may be why my attempt to go basic isn't working. I'm just not sure if I want to try to return it all the way to original condition and forgo Flex, or if I should persist in trying to get it alive with things configured as they are.
 

glitch

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With the Ohio Scientific boards, especially if I've just installed a new connector on a card, I will use a really broad flathead screwdriver or a piece of metal stock to press the connector(s) onto the pins. Just put your pushing device on the component side top edge of the Molex socket and work your way across the board, getting them started. Usually once the pins are started in, they push in pretty easily. The KK-156 connectors are *really* bad when they're new, all of the socketed chips creak like they're going to pop out!
 

g4ugm

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1771 chip does suggest that the supposed floppy controller is a single density floppy controller with hookups for both 5.25" and 8" drives.
Ah, didn't see the FAD ASSOCIATES logo at first. Yup, floppy controller from 1979 or 1980.

The breadboard looks weird but I do see a Motorola 6820 for parallel port as an option from SWTPC. Probably equivalent but home brewed.

Enjoy that new acquisition.

It would be nice to see the other side of that board. The 6820 is a PIA but why a 555, and why not other connector to allow access from external things...

P.S. Also enjoy...
 

Chuck(G)

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Yup, those molex connectors scared me away from the SWTPC 6800 the day I received the announcement of its availability. Over the years, I've learned that when you have those things, apply some dielectric grease (available from most auto parts and big box stores) to the pins. It'll save you grief later. More than once, I've pulled a card with those on some piece of equipment and ended up pulling the pin out of the PCB as well.

You might also consider screwing a stiffener bar to the PCB. If you can't find any, I can probably send you a couple. They're basically treated steel bar stock (about 5x10 mm) with threaded M3 screw holes every so often and have a plastic insulating strip.

You could probably make some out of 1/4" acrylic sheet as well.
 

dfnr2

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The 6820 is a PIA but why a 555, and why not other connector to allow access from external things...

I will guess a one-shot strobe for a centronics printer interface.

The board does have a "one hand clapping" look to it, though. There must be a connector somewhere, or perhaps a ribbon cable was soldered onto the back?

@falter: nice haul!

Dave
 

falter

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I will take some high res pics of both sides of each board so you guys can see. But the little breadboard one -- no, there is no connector to anything with it. Just wiring on the back side between ICs.
 

falter

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Oh maaaaaannnnnn

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Sooo close. So basically I swapped in the MP-C board, set it to 300 baud. I began to get suspicious that I had the RAM configured wrong. The RAMs were jumpered like so:

DRC 16k Bd 1 - Jumpered for 0000-3FFF
DRC 16k Bd 2 - Jumpered for 4000-7FFF
MP-M Bd 1 - Jumpers are - I can't figure it out. Supposed to be A to one of the pads marked 0 - 7.. instead A goes to a 7401, which is piggybacked on another chip. Pads 3 and 4 are also jumpered to this chip?
MP-M Bd 2- Jumpered from A - 5

So I thought.. aha.. no A000 in there.. maybe. Can't quite figure out the deal with those two MP-M boards. So I figure, ok, I'll try jumpering the DRC board for A000. Fire up the machine.. nope.. just a single bit of junk. Darn.

So then I'm like, ok, let's get real basic here and go back to the MP-Ms. So I took the first MP-M and changed the jumper so it was A to 0. Plugged it in alone with the CPU.. fired the machine up. Nope. No change. Then I took the other MP-M board, which is just all kinds of WTH with wires soldered everywhere and stuff piggybacked like crazy. I plug it in.. BOOM.. I get life! But it doesn't stop and allow input. It just keeps cranking out that prompt over and over again. But looking at SWTPC.COM's screenshots of the same terminal, it looks like it's 90% there. If I can just figure out why the thing is going bezerk, maybe I can get control. At this point, I'm thinking it must be a cabling issue. I would think if the RAM were totally screwed or the boards conflicting, we'd have nothing resembling the right prompt there. Thoughts? Interesting that the serial board does not work..
 

smp

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<...snip...>
Then I took the other MP-M board, which is just all kinds of WTH with wires soldered everywhere and stuff piggybacked like crazy. I plug it in.. BOOM.. I get life! But it doesn't stop and allow input. It just keeps cranking out that prompt over and over again.
<...snip...>
At this point, I'm thinking it must be a cabling issue.
<...snip...>
Thoughts?

A simple thought: The system monitor program seems to be acting as if it is getting constant, perhaps non-printing, input from the keyboard. Or constantly seeing <CR> characters coming from the terminal?

If you are certain that your terminal is operating properly, I suggest there might be a fault on the MP-M board making the system monitor think it is getting a steady stream of input? Is it possible to stick a logic probe in there and see what is coming into the circuit?

Just a thought...

smp
 
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falter

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I pulled the non-piggybacked RAM board. Now I get a normal prompt and it stops! But.. it's not receiving anything.. any keys I type essentially do nothing.
 

falter

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Victory is mine!

