Page 860 - Mechanical Engineers' Handbook (Volume 2)

5 Transfer of Digital Data 851

Data flow

Microcontroller
Digital system

0 1 1 0 1 1 0
Figure 35 Serial transfer of digital data.

Asynchronous Serial Data Transfer
In asynchronous serial communication the data are written to the serial line at a predefined
rate. Both the transmitter and receiver must be set for identical timing. This timing rate is
called bits per second (bps) or baud rate (e.g., 4800 baud or 9600 baud). The advantage of
asynchronous serial communication is that it only requires one wire (and ground) to com-
municate the data. This method is used in RS-232 (com ports on PCs) communication. The
microprocessor detects the first ‘‘edge’’ of the first data bit and then (because of the baud
rate) it knows how long to wait for the next bit. The baud rate is not exactly the rate the
data will be transferred because there are some overhead bits required for the transfer (e.g.,
parity). See Fig. 36.

Synchronous Serial Data Transfer
In synchronous serial communication the data are written to the serial line and a separate
line is used as a clock, or signal, to indicate the data are ready to be transferred. In this case
the rate of data transfer is controlled by the digital device that provides the clock. The
advantage is that the rate of transfer can be directly controlled and the transmitter and receiver
do not require precise coordination. The disadvantage is that an extra line (clock) is required.
See Fig. 37.

Data flow One byte of data

Digital

0 1 2 3 4 5 6 7
(LSB) (MSB)
Figure 36 Asynchronous serial data transfer.

Clock
12-bit regular shift
11 10 9 8 7 6 5 4 3 2 1 0 P1

P0

Figure 37 Synchronous serial data transfer.