I have a NEC PC-8001mkII computer (sometimes incorrectly called a "PC-8001mk2") which has a built-in disk controller for PC-8000-series double-density 5.25" drives. (The English and Japanese Wikipedia pages contain a bit of information about this model.)

The floppy disk connector on the back is a female 36-pin micro-ribbon connector, also known as a "Centronics" connector, similar to printer and SCSI drive connectors from the 1980s. That Wikipedia link shows the male and female connectors, and a picture of the female connector on the computer is below.

I'd like to know what the pinout for this connector is. Bonus points given if you can describe how to hook it up to an HxC 2001 or similar floppy emulator.

Given the number of pins, I'm guessing that it's moderately likely that NEC is directly running Shugart's 34-pin SA-400 interface through that connector. I have an oscilloscope; if you have suggestions for me about how to reverse-engineer the pinout for interface I'll happily take them.

NEC PC-8001mkII floppy disk connector


Starting with the working assumption that it's a variant of the 34-pin Shugart interface is a good start for reverse-engineering the connector.

Every even-numbered pin in the Shugart pinout (and the PC variant) is grounded. You can use a multimeter to test for continuity between the pins of the Centronics connector, and see if every other pin along the connector are electrically connected.

If you are able to open the computer, it would be useful to be able to trace the other pins back to the ICs that are connected to them. The part number of the chip could give you some information on the function of those pins. As an example, if you find some pins are driven by a 7438 chip, the fact that this is a chip with open collector outputs would indicate that these are generating outputs to be sent to the floppy drive.

With the computer running, you could issue commands that should operate the floppy drive, and observe the waveforms on the non-ground pins that you believe are outputs. control signals for Shugart floppy drives are active-low, so you should see the voltage drop to zero when you start trying to access the non-existent drive, and return high if/when the computer times out. One would expect to see this behaviour on the Drive Select lines (e.g. /DS0, pin 10), and on the /MOTOR-ON line (pin 16). There may also be activity on the /STEP (pin 20) and /DIRECTION (pin 18) lines as the floppy controller tries to step the drive's read/write heads back to track 0. (The controller expects to be told when it's there by the floppy drive, by an input signal on pin 26, and will continue stepping towards it until it gets there or gives up.)

If these tests match up with the 34-pin Shugart pinout, I'd suggest that what "walks like a duck and quacks like a duck" probably is a Shugart floppy interface, and it's worth assembling a cable to try it with an actual floppy drive.

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