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Use the release of reset as your trigger and set time scale to capture about 4us-8us across the sweep.This should get the first four cycles. U24p1 should be low during the first four cycles, U24p2 should be high. I’d expect U22p3, U22p11 to be low most of the time during the first four cycles...

You need to look at following signals during the 1st four cycles after RESET is released: 1) U24p1, U24p2 2) U22p3, U22p11 This will require a storage or digital scope. Mike

You‘ll also need to check U24 and U22 before passing off on item 4 Mike

The 909090 pattern means the CPU is running properly and has fetched a RST 7 instruction (0xFF) at some point. Upon fetching RST 7, the CPU jumps to location 0x38 and, in this case, also fetches an RST 7 (0xFF) at that location. From then on, the CPU pushes 0x39, 0x00 onto the stack over and...

Great!

Remove the PCB from the key board frame and you can the “type” directly on the PCB pads wih your fingers and that typically works. Mike

Since it‘s the original CUTER ROM, then the mods to the PIO board must be to make it looks the same to the CPU as the keyboard port on the 3P+S. Generally the 8255s on the PIO would require initialization by the CPU, but they can run in an input mode after reset without further initialization...

To support the IMSAI PIO instead of the 3P+S board that CUTER expects for the keyboard, you may have a modified version of CUTER. If not, then loading some custom init code for the PIO board may be required. Is CUTER a single ROM on your GPM or two 2708s? The sense switch settings are detailed...

Ask over in the Heath SEBHC group, someone there can help. https://groups.google.com/g/sebhc

LOL, as usual, hindsight is 20/20 - so frustrating. We kept testing resistance +/-16v to ground and there was no short. We forgot to test 16v to -16v. And this is one I’ve actually seen multiple times because these two pins are across from each other. I’ll head slap myself right along with you...

With the 1K resistor in-line to the motherboard, what is the voltage drop across the 1K resistor? I.e., is there +16v on the power supply terminal and +1v at the motherboard, or did you see the 1v reading at both the power supply terminal and at the motherboard terminal? Mike

So it does not sound like a diode short on the power rails then. Forgive me if I'm duplicating suggestions already tried in this thread, but without a connection to power or to the motherboard, use the resistance setting on your meter to verify open circuit between the DC- terminal and chassis...

Thank you both for your efforts. I've uploaded the complete version of the manual to the SWTPC section on the deramp website. Mike

Sounds like the power supplies are cycling into self protect mode over and over. A 165 ohm load at 16v is less than 1.6 watts, so a resistive load doesn’t appear to be the problem. Could be a diode short to ground in one of the ICs. Put your meter into diode test mode and see what you measure on...

Great work! Here’s a link to SWTPC notes about the binary format: https://deramp.com/downloads/swtpc/software/Two-Stage%20Binary%20Loader/BinaryLoader.pdf

I’m heavily into the middle of the restoration of an Altair 680. I’ll measure my boards and post some drawings. I’m going to make a video series about the 680 including getting a disk drive working with the 680 and running FLEX on the 680. Mike

I’ve been searching for the 8080 version ever since I got an iCOM 360 running. I found an old forum article that Bill Degnan had an FDOS disk and I have contacted him, but he has not been able to find the disk. Rich Cini also had an FD-360 controller and FDOS, but by the time I was looking for...

The rev 0 board did not have the 6850 ACIA, so console I/O was bit-banged. I haven’t seen a listing of the monitor for the rev 0 board, but I imagine it would be similar to the standard ASCII monitor but using the single bit I/O port logic as used in the Model 15 Teletype (Baudot) monitor (also...

For the most part, the 680b was the first operational version of the computer. The original 680 never quite worked and the 680b replaced it just a few months after the original 680 was announced. The front panel board did not change dimensions between the 680 and 680b. You must have a...

With the auto jump feature, it was primarily for S100 machines without a front panel. In my case it was in a Vector 1 computer that used this board for auto jump instead of having Vector’s own PROM/RAM board.