From my notes:

Five wires:
/CLS (= ready) signal. It will keep low while the card is moving
/RCP (= clock read input signal). Capture data in falling edge
/RDD (= data). Inverted logic so low level means '1'
*Another /RCP and /RDD for the second head. /CLS is common to both.

Vcc, GND ??

Check /RCP and /RDD with the oscilloscope in chopper mode, then you'll see clearly the clock (narrow and equally spaced pulses) and data (wider pulses).

Data word used to be 6 or 7 bits, can't remember.

There is a lot of info around in the web. I connected a card reader to my Commodore64 CIA serial register around 1988, long before the Internet exploded.

Txip.