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.
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 sampled 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).
Asynchronous and Synchronous Transmission
In the earlier days of printing telegraphy, "startstop" 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
startstop 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--it 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.
An 8-unit start-stop bit stream with a 2-unit stop element.
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.