I purchased a USB to serial converter, popped it open and observed the two chips inside: a PL-2303HX USB to Serial Bridge Controller for a USB to TTL serial converter, and a ADM211 for converting those TTL levels to RS232 levels. Which is fine if you wanted to actually talk RS232.

But I want to talk to a microcontroller, with TTL level signals. I thought about adding a few extra wires to the USB adapter, but I'd prefer to use the plug as-is. It should be possible, looking at the datasheets.

The ADM211 will accept up to +0.5V for a "high" signal, so a TTL output from the microcontroller should be fine to communicate back to a computer, but the signal will be inverted. That can be fixed in software.

The input is a bit trickier. An ATmega32 will only accept -0.5V on an input pin before it fries itself, but the ADM211 sends its signals at ±10V. A BS170 MOSFET though can handle a gate-to-source voltage of up to -20V - which is well outside what the ADM211 supplies. I found a circuit that uses exactly that component. The guts of it looks like this:

It looks like it should work as advertised. On a HIGH input from the microcontroller, the gate voltage will create a conducting channel within the FET, sending a low signal to the ADM211. A LOW signal will turn off the FET, bringing the output to 5V through the pull-up resistor.

I also thought about using a logic IC, so I looked at a 74HC00. The input voltage is the main problem - it's -0.5 to 7V. A resistor will deal with the 7V, but I'll have to think about the -0.5V.

I have no idea whether I'll give it a shot one day...