Audio Engineering

450 Chapter 15

Start

bit

Bit calls

Data

Hex

ASCII

00

63

hi

61

010110 10010110

Data

bits

Parity

bit

Stop

bit

Start

bit

Data

bits

Parity Stop

(b)

Figure 15.6(b) : Serial transmission requires the sender and receiver to use and recognize the

same signal format or protocol, such as RS232. For each byte, the composite signal contains

a start bit, a parity bit, and a stop bit using inverted logic (1   12 V; 0   12 V). The

time interval between each bit of the signal (the start bit, parity bit, stop bit, and data bits)

is fi xed and must be kept constant.

You should refer to a text on the use of transmission lines for a full discussion of this
point but for guidance here is a simple way of determining whether you will benefi t by
considering the transmission path as a transmission line.

● Look up the logic signal rise time, tR.

● Determine the propagation velocity in the chosen cable, v. This will be typically

about 0.6 of the speed of light.

● Determine the length of the signal path, l.

● Calculate the propagation delay, τ  l/v.

● Calculate the ratio of tR / τ.

● If the ratio is greater than 8, then the signal path can be considered electrically

short and you will need to consider the signal path’s inductance or capacitance,

whichever is dominant.

● If the ratio is less than 8, then consider the signal path in terms of a transmission line.

The speed at which logic transitions take place determines the maximum rate at which
information can be handled by a logic system. The rise and fall times of a logic signal are