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*  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:  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