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Old 07-08-2007, 08:41 PM
sje sje is offline
Join Date: Nov 2006
Posts: 62
Default Altair 680 front panel operation

Some questions about the Altair 680 front panel operation:

1) No single step operation?

2) No deposit next / examine next operation?
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Old 07-08-2007, 09:55 PM
Grant Stockly Grant Stockly is offline
Join Date: Jan 2005
Posts: 447

1) No single step operation?
No. I think this is a limitation of the 6800 processor.

2) No deposit next / examine next operation?
Nope. There is examine/deposit, but no auto increment. I think this is also a limitation of the processor design.

With the ROM monitor you can do memory operations very efficiently over the serial port.

If you type "M", then the rom monitor will respond with a space. You then type the 4 digit hex address, "0000", and the monitor responds with a space + the data + space. You can then press enter OR enter in NEW data for that location.

If you type "N", the monitor will respond with space + [previous M or N address + 1] + data + space. You can then press enter OR enter in NEW data.

A session would look like this:

.M 0000 00 01
.N 0001 2E 02
.N 0002 78 04
.N 0003 23 08
.N 0004 12 10
.N 0005 7E 20
.N 0006 2A 40
.N 0007 4D 80
.M 0000 01
.N 0001 02
.N 0002 04
.N 0003 08
.N 0004 10
.N 0005 20
.N 0006 40
.N 0007 80
Other ROM monitor commands are J (followed by 4 hex digits) for jumping to a specific address, and L for loading motorola S Record paper tape images.

The front panel is pretty handy for debugging problems with the computer, or for "fun", but it isn't as useful as the front panel on the Altair 8800.

Its still fun to watch the LEDs!
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Old 07-12-2007, 11:47 AM
sje sje is offline
Join Date: Nov 2006
Posts: 62

I agree that the Altair 680 front panel still has plenty of visual appeal even if a few functions require some additional manual intervention vs the Altair 8800.

It really is a good looking kit.
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Old 07-19-2007, 01:52 AM
sje sje is offline
Join Date: Nov 2006
Posts: 62
Default Single stepping the 6800

I think you're right about an inherent limitation in the 6800 concerning single step operation. I haven't been able to find any example circuits that will handle controlling a 6800 in step mode, either per instruction or per bus cycle. Other than the Altair 680, none of the early 6800 based personal computers (SWTPC, Sphere, etc.) had a front panel.

What is interesting is that the MOS 6502 is easy to step; the circuit needs only a pair of flip flops and a couple of switches. And the 6502 is pretty much the same as the MOS 6501 with a different pin-out; the 6501 itself was an almost drop-in replacement for the Motorola 6800, but the 6501 withdrawn from the market by MOS due to legal threats by Motorola. So where did the single step capability in the 6502 come from if it wasn't in the 6800?
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Old 01-31-2008, 05:36 PM
Bob Nelson Bob Nelson is offline
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Join Date: Jan 2008
Posts: 1
Default Single step on the 6800

I am pretty sure this was a restriction due to the CPU registers being implemented with dynamic memory technology. The processor had to keep running to insure they were being refreshed.

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Old 02-01-2008, 11:49 PM
rfeecs rfeecs is offline
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Join Date: Feb 2008
Posts: 6
Default Single stepping 6800 is possible

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.
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Old 02-04-2008, 05:33 AM
rfeecs rfeecs is offline
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Join Date: Feb 2008
Posts: 6
Default Yet more on single stepping the 6800

I have been surfing around sites about vintage computers, and stumbled across Dr. Robert Suding's page about the history of the digital group:

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.
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