Serial Communication
EIA RS232, RS422 and RS485
Recommended Standard 232
The purpose of this course note is to attempt to describe the main elements in Serial
Communication. This application note attempts to cover enough technical details of
RS232, RS422 and RS485.
EIA - Electroonic Industries Association
D-subminiature
•
The D-subminiature or D-sub is
a common type of electrical
connector. They are named for
their characteristic D-shaped
metal shield.
•
A, B, C, D, or E denoting the shell
size, followed by the number of
pins or sockets, followed by
either P (plug) – M(Male)
or S (socket) – F (Female)
RS232 is an asynchronous or synchronous serial
communications protocol
•
Personal computers first used DB25
connectors for their serial and parallel ports
EIA232 communication function and connector types for a personal computer and modem. DCE devices are sometimes called
"Data Communications Equipment" instead of Data Circuit-terminating Equipment.
RS-232C
RS-232 stands for Recommend Standard number 232 and C is the latest revision of the standard. The serial ports on most computers use a subset of the RS-232C
standard. The full RS-232C standard specifies a 25-pin "D" connector of which 22 pins are used. Most of these pins are not needed for normal PC communications, and
indeed, most new PCs are equipped with male D type connectors having only 9 pins.

RS232 on RJ45 (RS-232D)
MODEM COMMUNICATION BETWEEN TWO PC’S
RS232 DB9 pinout RS 232 Voltage levels
SIGNAL
VOLTAGE LEVELS
VOLTS
LOGICAL STATE
-3 to -25 1
+3 to +25 0
CONTROL
VOLTAGE LEVELS
VOLTS
LOGICAL STATE
-3 to -25 OFF
+3 to +25 ON
- The signal ground, ie. the reference level for all signals.
- The protective ground : Also known as Shield Ground. This is the chassis
ground connection between DTE and DCE. It should not be used as a signal
ground, and it must not be connected to signal ground. Connecting
protective ground on both sides makes sure that no large currents flow
through Signal ground in case of an insulation defect on one side
DB9 Null Modem Cable
Computer Computer
The Carrier Detect (CD) and the Ring
Indicator (RI) lines are only available in
connections to a modem. Because most
modems transmit status information to
a PC when either a carrier signal is
detected (i.e. when a connection is
made to another modem) or when the

Asynchronous (RS-232) serial link
universal asynchronous receiver/transmitter or UART
Electrical characteristics
Charge Pump Voltage Doubler
Charge Pump Voltage Inverter
By Reversing the position of the Vout and Vin, the regulator becomes fractional gain (1/2).




Example
The signal level inversion (logic 1 is -12V and
logic 0 is +12V)
0

The first step to connecting a device to the RS-232 port is to transform the RS-232 levels into 0
and 5 Volts, AND, this is done by RS-232 Level Converters.
The MAX-232. It includes a Charge Pump, which generates +10V and -10V from a single 5v
supply. This I.C. also includes two receivers and two transmitters in the same package
RS-232 Level Converters
RS-232 Level Converters