Cmos 74VHC04 Rs232 Level Shifter

This is a simple circuit i've used to interface with USB to RS232 converters. This does not adhear to the Rs232 specification. It uses the fact that most of the USB converters will treat a voltage less than a few tenths of a volt as "mark" and anything over +3v as "space" so we just need to invert Uart TX from the micro controller size and clip the rs232 transmit voltage to -0.7 and  +5.1 with the zener diode and invert (so we get 0 to Vdd) for the Uart RX on the micro controller side. 


Note that the spec for the 74VHC04 Hex inverter is that the negative volatge on the input pin should not be less than -0.5v. It's likely here to be more like -0.6v to -0.7v , but it seems to work OK up to about 400 kb/s, but not a circuit for production use.