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I'm writing software to initialise the EUSART1 module for RS232 on a PIC18F46K22. I'm using the usart.h library, built-in in the C18 compiler. There is some reference in MPLAB C18 C Compiler Libraries for the Open1USART() function on page 69 (73 in the PDF's numbering).

I now need to choose whether to use asynchronous or synchronous mode. What is the difference? Which should I choose when I just want to communicate with my pc, the common way (using RealTerm or so)?

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“Synchronous” means that one side of the communication transmits a clock signal along with the data signal. “Asynchronous” means that there’s just the data signal, and the sender and receiver are meant to be in agreement at what speed it is transmitted.

For communicating with your PC, you want asynchronous mode.

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  • \$\begingroup\$ So RS232 is always asynchronous, isn't it? \$\endgroup\$ – user17592 Apr 7 '13 at 18:16
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    \$\begingroup\$ It appears that the RS-232 standard as such does provide for a synchronous mode, but it’s not used in standard communication with a PC, and I don’t think there is even a pin for it anymore in contemporary connectors (The original RS-232 connectors used 25 pins). \$\endgroup\$ – microtherion Apr 7 '13 at 18:59

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