December 2001 Introduction 8 – 5
For parallel data transmission, the interface needs line drivers, e.g. a 36-pin
ribbon cable, instead of the USART illustrated above.
Advantages of serial data transmission:
n Economical
n Ideal for covering long distances
Disadvantages of serial data transmission:
n Slow
Advantages of parallel data transmission:
n Fast
Disadvantages of parallel data transmission:
n Somewhat more expensive
Data coding A common code for data transfer is the ASCII code (American Standard Code
for Information Interchange), which codes all characters with seven bits. This
means that, in all, 2
7
= 128 characters are coded.
Example:
The letter “z” is represented by the following combination of bits:
1 1 1 1 0 1 0 = 122 dec = 7A hex
When the letter “z” is transmitted via a serial interface, the appropriate
characters are sent one after the other.
0
1
1
0
1
0
1
1
0
1
1
0
1
0
1
1
LSB
MSB
0
1
1
0
1
0
1
1
0
1
1
0
1
0
1
1
LSB
MSB
0
1
1
0
1
0
1
1
Transmission length
Memory
Interface
buffer
Transmitter Receiver
Interface
buffer
Memory
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