IC1 generates the sampling clock of 256Hz

IC2 is a monostable used for pulse width modulation from 10usec to 1 msec
depending on the analog signal on PIN 5.
The pulse is isolated with the optocoupler and switches DTR to CTS.
DTR is an output and must be set to +10V by the software.
CTS is an input that can be read as a bit of the com port.

The UART of the com port can be programmed to generate 
interrupts on CTS level change.

A software driver (or DOS-program) could wait for the interrupt 
on the low-to high transition and then run a polling loop to calculate the
pulse lenght. The pulse lenght is a linear function of the input
voltage at IC2 PIN5.

This concept is as simple as a gameport ADC, but it is superior
to the gameport, because it does not suffer from 1/U nonlinearity,
it can generate interrupts and it provides the sampling clock.