Digital transmission has very significant advantages compared with analogue transmission
because the possibility of signal corruption during transmission is greatly
reduced. Many different protocols exist for digital signal transmission, and these are
considered in detail in Chapter 10. However, the protocol that is normally used for
the transmission of data from a measurement sensor or circuit is asynchronous serial
transmission, with other forms of transmission being reserved for use in instrumentation
and computer networks. Asynchronous transmission involves converting an
analogue voltage signal into a binary equivalent, using an analogue-to-digital converter
as discussed in section 6.4.3. This is then transmitted as a sequence of voltage pulses
of equal width that represent binary ‘1’ and ‘0’ digits. Commonly, a voltage level of
C6V is used to represent binary ‘1’ and zero volts represents binary ‘0’. Thus, the
transmitted signal takes the form of a sequence of 6V pulses separated by zero volt
pulses. This is often known by the name of pulse code modulation. Such transmission
in digital format provides very high immunity to noise because noise is typically much
smaller than the amplitude of a pulse representing binary 1. At the receiving end of
a transmitted signal, any pulse level between 0 and 3 volts can be interpreted as a
binary ‘0’ and anything greater than 3V can be interpreted as a binary ‘1’. A further
advantage of digital transmission is that other information, such as about plant status,
164 Signal transmission
Digital data Telephone Digital data
Fig. 8.10 Telephone transmission.
can be conveyed as well as parameter values. However, consideration must be given
to the potential problems of aliasing and quantization, as discussed in section 6.4.3,
and the sampling frequency must therefore be chosen carefully.
Many different mediums can be used to transmit digital signals. Electrical cable,
in the form of a twisted pair or coaxial cable, is commonly used as the transmission
path. However, in some industrial environments, the noise levels are so high that even
digital data becomes corrupted when transmitted as electrical pulses. In such cases,
alternative transmission mechanisms have to be used.
One alternative is to modulate the pulses onto a high-frequency carrier, with positive
and zero pulses being represented as two distinct frequencies either side of a centre
carrier frequency. Once in such a frequency modulated format, a normal mains electricity
supply cable operating at mains frequency is often used to carry the data signal.
The large frequency difference between the signal carrier and the mains frequency
prevents any corruption of the data transmitted, and simple filtering and demodulation
is able to extract the measurement signal after transmission. The public switched
telephone network can also be used to transmit frequency modulated data at speeds
up to 1200 bits/s, using acoustic couplers as shown in Figure 8.10. The transmitting
coupler converts each binary ‘1’ into a tone at 1.4 kHz and each binary ‘0’ into a tone
at 2.1 kHz, whilst the receiving coupler converts the tones back into binary digits.
Another solution is to apply the signal to a digital-to-current converter unit and then
use current loop transmission, with 4mA representing binary ‘0’ and 20mA representing
binary ‘1’. This permits baud rates up to 9600 bit/s at transmission distances
up to 3 km. Fibre-optic links and radio telemetry are also widely used to transmit digital