28119 Parallax Inc, 28119 Datasheet - Page 81

• Manufacturer Part Number: 28119
• Description: KIT UNDERSTANDING SIGNALS
• Manufacturer: Parallax Inc

Specifications of 28119

• Lead Free Status: Contains lead
• Accessory Type: Oscilloscope
• Interface Type: USB
• For Use With/related Products: BASIC Stamp® 2 and Board of Education
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Chapter 6: Asynchronous Serial Communication
The word asynchronous means “without a clock.” Asynchronous serial communication
only uses one line, the data line, to communicate. The most commonly known type of
communication that uses asynchronous communication is the RS-232 serial port on your
computer. With RS-232 serial communication, the data line is used for synchronization
and data. A consequence of this is that the receiver must always be watching the data line
for a “start bit.”
When there is no data sent, the data line is said to be in its idle state. When the receiver
sees that the data line has transitioned, it knows it’s time to initialize its timers and start
receiving data. To send and receive data, both the transmitter and the receiver must set
and read the data line at a very precise rate. This rate is called the baud rate. The baud
rate must be the same in both the sender and the receiver before communication begins.
The start bit lasts for one bit-time. After the start bit, each bit is sent, and each bit lasts
one bit-time. If the bit to send is a 0, the data line is set high. If the bit to send is a 1, the
data line is set low. See (Figure 6-1).
As you can see from the illustration above, the data bits are evenly spaced. Notice that
the time from the rising edge of the start bit to the middle of data bit 0 is 1.5 times the bit
time. When the receiver detects a start bit it will wait for 1.5 bit-times before reading in
data bit 0. As a result, the receiver need only wait one bit-time to read the center of each
subsequent bit. Reading the center of a bit is necessary to minimize errors.
Start
Bit
1
BIT 0
0
BIT 1
0
Data Being Transmitted
BIT 2
0
BIT 3
1
BIT 4
0
BIT 5
1
BIT 6
0
BIT 7
Figure 6-1:
Asynchronous
serial
communication
has no clock line