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I found my error. I had not clued into the instruction's 'now you can use a standard modem cable'. Not *null* modem. I changed it over to a regular modem cable and voila! Answering commands!

Only worrisome thing is it crashed on me.. started cranking out only 5s. I fiddled with the cable and made sure the wires were in good and tight.. then it started working again. I wouldn't be surprised though if this RAM board is flaky. I'll see if I can get it loading anything (like memory test) and see if we can get going.

For those curious, here's the SWTPC end of my 'standard modem cable':

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Initially I just hooked up the RX, TX and ground lines.. that did not work. It only started working when I connected up the DSR/DTR via jumper wire on the DB25 at the SWTPC end.

It's also working with the MP-S serial board now!
 

falter

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And here's the payoff:

https://www.youtube.com/watch?v=JDEtgEHqPiA

Turns out all I had to do on the CT1024 was cut two jumpers.

Unfortunately shortly after this video was taken the CT1024 keyboard quit again (stuck key somewhere). I also have noticed the Echo Off switch is not working, nor is the receiver on/off switch. So we have a bit more work to do there.

Still kind of cool to see the computer and the terminal it was intended to be used with together and talking!
 

falter

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Okay so, I definitely think I have some memory problems. I can load small TXT 'loader' programs via serial terminal and the machine will run them (ie. memory testing), but that's about it. On the memory test front, it generates an error right away when I run CDAT. The problem is, the only board that seems to work is a *nightmare* to contemplate solving. The chips are not socketed; the board is heavily modified with numerous changed traces, extra ICs, and all of the RAM is piggybacked with more RAM.

The second MP-M board is cleaner, but it also has some modifcations with trace cuts and jumper wires presumably to get along with the first. The system will not post properly with it in there regardless.

So then we have the DRC boards. These would be relatively easy to fix up since all the chips are socketed, but the machine will not post with either or both of them, no matter how they are jumpered address wise. So either they're bad, or there's something I'm missing. I have a feeling the MP-M board that works covers multiple address ranges and that maybe I'm not getting the right ones here. I don't know for sure though.

Anyway, if anyone has any thoughts, I'd be most appreciative!
 

falter

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I've sort of got it figured out now. I assumed the first board must have A000 since the machine starts on it. When I ran the test program, it would fail on testing $4000 right away. So I realized that perhaps if I configured one of the DRC boards for $4000-7FFF that would fill that space. And it did. Next, the tester failed $0000-3FFF. So I configured another DRC board to hit that space. No problem. So now the test fails at $8000, and sure enough, if I go to edit that memory space it gives errors, so it's not seeing it. But I can't figure out how to configure the last MP-M board to cover that space. And I'm not really understanding how the addressing works. The DRC boards are 16k, so I'm assuming that means they cover a wider address range than the smaller MP-M boards. I don't know how much RAM the piggybacked MP-M has in total, but I'm guessing if it was 4K originally it's now 8K if it's doubled? So maybe I only have $A000-FFFF or something like that? But I need to get 8000 up to A000? Maybe that last MP-M board covered it.. but it doesn't seem to be doing it now. And actually, if I try to edit any memory from C000 onwards, that comes up no good either.. so it's like that big MP-M board is just doing A000-B000. Confusing!
 

falter

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After much fumbling about, I finally figured out that my massively expanded MP-M board was in fact the root of a lot of problems with my 6800 system. I also learned that the other MP-M had been modified for the purposes of using higher memory addresses. I didn't know this at the time and modified it back to stock, trying to use it as one more board. After I realized what was going on I ended up modding it back to use the higher addresses, so I could take it out of play and just use it and my two 16k DRC boards. It's a good thing I took lots of pictures of how it was modded before -- it roughly but not exactly conforms to the mod suggested by SWTPC in their guidebook. When I tried going by the guide it didn't work -- looking back at pictures told me to change a couple more things. Anyway, that's all good now! I'm not sure if I'll ever fix the 'piggybacked' MP-M board. It looks woefully complicated and every chip is soldered, which would make it a real nightmare to diagnose and fix.

Now I am thrilled because I am able to load things like Altair 680 BASIC and that Space Voyage game. To use my CT-1024 I have to first load the S19s in (at a grisly 110 baud) on my Thinkpad 380XD and then switch the cable to the CT1024. It works. But yeah.. slow. And unfortunately this CT1024 was built to stock specs, so it cannot go above 110 without modification.

The other problem with the CT-1024 in stock config is there's no scrolling. So you have to be at the ready with the EOF button to clear the screen. Not very practical! My homebrew CT1024 does some form of scrolling so I might try that out next.

Anyway, here's a couple of 'victory' videos. Seeing Allen and Gates' BASIC was a real treat.

MITS 680 BASIC - https://www.youtube.com/watch?v=uNgUi6XDwv4

Space Voyage briefly - https://www.youtube.com/watch?v=jAL7YOAfH-E
 
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