the Morse code will recall that the spaces between dots and dashes told the operator where letters ended and where words ended. The sending device or transmitter delivers a continuous series of characters to the line, each consisting of five, six, seven, eight, or nine elements (bits) per character. A receiving device starts its print cycle when the transmitter starts sending and, if perfectly in step with the transmitter, can be expected to provide good printed copy and few, if any, errors at the receiving end. It is obvious that when signals are generated by one machine and received by another, the speed of the receiving machine must be the same or very close to that of the transmit- ting machine. When the receiver is a motor-driven device, timing stability and accuracy are dependent on the accuracy and stability of the speed of rotation of the motors used. Most simple data-telegraph receivers sample at the presumed center of the signal element. It follows, therefore, that whenever a receiving device accumulates timing error of more than 50% of the period of one bit, it will print in error. DATA COMMUNICATIONS Character 1 Character 2 Character 3 11234523423 Character 1 Character 2 Character 3 Figure 10.5 Five-unit synchronous bit stream with timing error. The need for some sort of synchronization is illustrated in Figure 10.5. A five-unit code is employed, and it shows three characters transmitted sequentially. 5 The vertical arrows are receiver sampling points, which are points in time. Receiving timing begins when the first pulse is received. If there is a 5% timing difference between the transmitter and receiver, the first sampling at the receiver will be 5% away from the center of the transmitted pulse. At the end of the tenth pulse or signal element, the receiver may sample in error. Here we mean that timing error accumulates at 5% per received signal element and when there is a 50% accumulated error, the sampling will now be done at an incorrect bit position. The eleventh signal element will indeed be sam