|Serial communication is basically the transmission or reception of
data one bit at a time. Today's computers generally address data in
bytes or some multiple thereof. A byte contains 8 bits. A bit is
basically either a logical 1 or zero. Every character on this page is
actually expressed internally as one byte. The serial port is used to
convert each byte to a stream of ones and zeroes as well as to convert a
streams of ones and zeroes to bytes. The serial port contains a
electronic chip called a Universal Asynchronous Receiver/Transmitter
(UART) that actually does the conversion.
The serial port has many
pins. We will discuss the transmit and receive pin first. Electrically
speaking, whenever the serial port sends a logical one (1) a negative
voltage is effected on the transmit pin. Whenever the serial port sends
a logical zero (0) a positive voltage is effected. When no data is being
sent, the serial port's transmit pin's voltage is negative (1) and is
said to be in a MARK state. Note that the serial port can also be
forced to keep the transmit pin at a positive voltage (0) and is said to
be the SPACE or BREAK state. (The terms MARK and
SPACE are also used to simply denote a negative voltage (1) or a
positive voltage(0) at the transmit pin respectively).
When transmitting a byte, the UART (serial port) first sends a
START BIT which is a positive voltage (0), followed by the data
(general 8 bits, but could be 5, 6, 7, or 8 bits) followed by one or two
STOP BITs which is a negative(1) voltage. The sequence is repeated
for each byte sent. Figure 1 shows a diagram of a what a byte
transmission would look like.
At this point you may want to know what is the duration of a bit. In
other words, how long does the signal stay in a particular state to
define a bit. The answer is simple. It is dependent on the baud rate.
The baud rate is the number of times the signal can switch states in one
second. Therefore, if the line is operating at 9600 baud, the line can
switch states 9,600 times per second. This means each bit has the
duration of 1/9600 of a second or about 100
When transmitting a character there are other characteristics other than
the baud rate that must be known or that must be setup. These
characteristics define the entire interpretation of the data stream.
The first characteristic is the length of the byte that will be
transmitted. This length in general can be anywhere from 5 to 8 bits.
The second characteristic is parity. The parity characteristic can be
even, odd, mark, space, or none. If even parity, then the last data bit
transmitted will be a logical 1 if the data transmitted had an even
amount of 0 bits. If odd parity, then the last data bit transmitted will
be a logical 1 if the data transmitted had an odd amount of 0 bits. If
MARK parity, then the last transmitted data bit will always be a
logical 1. If SPACE parity, then the last transmitted data bit
will always be a logical 0. If no parity then there is no parity bit
The third characteristic is the amount of stop bits. This value in
general is 1 or 2.
Assume we want to send the letter 'A' over the serial port. The
binary representation of the letter 'A' is 01000001. Remembering that
bits are transmitted from least significant bit (LSB) to most
significant bit (MSB), the bit stream transmitted would be as follows
for the line characteristics 8 bits, no parity, 1 stop bit, 9600 baud.
LSB (0 1 0 0 0 0 0 1 0 1) MSB
The above represents (Start Bit) (Data Bits) (Stop Bit)
To calculate the actual byte transfer rate simply divide the baud rate
by the number of bits that must be transferred for each byte of data. In
the case of the above example, each character requires 10 bits to be
transmitted for each character. As such, at 9600 baud, up to 960 bytes
can be transferred in one second.
The above discussion was concerned with the
"electrical/logical" characteristics of the data stream. We will expand
the discussion to line protocol.
Serial communication can be half duplex or full duplex. Full duplex
communication means that a device can receive and transmit data at the
same time. Half duplex means that the device cannot send and receive at
the same time. It can do them both, but not at the same time. Half
duplex communication is all but outdated except for a very small focused
set of applications.
Half duplex serial communication needs at a minimum two
wires, signal ground and the data line. Full duplex serial communication
needs at a minimum three wires, signal ground, transmit data line, and
receive data line. The RS232 specification governs the physical and
electrical characteristics of serial communications. This specification
defines several additional signals that are asserted (set to logical 1)
for information and control beyond the data signals and signal ground.
These signals are the Carrier Detect Signal (CD), asserted by modems to
signal a successful connection to another modem, Ring Indicator (RI),
asserted by modems to signal the phone ringing, Data Set Ready (DSR),
asserted by modems to show their presence, Clear To Send (CTS), asserted
by modems if they can receive data, Data Terminal Ready (DTR), asserted
by terminals to show their presence, Request To Send (
RTS), asserted by terminals if they can receive data. The section RS232
Cabling describes these signals and how they are connected.
The above paragraph eluded to hardware flow control. Hardware flow
control is a method that two connected devices use to tell each other
electronically when to send or when not to send data. A modem in general
drops (logical 0) its CTS line when it can no longer receive characters.
It re-asserts it when it can receive again. A terminal does the same
thing instead with the RTS signal. Another method of hardware flow
control in practice is to perform the same procedure in the previous
paragraph except that the DSR and DTR signals are used for the
Note that hardware flow control requires the use of additional wires.
The benefit to this however is crisp and reliable flow control. Another
method of flow control used is known as software flow control. This
method requires a simple 3 wire serial communication link, transmit
data, receive data, and signal ground. If using this method, when a
device can no longer receive, it will transmit a character that the two
devices agreed on. This character is known as the XOFF character. This
character is generally a hexadecimal 13. When a device can receive again
it transmits an XON character that both devices agreed to. This
character is generally a hexadecimal 11.
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