Information Technology Reference
In-Depth Information
inserting it into the shift register at the correct bit position.
A single parity bit can only detect an odd number of errors, that is, 1, 3, 5, and so on. If
there is an even number of bits in error then the parity bit will be correct and no error will be
detected. This type of error coding is not normally used on its own where there is the possi-
bility of several bits being in error.
Baud rate
One of the main parameters, which specify RS-232 communications, is the rate of transmis-
sion at which data is transmitted and received. It is important that the transmitter and receiver
operate at, roughly, the same speed.
For asynchronous transmission the start and stop bits are added in addition to the 7 ASCII
character bits and the parity. Thus a total of 10 bits are required to transmit a single charac-
ter. With 2 stop bits, a total of 11 bits are required. If 10 characters are sent every second and
if 11 bits are used for each character, then the transmission rate is 110 bits per second (bps).
Table 13.2 lists how the bit rate relates to the characters sent per second (assuming 10 trans-
mitted bits per character). The bit rate is measured in bits per second (bps).
Bits
ASCII character
7
Start bit
1
Stop bit
2
Total
10
Table 13.2
Bits per second related to characters sent per second
Speed (bps)
Characters per second
300
30
1200
120
2400
240
In addition to the bit rate, another term used to describe the transmission speed is the
baud rate. The bit rate refers to the actual rate at which bits are transmitted, whereas the baud
rate relates to the rate at which signalling elements, used to represent bits, are transmitted. As
one signalling element encodes one bit, the two rates are then identical. Only in modems
does the bit rate differ from the baud rate.
Bit stream timings
Asynchronous communications is a stop/start mode of communication and both the transmit-
ter and receiver must be set up with the same bit timings. A start bit identifies the start of
transmission and is always a low logic level. Next, the least significant bit is sent followed
by the rest of the bits in the character. After this, the parity bit is sent followed by the stop
bit(s). The actual timing of each bit relates to the baud rate and can be found using the fol-
lowing formula:
1
Time
period
of
each
bit
=
second
baud
rate
