USB to RS232 and TTL UART Adapters
These allow for the PC to communicate over a USB port with UART serial devices using either RS232 or TTL voltage levels, and come in two types:
1. Adapter Type
These adapters have a USB-to-UART converter and a RS232 line-driver chip and are mostly supported by Linux.
Something to watch for is adapters containing fake Prolific chips; Prolific have modified their Windows drivers to no longer support the fake chips, but this can be overcome by installing old versions of the drivers. Linux doesn’t not have this problem and supports the fake chips as well as the real ones. See here for more information.
Also, some adapters have the RX and TX lines reversed on the DB-9 plug, so it is best to check which is which before wiring the socket.
2. Board Type
These boards have pins for the TX and RX lines at 5v levels as well as 5v and 3.3v power derived from the USB 5v line.
Many of these also have a pin marked RST which is the RS232 DTR line. These can be used for programming Arduino devices.