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Old 01-05-2009, 05:54 PM
FlashCorliss FlashCorliss is offline
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Location: Atlanta, GA
Posts: 4
Default I can't get the Monitor to start...

I got an Altair 680 kit for Christmas from my wife (and Martin). I was very surprised!

However, it was missing all of the sockets, the power cord, fuse holder, back power switch, and most of the parts required to wire up the RS-232 interface. Sadly, I couldn't build it on Christmas day (DigiKey was closed) - Darn.

So, after extravagant overnight shipping, and many hours of meticulous kit building, I was ready to power the 680 up.

LEDs blink, memory can be accessed, read, and written, the power supplies are proper, but I get no "." on the RS-232. The computer is jumpered for 9600 baud, 8, n, 1, terminal, RS-232. The DB25 output (pin-3) shows -12, the input (pin-2) reads close to zero, and ground is ground (pin-7). I connected the DB25 of the Altair 680 to a DB9 for my laptop as so...

DB25 pin 2 - 3 of DB9
3 - 2
7 - 5

I've used several different terminal programs with 3 different laptops, and still no monitor prompt. I'm running with only the 6800 and the ACIA Prom (the memory expansion board and VTL roms have been removed). I have not probed around with a scope yet - was hoping not to. I took the 6850 chip out and jumpered pins 2 and 6 together and I was able to get the laptop to see it's own characters (a loopback through the RS-232 circuit). So I feel confident that the wiring is correct. A logic probes reveals nothing on the output of the 6850 (pin-6).

Any ideas? When the 680 in set to RUN, the upper LEDs (A9-A15) appear to be counting up in binary fashion. In fact, with the ACIA MON rom removed, the A13-A15 LEDs appear to count (I guess it's just random code).

Did everybody else's 680 work okay? What else can I check?

Thanks, Flash

Last edited by FlashCorliss; 01-05-2009 at 07:53 PM.
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Old 01-05-2009, 10:43 PM
Grant Stockly Grant Stockly is offline
Join Date: Jan 2005
Posts: 447

I never include the actual power cord, but all of those other parts should have been in the box when it was shipped. Do you know if they were missing when it was originally shipped?

There are quite a few things that can cause it to operate wrong like you are describing. A correctly functioning CPU isn't even needed to examine-deposit memory.

Do you have an oscilloscope?
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Old 01-05-2009, 11:02 PM
FlashCorliss FlashCorliss is offline
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Join Date: Jan 2009
Location: Atlanta, GA
Posts: 4

I'm not sure what all was originally in the box as I got it second hand - serial number 5, I believe. The guy my wife bought it from did offer to buy sockets when I asked him about the shortage. But since I wanted the gold Mill-Max variety anyway, it was less of a hassle to buy my own from DigiKey.

I had parts left over too; 2 IC chips, some resistors, lots of mounting hardware... I'll just put them in the bin for when I build the Altair 8080. :-)

Yes, I have an oscilloscope. What should I check?

I was toying with the idea of making my own ROM (to replace the MONITOR) that simply had 00,00 at the FFFE and FFFF locations. I'm thinking that will force the computer to jump to the bottom of the RAM on a reset and start executing from there. That way I can toggle in simple OP codes and check the outputs on the LED's (kind of like the COSMAC ELF or Altair 8080). I'm hoping that will let me know if the processor is running away or not. Since nothing comes out the RS-232, I'm flying blind...

I await your advice...


Last edited by FlashCorliss; 01-07-2009 at 01:42 AM.
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Old 01-16-2009, 02:19 AM
FlashCorliss FlashCorliss is offline
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Join Date: Jan 2009
Location: Atlanta, GA
Posts: 4
Default Behold the Power of the Altair 680!

So, I got serious...

Digital Storage Scope, logic probe, DVM, 16 bit logic analyzer, PROM burner... I was going to get this fixed.

As I was getting ready to attach the logic analyzer, you know - measuring voltages and clocks, my logic probe revealed no signal on the Monitor ROM A2 line - curious. So I jumpered that pin to the socket next to it, and Voila, a "." on the terminal. I yanked the motherboard and discovered that I had missed the solder pad completely. A little dab and she's good to go.

It's a shame really. I was looking forward to using the logic analyzer:-(.

But in preparation for the surgery, I found out a lot of stuff about the Altair. Like just how funky the 1702A ROMs are, and even funkier their programming algorithim. Like how simple the Altair 680 design is, yet very complex if you don't know where the processor is going (mine was running away). And, just how awesome this kit is! Grant, you are the man!

In hindsight, stepping through the first handful of Op Codes, starting at 0xFF00, and comparing them to the source listing in the manual would have revealed the stuck (missing) A2 line. Poo

Oh well, it wasn't a complete waste of time. I got to play with my ELAB-080 that I have had for years but was still in the box. I'm kind of hoping something else will break so I can use the ELAB again.

(P.S. Grant, do you show me on your waiting list for the 8800? I can't wait to build another kit!)
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