Altair 8800 build - troubleshooting Front Panel LED behavior

[Luke Anderson]

Just looking for a pointer, an indication of where to start looking here. I have a logic probe and multimeter. Thanks in advance.

I went ahead and entered a simple "jump to" loop program via the Altair front panel:

Address 000: 303 (jump instruction)

Address 001: 000 (low byte)

Address 010: 000 (high byte)

When I step cycle through this program, it works as expected: starting with the first address, I can see the jump instruction light up, and the next two addresses show zeroes, and then it jumps back to Address 000. If I continue to step, it continues to loop the same way.

When I switch the computer to run mode, I would expect only the following LEDs to light up: 

D7 D6  .. .. .. ..   D1 D0

                           A1 A0

However, when the computer is running, ALL of the "D" LEDs are constantly illuminated. A1 and A0 are the only address LEDs illuminated, so that part is correct. But there shouldn't be any illumination of D5 - D2.

[Patrick Linstruth]

The S100 bus has separate data in and data out lines. The CPU is bidirectional. The front panel only displays the data in signals.

When your program is running, in addition to fetching instructions C3 00 00, the CPU is also putting status bits on the data out bus and processing those instructions. During this time, the CPU isn’t putting any signals on the bus that would cause your memory or IO device to gate anything on the data in bus. With nothing driving data in D0-D7 on the bus, they are being pulled up by the CPU card with resistors, so all D0-D7 LEDs light up. When the CPU gets back to an opcode or operand (memory) fetch, your D0-D7 will again show your code because your memory board is now gating the value on the data in bus.

You are seeing the CPU switch back and forth between memory read and other states causing the D0-D7 LEDs to glow. You should see the same thing by pulling all but the CPU board, D0-D7 lit.

This is normal behavior.

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