Progress on 680

[Charley Jones]

I’ve hit a short wall on my 680 build. D5 is now working, but A4 is still not lighting under run mode. It will light with halt when A4 is up. Would anyone be available to help me track down what could be wrong? I’ve resoldered every connection. A few were really difficult to get to.

Sent from my iPhone 15pm!
Charley Jones, PMP

[Chris Davis]

Did you read my reply to your previous message?  There is nothing wrong.  It sounds like it's operating normally.

[Chris Davis]

When the Altair 680 monitor ROM is idle, it continuously loops through a small set of ROM addresses while polling the serial interface. None of these addresses have bit A4 set. Since the front-panel LEDs display a time-averaged view of address activity, the A4 LED remains dark while the other address LEDs appear illuminated. This is normal and matches original hardware behavior.  

[Charley Jones]

Oh well,  chasing a non problems. Thanks Chris.  Beautiful build.

Sent from my iPhone 15pm!

Charley Jones, PMP

On Dec 31, 2025, at 1:14 PM, Chris Davis <famousd...@gmail.com> wrote:

When the Altair 680 monitor ROM is idle, it continuously loops through a small set of ROM addresses while polling the serial interface. None of these addresses have bit A4 set. Since the front-panel LEDs display a time-averaged view of address activity, the A4 LED remains dark while the other address LEDs appear illuminated. This is normal and matches original hardware behavior.  

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