Now that I am ready to do the tests I found that I have the wrong computer! My Casio is a PV-16 and it provides the cassete circuits internally.
This little computer have been sitting on a drawer for most of the time that I have acquired it and it is extremely similar to the PV-7.
Well that should not bother the tests. I should use a microcontroller to replicate the signas coming from the computer and an oscilloscope to acquire the signals read from a tape recorder.