Information Technology Reference
In-Depth Information
For example, if the baud rate is 9600 baud (or bps) then the time period for each bit sent is
1/9600 s or 104
s. Table 13.3 shows some bit timings as related to baud rate. An example of
the voltage levels and timings for the ASCII character 'V' is given in Figure 13.6.
µ
Table 13.3
Bit timings related to baud rate
Baud rate
Time for each bit (
µ
s)
1 200
833
2 400
417
9 600
104
19 200
52
+12 V
Stop
bit
Start
bit
0
1
1
0
1
0
1
-12 V
ASCII 'V' 101 0110
Baud rate: 9600 baud
S
120
µ
Figure 13.6
ASCII 'V' at RS-232 voltage levels
13.3 Communications between two nodes
RS-232 is intended to be a standard but not all manufacturers abide by it. Some implement
the full specification while others implement just a partial specification. This is mainly be-
cause not every device requires the full functionality of RS-232, for example a modem re-
quires many more control lines than a serial mouse.
The rate at which data is transmitted and the speed at which the transmitter and receiver
can transmit/receive the data dictates whether data handshaking is required.
13.3.1 Handshaking
In the transmission of data, there can either be no handshaking, hardware handshaking or
software handshaking. If no handshaking is used then the receiver must be able to read the
received characters before the transmitter sends another. The receiver may buffer the re-
ceived character and store it in a special memory location before it is read. This memory lo-
cation is named the receiver buffer. Typically, it may only hold a single character. If it is not
emptied before another character is received then any character previously in the buffer will
be overwritten. An example of this is illustrated in Figure 13.6. In this case, the receiver has
read the first two characters successfully from the receiver buffer, but it did not read the third
character as the fourth transmitted character has overwritten it in the receiver buffer. If this
condition occurs then some form of handshaking must be used to stop the transmitter sending
characters before the receiver has had time to service the received characters.
Hardware handshaking involves the transmitter asking the receiver if it is ready to receive
