Wide spread. Its very easy to work with. UART aka Serial TTL or just RX/TX is native to almost all chips. There is no standard protocol though, so unless its a modem (AT commands) or you have a manual you will likely have a hard time communicating with it. If you want to see what is going over the wire get a logic analyzer. Typically ~$30. Connects to USB and comes with software to decode the messages. Its read only so to speak.