I'm trying to build an interface to talk to this particular battery charger that has a single wire bidirectional serial port.
Someone created the following schematic based on the official interface you can buy. It may not be 100% accurate but it's a good starting place. The zener voltages listed were measured values, I imagine most or all of them are actually 3.3V zeners or thereabouts because that's supposedly the signal wire communication voltage. The interface is an RS-232 to single wire serial converter with optoisolation.
This circuit is beyond my very basic understanding of electronics. The interface I want to build does not need the RS-232 conversion components because it will have straight TTL serial TX/RX and I do not need the optoisolation because I'm using separate unconnected power sources for each side (batteries).
My plan is to use an Atmel ATmega with two serial ports to provide the interface. The software on the ATmega will do nothing more than pass data between the two serial ports. The ATmega runs at 5V so I'm using a CD4050 to convert the single wire signal down to 3.3V.
The following is the schematic I have started. I believe I need some pull-up or pull-down resistors and possibly signal inversion but I'm not sure, this is where I need help understanding the original circuit.