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Async & Sync     

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ASYNCHRONOUS VS. SYNCHRONOUS
DATA TRANSMISSION

The "normal" (that is, the "typical") RS-232 implementation is considered asynchronous. That is, the timing between individual characters is irrelevant because each character has its own start bits. The receiver only has to remain synchronized with the transmitter for the length of time required to receive the data, parity, and stop bits. If the receiver clock rate wanders slightly it’s OK, since the clock is synchronized at the beginning of every character. On the other hand, a design could be implemented that required longer streams of characters to be sent in one continuous block. In this case, the transmitter could use the timing lines on the RS-232 interface to implement a synchronous connection. The advantage of the synchronous implementation is that it can be significantly more efficient for the transmission of large quantities of data.

Consider the transmission of a 1K block of data using asynchronous RS-232, 1 start bit, 8 bit words, no parity, and 1 stop bit. This means that each of the 1024 bytes of data requires 10 bits on the cable. The 1K of data is made up of 8192 bits (1024 * 8 = 8192). It requires 10,240 bits to transmit these bytes (10 bits per character, 1024 characters). This results in an efficiency ratio of 80% (8192/10240 = .80). That is, 80% of the transmission is data, 20% is overhead (in the form of the start and stop bits). This is assuming that we are not configured for 2 start or stop bits or a parity bit. This is, essentially, "best case" for asynchronous RS-232.

If, on the other hand, we send synchronous blocks of 1K, each block will have start bits but the data will be sent in one continuous stream. In this case there are, say, 2 start bits, and, perhaps, another (let’s say) four bytes of overhead followed by, let’s say, a four byte checksum field and then 2 stop bits. Now we’ve added 8 bytes plus four bits, or, 68 bits of overhead. The overhead, however, is for the entire 1024 bytes of data! That means that to transmit the 8192 bits of data it requires 8260 bits. That is 99% efficiency! (8192/8260 = .9918). In current implementation there is a very limited use for synchronous RS-232. This is because other interconnection technologies are either less expensive or more popular or because asynchronous RS-232 is simply cheap and easy.

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