RS-232 protocol is introduced by EIA (Electronics Industries Association) in 1969 for serial communication. Protocol is nothing but just simple set of rules decided to use while communicating.
- DTE and DCE
DTE stands for Data Terminal Equipment. A computer is a DTE. DCE stands for Data Communication Equipment. A modems, microcontrollers,etc are a DCE. DTE normally comes with a Male Connector, while DCE comes with a Female Connector. However, that is not always true. Use the simple way below to confirm: Measure Pin 3 and Pin 5 of a DB-9 Connector with a Volt Meter, if you get a voltage of -3V to -15V, then it is a DTE device. If the voltage is on Pin 2, then it is a DCE device.
Note: The result for a DB-25 Connector is reversed (Please refer to DB-9 to DB-25 conversion table below).
- RS-232 Pin outs (DB-9)
A male DB-9 connector viewed from the front. Reverse or back view of male connector for Female Connector.
|
DTE Pin Assignment (DB-9) |
DCE Pin Assignment (DB-9) | ||||
|
1 |
DCD |
Data Carrier Detect |
1 |
DCD |
Data Carrier Detect |
|
2 |
RxD |
Receive Data |
2 |
TxD |
Transmit Data |
|
3 |
TxD |
Transmit Data |
3 |
RxD |
Receive Data |
|
4 |
DTR |
Data Terminal Ready |
4 |
DSR |
Data Set Ready |
|
5 |
GND |
Ground (Signal) |
5 |
GND |
Ground (Signal) |
|
6 |
DSR |
Data Set Ready |
6 |
DTR |
Data Terminal Ready |
|
7 |
RTS |
Request to Send |
7 |
CTS |
Clear to Send |
|
8 |
CTS |
Clear to Send |
8 |
RTS |
Request to Send |
|
9 |
RI |
Ring Indicator |
9 |
RI |
Ring Indicator |
DB-9 to DB-25 Conversion
|
DB-9 |
DB-25 |
Function |
|
|
1 |
8 |
DCD |
Data Carrier Detect |
|
2 |
3 |
RxD |
Receive Data |
|
3 |
2 |
TxD |
Transmit Data |
|
4 |
20 |
DTR |
Data Terminal Ready |
|
5 |
7 |
GND |
Ground (Signal) |
|
6 |
6 |
DSR |
Data Set Ready |
|
7 |
4 |
RTS |
Request to Send |
|
8 |
5 |
CTS |
Clear to Send |
|
9 |
22 |
RI |
Ring Indicator |
- RS-232 Signals
The graphic above illustrates a typical RS-232 logic waveform (Data format: 1 Start bit, 8 Data bits, No Parity, 1 Stop bit). The data transmission starts with a Start bit, followed by the data bits (LSB sent first and MSB sent last), and ends with a “Stop” bit.
The voltage of Logic “1” (Mark) is between -3VDC to -15VDC, while the Logic “0” (Space) is between +3VDC to +15VDC.
-
Setting the baud rate
- Baud rate is generally measured in bit per second and this is used in modem terminology. Baud rate is nothing but signal state change per second.
- For faithfull communication between computer and microcontroller, we need to synchronize the baud rate of both devices.
- For round figure baud rate and neat communication, we have to choose frequency of oscillator according to following formula.
- This is all about RS-232 protocol. Next tab circuit diagram shows how to connect your personal computer with microcontroller with line driver IC.