Altair 680 front panel operation

[rfeecs]

To quote the 6800 datasheet:

To debug programs it is advantageous to step through programs instruction by instruction. To do this, HALT must be brought high for one MPU cycle and then returned low as shown at point B of Figure 13. ...

Looking at the schematic for the Altair 680, the address LEDs are tied directly to the switches. They simply display how the switches are set. Probably wanted to stay as simple as possible.

[rfeecs]

I have been surfing around sites about vintage computers, and stumbled across Dr. Robert Suding's page about the history of the digital group: http://www.ultimatecharger.com/dg.html

He says:
I never cared much for the 6800 because its address bus was dynamic, and when I tried to single step it, all I got was all 1's for address out to my static RAM...I preferred the 6501 to the 6800 because if I had a problem CPU PCB, I could pop out the dynamic 6800 and put in a static 6501 and new boot code EPROM and quickly single step through the problem software.

I went back to the datasheets for the 8080, the 6800 and the 6500 series:

The 8080 has a READY line that will put it into a wait state where it will hold the address on the bus as long as READY is low. The Altair 8800 front panel uses this for all its functions to hold the processor including single stepping. It also allows the front panel to display the address LEDs directly from the processor address bus.

The 6501 and 6502 have a RDY line which performs a similar function, and holds the address for slow memory or single stepping.

The 6800 has a HALT line (on the same pin as the 6501 RDY line). This can be used to halt the processor for one or more cycles. So it can be used for single stepping, but the address lines are not held active once the instruction cycle is complete. So front panel circuitry like the 8800 has would not work with the 6800. The 6800 would require latching the address lines to display them from the address bus.

Interestingly, the Apple-1 operation manual shows a schematic for displaying the address with LEDs using latches. It also shows the two flip-flop single stepping circuit. The M6800 applications manual shows a circuit with 3 flip flops and 4 nand gates.