I'm re-implementing a 1980's Microbee computer on an FPGA (see [here]( http://www.toptensoftware.com/fpgabee)) and trying to figure out how to do the cassette port. Here's the schematics for the original Microbee cassette interface:

[![MicrobeeSchematic][1]][1]  
<sub>(source: [toptensoftware.com](http://www.toptensoftware.com/fpgabee/MicrobeeCassettePortSchematic.png))</sub>  

I also found this description of it in a technical manual:

> The cassette data output consists merely of an RC network which
> accepts a signal from  DB1, pin 28 of the PIO. The signal is
> attenuated and then decoupled prior to sending it to  the cassette
> recorder MIC input. This signal appears on pin 3 of the 5 pin DIN
> socket.   
> 
> The cassette data input circuit is slightly more
> complicated. The input from pin 5 of the  DIN socket passes first to
> an attentuator -decoupler. Following this is a CA3140 op-amp, to 
> allow a wide range of input levels to be squared up before the signal
> is passed to pin 27 of  the PIO, DBO. The two diodes across the
> inverting and non-inverting inputs to the op-amp  clip any input
> signals greater than the diodes' forward voltage in either direction.
> The  47pF capacitor is required by the CMOS op-amp for
> precompensation.

My questions:

1. What does *"de-coupled"* in the description mean?
2. Would the same circuit work if connected to two of the I/O pins on a Xilinx Spartan 6 FPGA (through the PMod connector on a Nexys3) and if not, could it be adapted to make it work?

---

First attempt, based on comments in answers, but the output resistor shouldn't be in series.

*I'm leaving this here for context and instructional reasons, please see the next schematic* 

![MicrobeeSchematic2](http://www.toptensoftware.com/fpgabee/MicrobeeCassettePortSchematic2.png)

New questions:

1. Is the polarity of the comparator correct?
2. For the [MCP6546](http://ww1.microchip.com/downloads/en/DeviceDoc/21714f.pdf), does Vss go to ground and Vdd to 3.3V?
3. I'm not sure what to make of the "dotted out" resistor across the tape inputs in the original circuit.

---

Incorporating feedback regarding the output of the comparator being open-drain:

![MicrobeeSchematic3](http://www.toptensoftware.com/fpgabee/MicrobeeCassettePortSchematic3.png)

What alternatives could I use for the MCP6546, which I've not been able to find in retail stores here in Australia.  I can get LM311 or LM393 which, from what I can tell are similar. Will these work just as well?

 [1]: https://i.sstatic.net/AtlwC.png