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RS-232
Websites
RS-232 Serial Communications is not that hard.
http://www.sandman.com/rs232.html
For RS485, RS422, RS232 and RS423.
http://www.rs485.com/rs485spec.html
RS-232 was created for one purpose, to interface between data terminal equipment and data communications equipment employing serial binary data interchange.
http://www.ctips.com/rs232.html
Protocols and computer networks, by Dr. D. Koren of Tel-Aviv University.
http://www.rad.com/networks/1995/rs232/rs232.htm
Documentation with download.
http://www.ent.ohiou.edu/~welker/rs232/
Detailed explanations on interfacing the serial port.
http://www.beyondlogic.org/serial/serial.htm
A tutorial with signal names and definitions.
http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html
Overview of the RS-232 standard and cabling.
http://www.hw-server.com/rs232