restarting AltairPC project in near future because of this

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restarting AltairPC project in near future because of this

Postby vbriel » Jun Sun 08, 2008 9:08 am

Image

Ok, here's a photo on my wirewrap board. Can't see the mess on the bottom but that's ok, trust me, it's a rat's nest.

The LED's on the right are DATA BUS and on the Left is 8 ADDRESS BUS LED's. After not being happy with the AltairPC project because the board was just some emulated board that couldn't do much I decided to start from scratch. I wanted an actual CPU that could handle more that 256 bytes and yet programmed on the front panel just like the Altair 8800. Well, there lies the problem.

Without getting too deep into the project here is a down and dirty explanation of how the Altair 8800 examines memory. What many people think it does is just asserts the switches to the address bus and reads the data from that memory location. Wrong! What it actually does is runs the cpu, and performs a JMP command to jump to that memory location, then stops the CPU at that address thus having RAM display the contents of memory where the CPU's program counter is at.

So, my goal was to take the 50ish chips that the Altair used to handle doing things like EXAM, DEPOSIT, EXAM NEXT, DEP NEXT and have a microcontroller control the CPU to do this. This means that I need to make the u-controller assert JMP commands at the right time and keep track of the program counter, so I can do a DEP NEXT and EXAM NEXT.

After some sleepless nights I was able to get it to work. I could actually assert JMP commands into the CPU and have it go to those locations. Writing to RAM is simple, OE and RD high, WR low, send the data to the data bus, small pause and turn off WR signal.

I tested by typing in a simple A+B add program from the Altair and ran it to verify that it works. It is running code as it should so my little wirewrap board is basically a small Altair 8800!

What is different. Well, for the wirewrap board I only use 8 address LED's and 8 switches so you can only address 256 bytes but it would only take a little wirewrap to add more. Also, in place of the 8080A CPU I used a Z80. The main reason was the Z80 runs on a single 5V supply, the 8080 doesn't. The Z80 runs all of the 8080 instruction set plus has an extended instruction set of its own. Plus I hadn't done a Z80 project since school and it was fun to work with.

What's next:

PCB prototype! I've got much of the design into software now so I just need to finish it and do the board layout. Some of the status LED's on the Altair don't apply because they were 8080 specific LED's and not used on a Z80 so I'm working on the logistics of that too.

Replica 1 front panel? Yup, the same theory is possible because I can control the CPU. There is a key signal missing on the 6502 that I have to work out but I think it is possible to do the same thing.

I'll keep you informed of my progress.

Vince
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Postby Bill Loguidice » Jun Mon 09, 2008 11:11 am

Great news, Vince. I've been hoping this would pick up steam again.
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Postby jmmm » Jun Mon 09, 2008 11:55 am

Dear Vince,

It might be nice to add a LED Output Port like on an Imsai.

JMMM
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Postby mcphja2 » Jun Mon 09, 2008 8:54 pm

This would be a great kit. I can already imagine some modifications and case designs.

I have always thought this is what the "IMSAI Series Two" should have been.
I have an old Altair backplane and a few cards. Maybe some enterprising people will find a way to interface old hardware.
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Oh! Mini Altair kit....

Postby Aaron Teeling » Jun Mon 09, 2008 9:56 pm

I've been jonesing for a new Briel Computers project.

Please make it S-100 card compatible.. It just opens it up to so many new possibilities.
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Postby vbriel » Jun Mon 09, 2008 11:47 pm

Ok, this opens up some questions before I go forward. What would you like to see out of this project. S100 is possible but I wasn't designing this as a S100 system. I was aiming at a SBC that could be installed however you you want, for example, on the front of a PC case. Grant already has a great exact Altair replica that is S100 bus. Any Z80 system can be modified to run a S100 bus so even this could be made to work if you wanted. The problem with S100 is 8, -8 16 and -16Volts to drive the thing. This requires a large power supply or at least a switching type that takes a huge space. So, it would be possible to adapt to S100 I suppose if that's what everybody wants.

My goal was to also emulate the 2SIO board and load Altair BASIC, that would be cool. The AVR micro-controller that emulates the front panel has a built in UART that would be pretty simple to emulate the 6850 in a 2SIO. This is the final bit of firmware that I'm writing.

Oh, there are 2 AUX switches on the Altair, I'm going to use one as a "slow down" mode. Anybody who has seen an actual altair in operation knows the lights blink very fast when running. I can slow down the CPU so the blinking lights are more pleasing to watch.

