Serial Data Communication

• Component
All data communications systems have the following components:
• The source of the data (e.g. a computer). Also required is circuitry that
converts the signal into one that is compatible with the communications link,
called a transmitter or line driver
• The communications link (twisted-pair cable, coaxial cable, radio, telephone
network etc), which transfers the message to the receiver at the other end
• The receiver of the data where the signal is converted back into a form that
can be used by the local electronics circuitry

• Transmission modes
1. Simplex

2. Duplex
- Half duplex

- Full duplex

BASIC PRINCIPLE
a. Coding
Encoding is the process of converting the message data
into a standard binary code for transfer over the data
communications link. This code assigns a binary field of 7-bits to
represent each character, giving 128 (27) unique characters
made up of:
• Upper and lower case letters, and numerals 1 to 9
• Various punctuation marks and symbols
• A set of control codes (the first 32 characters) that are used by
the communications link itself and are not printable

• Format of data communication

Format of a typical serial asynchronous data message

In summary, the optional settings for asynchronous
transmission of characters are:
• Start bits 1
• Data bits 5, 6, 7, 8
• Parity bits even, odd, mark, space or none
• Stop bits 1, l½ or 2
As there cannot be half a bit, 1½-stop bits means that
the mark length is 50% longer than for one stop bit.

• Data transmission speed
Factor:
• Type and complexity of the circuitry at each
end (interface)
• Communication link (twisted-pair, coaxial cable,
radio etc)
• Distance between the sender and receiver
• Amount of data being transferred
• The overhead associated with the data transfer
• The acceptable rate of error

RS-232-C
• Characteristics
1. Electrical
The RS-232 standard is designed for the connection of
two devices: data terminal equipment (DTE), such as a computer
or printer, and data communications equipment (DCE), such as a
modem.

Connection between DTE and DCE

The electrical connections are divided into the
four groups shown below:
• Data lines
• Control lines
• Timing lines
• Special secondary functions

• Function
• Pin 1: Protective ground (shield)
• Pin 2: Transmitted data (TXD)
• Pin 3: Received data (RXD)
• Pin 4: Request to send (RTS)
• Pin 5: Clear to send (CTS)
• Pin 6: Data set ready (DSR)
• Pin 7: Signal ground (common)
• Pin 8: Data carrier detect (DCD)
• Pin 20: DTE ready (or data terminal ready)
• Pin 22: Ring indicator
• Pin 23: Data signal rate selector (DSRS)

RS-485
• Application

The RS-485 standard was designed for two-wire, half
duplex, balanced multidrop communications, and allows up to
32 line drivers and 32 line receivers on the same line. It
incorporates the advantages of balanced lines with the need
for only two wires (plus signal common) cabling.
RS-485 provides reliable serial communications for:
• Distances of up to 1200 m
• Data rates of up to 10 Mbps
• Up to 32 line drivers permitted on the same line
• Up to 32 line receivers permitted on the same line

• Range
The line voltages range between –1.5 V to –6 V
for logic ‘1’ and +1.5 V to +6 V for logic ‘0’. The
line driver for the RS-485 interface produces a 5
V differential voltage on two wires. For full-
duplex systems, four wires are required. For a
half-duplex system, only two wires are required.

Converter (RS-232 to RS-485)

Typical specifications for an RS-232/RS-485 converter are:
• Data transfer rate up to 1 Mbaud
• DCE/DTE switch selectable
• Converts all data and control signals
• LEDs for status of data and control signals
• Powered from AC source or self-powered from pins 9 & 10 of EIA-232 port
• Optically isolated (optional)
• DB-25 connector (male or female)
• DB-37 connector (male or female)

PROTOCOLS
A protocol is essentially a common set of rules
governing the exchange of data between the transmitter
and receiver of a communications network, and is normally
associated with the packaging of data transmitted on the
communications interface.
Important feature:
• Initialization
• Framing and frame synchronization
• Flow control
• Line control
• Error control
• Timeout control

FLOW CONTROL PROTOCOLS
1. Character flow protocols (XON/XOFF)
This is a popular flow control protocol that has two
characters assigned as XON (start) or XOFF (stop).
Typically, the ASCII characters DC3 (Ctrl-S) and DC1
(Ctrl-Q) are assigned to XOFF and XON respectively.

2. Whole line protocols (ETX/ACK)
The ETX/ACK protocol, designed by IBM, is based on
the transmitter appending an ETX character after
each line of data and waiting for the receiver’s ACK,
requesting the next line of data.

ASCII-based PROTOCOLS
- Protocols Structure

ERROR DETECTION
• Character redundancy checks
• Block redundancy checks
• Cyclic redundancy checkS

TROUBLESHOOTING
• Check the basic parameters
• Identify which is DTE or DCE
• Clarify the needs of the hardware
handshaking
• Check the actual protocol used

TESTING SERIAL DATA
COMMUNICATION CIRCUITS
When a data communication link won’t work, the following five
very useful devices can be used to assist in analyzing the
problem:
• The breakout box

• Protocol analyzer
A protocol analyzer is used to display the actual
bits on the data line, as well as the special control
codes, such as X-ON, X-OFF, LF, CR, etc.
This helps to confirm that the transmitting
terminal is sending the correct data and that the
receiving device is receiving it. The protocol analyzer is
useful in identifying incorrect setting of baud rate,
parity, stop bit, noise or incorrect wiring and
connection.

Serial Data Communication

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