RS-232 C is a standard connection between a modem and the devices, electrical specifications, data exchange method, and timing. They perform different functions at the sending and receiving ends. At the sending end, a serial digital signal is used to modulate an analog signal (using, for example, frequency modulation), which is sent out on the
communication circuit; on the receiving end, the analog signal is detected and demodulated to reconstruct the original serial digital signal.
Serial data is passed between devices by either the asynchronous method or the synchronous method.
In the asynchronous method, data is handled in units of one character (6, 7, or 8 bits). The data is delimited by appending a start bit before each character, and a stop bit after each character. The receiving device recognizes the beginning of the character by detecting the start bit, and from that time reads each bit making up the character at fixed time intervals (determined by the transmission speed).
Synchronous transfer, on the other hand, sends data in units of character strings, which are made up of a .number of characters. As shown in the above figure, no start bits or stop bits are added as is done when the transfer unit is one character. Bit position is detected by synchronization with a special clock signal, and characters are distinguished by
detecting a special synchronization character at the head or in the middle of the character string and then counting bits as the data is received.
HDLC (High Level Data Link Control Protocol):
The synchronous transfer method called the High Level Data Link Control Protocol (HDLC) is a standardized method. This method uses frames of variable bit length as the data transmission unit rather than single characters or character strings. It is being used in wide area networks for connecting computers. In this method,a special bit string (01111110), called a flag sequence, is placed at the beginning and end of the frame. These flags are used to detect the frame and the bit positions.
The RS-232 C interface can be used for direct connection of nearby devices (within 15 meters) as well as for the widely used standard modem connection. Thus it is standard equipment on many computers and control devices.