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MITS Altair 680


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  MITS Altair 680 by Bill Degnan - 02/16/2007 20:34
directory of MITS Altair 680 pictures:


top/front corner view, main board - 680 MAIN BOARD REV 1-6X
same, closeup
same, another closeup of board ID
same, another closeup
front panel of MITS Altair 680. Compare colors with Byte back cover July 1976
another front panel picture of MITS Altair 680.
front panel of MITS Altair 680 (grainy pic).
MITS Altair 680 Main board view.
Altair 680 rear view. Apparently there once was a serial card installed in this unit.
MITS Altair 680 Main board view.
MITS Altair 680 serial number E11952.
View of 6800 and 6850 main board chips

Thanks for your replies. In response to all replies...

I agree that the system has been made from a kit, the pictures indicate this.
I purchased a ACIA Monitor ROM from Steve today.
I hope to find a KCACR card, RAM expansion, and a BASIC Prom. If anyone has these for sale, please let me know.
I do not have a user manual that details the use of the toggle switches, but I have some leads. In the mean time I have books and magazines.

I humbly appreciate assistance!

TESTING PROCEDURE: (I wrote to the Ebay seller and received the following response)
"..Programming the 680 from the toggle switches is a little different than the 8080.

1. Put the halt/run switch in the halt mode and press Reset.
2. Use the address switches to select the address you want to program.
3. Set desired data bit pattern on the data switches.
4. Press Dep (deposit).
5. Repeat for whatever program you want to load.

Data at any location can be viewed on the data lights in halt mode by setting the address switches to that memory location.

When you have put your program into memory, you can press Reset and then move the halt/run switch to run to execute.

This should force the 680 to begin execution at the reset vector location - address stored in 0xfffe/0xffff.

The 680 doesn't realy have a halt mode. The reason you see data lights in halt mode at address location designated by the address switches is that the 680 is running all the time. I believe someone came out with a single-step modification, but it was not widely used.

This is from memory - an Altair 680 manual would probably give you a better tutorial. Hope this helps.

I believe that HLT then RESET then RUN looks for an address to begin execution at 0xFFFE/0xFFFF.
One would assume that the jump vector would be 0x0000. However, in the absence of an EPROM at
the last page of memory, this is probably not the case. Most likely, the reset vector will read 0xFFFF
(no EPROM - everybody goes high). In this case, program execution begins at 0xFFFF. The instruction
at 0xFFFF will be 0xFF ( STX ). This is an EXT instruction so it will treat the next 2 bytes
( at 0x0000 and 0x0001 because memory wraps around ) as address bytes.

When I tested the unit, I started with a pair of NOPs at 0x0000 and 0x0001. This gave the processor
an address (0x0101) to go with the STX command and my test code could start cleanly at address 0x0002.

If your put an address for the STX command in 0x0000 and 0x0001 and start your code at 0x0002 you should
be ok.

Of course, a better solution is to find a 1702 EEPROM and run the monitor program so you don't have to
remember if the low address byte or the high address byte comes first.

I hope this info is helpful and not totaly erroneous. .." - Ebay seller

  MITS Altair 680 Update by Bill Degnan - 02/23/2007 23:53
My system:
The baud rate of the motherboard is 1200 (jumpered in)
The terminal bit is high (indicates that it expects a terminal).
RAM Memory is jumpered to 0000 location (1K)
Stop bits = 2
According to the Monitor guide F002 are the toggle settings for the switches (first 4 up, second-to-last up, the rest off/down)
I took two known working terminals, set to 1200 baud each and followed the start up directions.

I am not getting the expected dot prompt. There is nothing on the screen.

  Not IC 74L12 by Bill Degnan - 03/05/2007 23:11
IC in position L should be missing unless using baudot teletype.

Pictures have been updated.



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