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Can you explain the concept why it is synchronous, other than the link you have given
Asynchronous communication is used on RS-232 based serial devices such as on an IBM-compatible computer's COM 1, 2, 3, 4 ports. Asynchronous Transfer Mode (ATM) also uses this means of communication. Your PS2 ports on your computer also use serial communication. This is the method is also used to communicate with an external modem. Asynchronous communication is also used for things like your computer's keyboard and mouse.
---------- Post added at 22:25 ---------- Previous post was at 22:21 ----------
The Rs232 uses asynchronous communications (most PC-based systems). If you are using synchronous communications, the null modem will have additional connections for timing signals, and a DB25 connector would be necessary.
Strictly speaking, RS-232 standard specifies electrical characteristics and other interface details, but does not specify protocols, encoding, etc.
It can be used both in synchronous or asynchronous links.
Nevertheless, many serial links that usually use RS-232 (e.g. COM ports in PC) are asynchronous: they send characters in start-data-stop format.
Regards
Zorro is correct about the RS232 only being an electrical specification, how the data is sent is entirely up to the program at each end.
Almost all present day serial links are asynchronous, the data is framed between start and stop bits so the receiving end can located and correctly isolate the data from the stream without knowing exactly when it is due to arrive. Older PCs with 25-pin D connectors had the option to use both modes, if synchronous mode was selected, the transmit clock was sent to the connector and the UART clock was derived from the incoming connection.
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