Starting the debugging of IOB6120 add on to the SBC6120 PDP-8 replica

NEED TO USE LOGIC ANALYZER TO TRACK ACTIVITY OVER THE BUS

The SBC6120 features a 50 pin expansion connector, carried around through the front panel FP6120 to a connector where the IOB6120 is connected. A few of the pins are power and ground, but most are control signals and the main bus. I want to track these, which will require three pods on my HP logic analyzer, each offers up to 16 probes.

The next channel is mechanical - how to access the various signals. With through hole ICs like DIP chips, I can put a grabber around a chip lead, but with surface mount this is quite difficult or impossible depending upon the pin pitch.

If I can build an extender, something that plugs into the FP6120 connector, has a second end to each signal wire that plugs into the IOB6120, and allows attachment of the logic analyzer probes somewhere, that will let me do the job. I would need a female 50 pin on a cable, like an internal SCSI connector. In addition, a few male-to-male adapters which could simply be 2x25 header strips.

LIKELY CANDIDATES AND FIRST AREA TO CHECK

The likelihood is that I have one or more counterfeit chips on the board. Since they have their outputs tied together on the bus, presumably they sit in high impedance until the chip enable line is asserted. If these are not real chips, they likely won't have high impedance on those pins which will jam the bus. 

The four SRAM chips as well as the Flash memory chip are the ones I suspect the most. All other chips were sourced from Digikey or Mouser, reputable suppliers. 

QUICK CHECKS OF SUSPECT CHIPS

To check the suspect chips, I will measure the impedance of the bus lines which should all be high-Z. I can provide power to the NVRAM battery contact to power the SRAMs and again check the state of the bus lines. Pulling the chip enable low at the PCB activates the outputs; again they should only activate with contents. Finally, I can provide 5V to the board, test the bus to see if the flash is stomping on the bus, then pull the flash enable low at the PCB to check for its outputs.

PLAN B AND C IF CHIPS ARE FAKE

The obvious plan B is to attempt to buy from another eBay source and hope that the results are better this time around. If that doesn't work, I will be forced to a complex alternative wherein I will create small PCBs that match the footprint of the pads on the main PCB, then place some tinier alternative chip that will be functionally and electrically identical albeit with a different assignment of signals to pins. 

For the Flash it also requires me to modify the programming which writes the flash image, or write it myself which is the more straightforward approach. Hopefully the IOB6120 treats the flash as pure SRAM for everything except writing new ROM contents via the FL command.