In IBERCOMP's manual of altair535 part two there is one example whith code and the circuit.
This example if for conecting an standard cassette to an 80535.
The problem is that it is writen in spanish.

I dont know if they sold the manual without the complete kit of hard.
Ask them in www.ibercomp.es

The manual is named:
"ALTAIR 535
MANUAL DEL USUARIO
PARTE SEGUNDA"

"8.10 Interface a un cassete"

Regards.