Anyways, just ramblings, let me know what you want out of this project, it is moving along very fast now.

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Postby CodeJockey » Jun Tue 10, 2008 8:28 am

Your original idea sounds good to me. Keep it simple, and allow for an expansion connector. That way, if the end user wants S-100 compatability (or another bus) in the future an interface card could be designed.
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Postby Bill Loguidice » Jun Tue 10, 2008 12:45 pm

For what my opinion is worth as a non-hardcore hacker, I really just want a low cost and working (functional and usable) blinking light and switches panel that can also possibly output to some type of display (for instance, how the Micro-Kim interfaces with a PC)... Anything else would be gravy for me.
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Altair Replica... the Vince Briel way

Postby Aaron Teeling » Jun Tue 10, 2008 2:25 pm

Well I purchased a computer from Grant.. they are nicely done but also extremely expensive. In the end, it's ALL been done before.

Blinking lights are cool too and yes I would like to have a switched front panel, but like all your other projects, The computer should be able to be functional too. Something that you can add a original S-100 card and make it operate.

In the end, it's all up to you, but the big wish list for me is:

Z80 computer with CP/M OS.
IDE drive compatibility (can be done as a separate kit)
Either onboard keyboard/video or via terminal display
An option to hook up a blinking light front panel (add on board? Mini board?)
USB serial capability
S-100 bus capability
Reduced chip count kit or kits.

In my mind, I see your Altair Replica (or Micro-Altair) SBC could be shoehorned into your AltairPC case and still have enough room to put your modern motherboard in the back.

As for the power supply, I can only assume that the use of a PC switching PS would be used, although the 16V requirement may be a bit tough. Perhaps do like you did with the Replica SE, make it an option if you plan to go the S-100 bus route.

All of this, of course is me running my mouth. I don't have the talent to pull this off and you are in the best position to know what is or is not doable. I just believe that the few of you that build these machines are not necessarily in competition.
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Postby vbriel » Jun Tue 10, 2008 4:15 pm

Well, the CPU and front switch section testing is complete. My design so far is to have a serial port and 32K RAM which should be plenty but it is expandable. I'm going to read the manual on the 2SIO interface so that I can emulate it with the micro controller's built in UART.

Aaron, do you have a 2SIO board for your Altair? I've got some questions about loading basic if you do.

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I don't have an Altair 8800...

Postby Aaron Teeling » Jun Tue 10, 2008 7:18 pm

.... I kicked down for the Altair 680 instead. Just received it about a week ago and it's in tiny pieces. Looks like a cool project though.

I'm sure there is someone in the Vintage Computer Fourm that may be able to answer your question though.
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Postby vbriel » Jun Wed 11, 2008 6:43 am

Well, reading through a guys site who did a disassembly of BASIC, I see where port FF is checked and I don't know why. I need to read the basic manual to find out why. Port 255 on the altair would be the "sense switches" better known as A15-A8 switches or maybe even A15-A0? If I know what BASIC is looking for I can make sure the microcontroller sends the proper response back.

Making the 6850 emulation and figuring out the reason basic checks the front panel is a final major hurdle. I'm going to start with port 255 being ignored or replying with a 0. This should work but I'm not positive.

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Postby vbriel » Jun Wed 11, 2008 1:48 pm

Not that I'm giving up before I even try but I may just attach a 6850 UART to the Z80 to act as a SIO (single instead of 2SIO) for the serial port. This will give it a better chance at full compatibility with BASIC. The change is 2 more chips, not a major deal.

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Postby vbriel » Jun Thu 12, 2008 9:18 pm

Just an update:

I'll be getting a 6850 ACIA in tomorrow and I'll wirewrap it and a 74HC4060 up to the Altair wirewrap board I made. If I can get BASIC to load and run then the R&D of this project circuit board will be 99% done. This is the last feature that I want the board to do. It must be able to load and run BASIC.

I will not go forward with this project unless BASIC loads. Shouldn't be an issue though.

As a note, I'm also going to try a simple circuit to use nicad battery backup on the RAM chip so that it will retain data when turned off. This would be cheaper than NVRAM chip and a nice feature.

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Postby mcphja2 » Jun Thu 12, 2008 10:53 pm

Sounds great. Who wouldn't love to be able to toggle in binary programs while riding the train to work!
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