RS232

What is RS232 Protocol?

In RS232, ‘RS’ stands for Recommended Standard. It defines the serial communication using DTE and DCE signals. Here, DTE refers to Data Terminal Equipment and DCE refers to the Data Communication Equipment. Example of DTE device is a computer and DCE is a modem. Formally, it is specified as the interface between DTE equipment and DCE equipment using serial binary data exchange.

RS232 describes the common voltage levels, electrical standards, operation mode and number of bits to be transferred from DTE to DCE. This standard is used for transmission of information exchange over the telephone lines.

Electrical Standards

The electrical specifications for RS232 are updated in the year 1969. It specifies the electrical voltages, slew rate, line impedance, operation mode, and baud rate.

Voltage Levels

The line voltages of RS232 range from -25V to +25V. They are categorized as signal voltage and control voltage.

RS232 Voltage Levels

The signal voltage between +3V to +25V represents logic ‘1’ and the signal voltages between -3V to -25V represents logic ‘0’. Whereas the control voltage signals use negative logic, i.e., logic ‘1’ indicates -3 to -25 volts and logic ‘0’ indicates +3V to +25V. The voltage from -3V to +3V is considered as an indeterminate state

ps:

控制电压:

控制电压则是指在控制线上（如RTS、CTS、DSR、DTR和DCD等）传输的电压，用于控制通信过程中的各种状态和行为。在RS232标准中，控制电压的有效范围通常也是+3V至+15V之间，表示信号的有效状态（ON或接通）。然而，与信号电压不同，控制电压的具体值可能因不同的控制信号和用途而有所差异。

Baud rate:(...)

Communication Interface

RS232 determines the communication between the DTE and DCE using DB9 and DB25

connectors. The D-sub connectors (DB9, DB25) comes with male and female cable. DB9 connector has 9 pins and DB25 connector has 25 pins with each pin having its own function.

DB9 Male and Female Pinouts

DB25 Pinout

Functional Description

Apart from the electrical characteristics, RS232 defined the functions of signals that are used in the serial interface. Some of them are common ground, Data, control and timing signals. Here is a list of signals used in RS232 pinout.

Signal Name	Function
Protective Ground	This signal is connected to chassis ground of metallic connector.
Common Ground	Zero reference voltage level for all the control signals.
TxD (Transmit Pin)	To transmit data from DTE to DCE.
RxD (Receive Pin)	Sends data from DCE to DTE.
DTR (Data Terminal Ready)	DTE is ready to accept request.
DCD (Data carrier Detect)	DCE accepts a carrier from a DTE located at remote location.
DSR (Data Set Ready)	DCE is prepared to send and receive the information.
RI (Ring Indicator)	Detects the incoming ring tone on the telephone line.
RTS (Request to Send)	DTE call for DCE to send the data.
RTR (Ready to Receive)	DTE is geared up to receive data coming from DCE.
CTS (Clear To Send)	DCE is in a ready state to accept data coming from DTE.

Other than above signals, (primary signals) RS232 provides secondary signals such as secondary DTE, secondary RTS, secondary DCD, secondary TxD and secondary RxD for optional connection of DTE and DCE.

cable:

Protocol Format :

other:

Why to use Handshaking?

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Difference between RS232 and UART

The main difference between RS232 and UART protocol is the voltage levels. Apart from this, they both supports half duplex and full duplex communication.

Microcontrollers don’t tolerate RS232 voltages and may be damaged. To avoid this, UART (Universal Asynchronous Transmitter Receiver) is used. It sends and receives the data in serial form. To do the level conversion of voltages, RS232 driver IC such as MAX232 is used between the UART and serial port.