How are characters transmitted?
A digital computer only recognizes the states yes and
no or 0 and 1. They are represented e.g. by 0 V and
+5 V in a memory cell (bit).
Eight bits are combined into one byte. 256 different
combinations of 0 and 1 can be generated with one
byte.
An international standard (ASCII code) defines which
character corresponds to which bit combination. An A
is represented, for example, as 01000001 or a 5 as
00110101.
Parallel interface
The eight bits of a byte can be transmitted simulta-
neously on parallel lines. This is carried out e.g. be-
tween the PC and printer via the Centronics interface.
Parallel interfaces are fast, but require expensive cables
with at least nine cores and often only permit short
cable lengths.
Serial interface
If the eight bits are sent in succession (i.e. serially) on a
line, the 0s and 1s arrive at the other end in succession.
The receiver combines them into a byte again. Low-
cost 2-core cables are sufficient for this type of trans-
mission.
An understandable example of a serial transmission is
the Morse code. In this case the letters are coded as a
sequence of long and short signals.
Transmission rate
The receiver must know exactly how fast the 0s and 1s
follow in succession. Otherwise it cannot recognize e.g.
if two 0s arrive immediately in succession. The trans-
mission only functions if the transmitter and receiver
are set to the same transmission rate (baud rate). This is
specified in bit/s or baud.
If the train is too fast for the conveyer belt, the wagons
remain empty. If it is too slow, several digits fall into
one wagon or fall under the wheels!
01010011
AUS
A
01000001
U
01010101
01010011
AUS
1
010
A
01000001
1
1
0
0
Networking
2VERNETZ.CHP, Stand Dezember 22, 1999
21
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