448 Chapter 15
misbehavior or errors and it is part of the skill of the design and layout of such circuitry
that this risk is minimized.
Logic elements made using CMOS technologies have better input noise margins because
the threshold of a CMOS gate is approximately equal to half of the supply voltage.
Thus, after considering the inevitable spread of production variation and the effects
of temperature, the available input range for a logic low (or 0) lies in the range 0 to
1.5 V and for a logic high (or 1) in the range of 3.5 to 5.0 V (assuming a 5.0-V supply).
However, the output impedance of CMOS gates is at least three times higher than that for
simple TTL gates and thus in a 5.0-V supply system interconnections in CMOS systems
are more susceptible to reactively coupled noise. CMOS systems produce their full
benefi t of high noise margin when they are operated at higher voltages but this is
not possible for CMOS technologies intended to be compatible with 74 00 logic
families.
15.5 Transmitting Digital Signals .................................................................................
There are two ways in which you can transport bytes of information from one circuit or
piece of equipment to another. Parallel transmission requires a signal line for each bit
position and at least one further signal that will indicate that the byte now present on the
signal lines is valid and should be accepted. Serial transmission requires that the byte
be transmitted one bit at a time and in order that the receiving logic or equipment can
recognize the correct beginning of each byte of information it is necessary to incorporate
some form of signaling in serial data in order to indicate (as a minimum) the start of each
byte. Figure 15.6 shows an example of each type.
Parallel transmission has the advantage that, where it is possible to use a number of
parallel wires, the rate at which data can be sent can be very high. However, it is not
easy to maintain a very high data rate on long cables using this approach and its use for
digital audio is usually restricted to the internals of equipment and for external use as an
interface to peripheral devices attached to a computer.
The serial link carries its own timing with it and thus it is free from errors due to
skew and it clearly has benefi ts when the transmission medium is not copper wire but
infra-red or radio. It also uses a much simpler single circuit cable and a much simpler
connector. However, the data rate will be roughly 10 times that for a single line of a