pled in error, and all subsequent elements will be errors. If the timing error between transmitting machine and receiving machine is 2%, the cumulative error in timing would cause the receiving device to receive all characters in error after the 25th element (bit). 10.7.2 Asynchronous and Synchronous Transmission In the earlier days of printing telegraphy, "start-stop" transmission, or asynchronous operation, was developed to overcome the problem of synchronism. Here timing starts at the beginning of a character and stops at the end. Two signal elements are added to each character to signal the receiving device that a character has begun and ended. For example, consider the seven-element ASCII code (see Figure 10.1) configured for start-stop operation with a stop element which is of 2 bits duration. This is illustrated in Figure 10.6. In front of a character an element called a start space is inserted, and a stop mark is inserted at the end of a character. In the figure the first character is the ASCII letter upper case U (1010101). Here the receiving device knows (a priori) that it starts its timing 1 element (in this case a bit) after the mark-to-space transition- counts out 8 unit intervals (bits) and looks for the stop-mark to end its counting. This is a transition from space-to-mark. The stop-mark, in this case, is two unit intervals long. It is followed by the mark-to-space transition of the next start space, whence it starts counting unit intervals up to 8. So as not to get confused, the first seven information bits are the ASCII bits, and the eighth bit is a parity bit. Even parity is the convention here. Start Start space space Timing Parity New timing starts starts Figure 10.6 An 8-unit start-stop bit stream with a 2-unit stop element. 5 A 5-bit code. The unit and bit are synonymous in this text. A code element carries out a function. It may be one or more bits in duration. 10.7 BINARY TRANSMISSION AND THE CONCEPT OF TIME In such an operation timing error only accumulates inside a character. Suppose the