Another 680 limps in Las

[Charley Jones]

When I turn on the device in run mode, all switches down, all lights but HLT, A4, and D5 blink. If I hit HLT, then HLT lights. Good. Re-soldered switches s and LED A4 and D5. No change.

In HLT, A4 up, it does light

Going to take the board off and re-solder header pins. And go over the resistor packs again.

[Chris Davis]

That sounds like the correct behavior, what are you expecting to see?  The LEDs are showing the current memory locations. In RUN mode the 256-byte monitor ROM is running in the FF00-FFFF range in a loop waiting for keystrokes.

[Tom Lake]

I got my kit today and assembled it. I found that, in order to get all the LEDs though the front panel holes, I had to solder one leg of each LED before sliding the FP over them otherwise they flopped around and I could never line them all up at once. 

My case won't arrive until Monday or Tuesday, though.  I have a question about which lights are supposed to come on, though. On mine, A1 is on all the time no matter what mode I'm in and A0 is off no matter what (even when I raise the AC switch)

The other address LEDs go on when I raise the corresponding switch and go off when I lower it when in HLT mode. I see the data lights change when I raise or lower A0 or A1 so I'm sure the switches are good.
A0 could be a bad LED but I sort of doubt it. A1? I don't know why it would always be on.

Any ideas?

[Tom Lake]

Figured it out! I had a spare Due so I figured, what the heck. It couldn't hurt to try it. I swapped out the one that came with the kit and - voila! A0 and A1 LEDs now work as they're supposed to.
It seems like it was a bad Due.

I do have another question, though. When I am in RUN mode, LEDs A8-A11 are off as well as A4. If the ROM is cycling through FF00-FFFF shouldn't all the upper LEDs be on? All the LEDS A0 - A15, D0-D7 and the three status LEDs come on when I raise AC so I know the LEDs are good.

[Charley Jones]

Hey Tim, in my experience, it’s only A4 that you see off in run mode.  If you halt, the address light will become active when the switch below it is in the up position.

I hope that helps.

Run mode:

Not what I was expecting either.  All lights on but hlt and A4 is normal. 

Sent from my iPhone 15pm!

Charley Jones, PMP

On Jan 17, 2026, at 8:13 PM, Tom Lake <tl...@twcny.rr.com> wrote:

Figured it out! I had a spare Due so I figured, what the heck. It couldn't hurt to try it. I swapped out the one that came with the kit and - voila! A0 and A1 LEDs now work as they're supposed to.

--
You received this message because you are subscribed to the Google Groups "Altair-Duino" group.
To unsubscribe from this group and stop receiving emails from it, send an email to altair-duino...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/altair-duino/df558074-b07b-4074-a6af-2b6e3702858en%40googlegroups.com.

[Chris Davis]

The front panel updates the address LEDs by sampling the address bus about every 10 ms. At that rate the CPU has already executed thousands of cycles between updates, so each sample reflects whatever address happened to be on the bus at that instant — not necessarily the current instruction address. Since the monitor’s idle loop never fully asserts the third address nibble (A11–A8), and many normal bus accesses have that nibble at zero, it can appear dim or off in RUN mode. This is normal behavior and does not indicate the CPU is executing in the wrong memory range.

The current address does have the third nibble "off" in many legitimate accesses, such as reading the stack page or the ACIA (F000 & F001).

[Tom Lake]

Thanks. I was wondering why some people have all address lights except A4 on and I not only have A4 off but A11-A8 as well. We're running identical hardware and software, I believe.

[Chris Davis]

Sometimes I turn it on and the third nibble is off, sometimes it's on.  It just depends on where that 10ms window hits.