CH340G USB to RS232 TTL module schematic (D-SUN-V3.0)
Cheap ($1) chinese USB to RS232 TTL modules from eBay. Works fine and is cheap enough to be thrown anywhere.
Windows drivers download: http://www.arduined.eu/ch340g-converter-windows-7-driver-download/
CH340G datasheet (third-party translation): http://cdn2.boxtec.ch/pub/diverse/ch340g-datasheet.pdf
Schematic (reverse engineered):
Source version (KiCad): https://github.com/NiceCircuits/modules_from_china/tree/master/CH340G_D-SUN-V3.0
PCB with marked components: