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RS232

Page history last edited by wilbo666 4 years, 8 months ago


 

Introduction

This page provides information in regards to the RS232 standard often used in serial communications.

 

Signal Levels

The following applies to all RS232 signals, TxD, Rxd, CTS, RTS, etc.

 

Logic 1 = -3V to -15V

Logic 0 = +3V to +15V

Undefined = -3V to 3V

 

 

Idle State

The idle state of a RS232 Transmit pin (TxD) is a logic 1 (-3V to -15V).

 

RS232 DTE & DCE

DTE = Data Terminal Equipment, i.e. PC.  DTEs should be fitted with Male connectors.

DCE = Data Communications Equipment, i.e. MODEM.  DCEs should be fitted with Female connectors.

 

 

RS232 Serial Port Pinouts

The most common RS232 serial connectors are DB9 and DB25.

 

DB9 Pin
DB25 Pin  Acronym Full name Direction (DTE) Meaning
1
8
DCD
Data Carrier Detect
DTE <-- DCE
Modem connected to another
2
3
RxD
Receive Data
DTE <-- DCE
Receives bytes into PC (DCE to DTE)
3
2
TxD
Transmit Data
DTE --> DCE
Transmits bytes out of PC (DTE to DCE)
4
20
DTR
Data Terminal Ready
DTE --> DCE
I'm ready to communicate (DTE)
5
7
SG
Signal Ground                                   
6
6
DSR
Data Set Ready
DTE <-- DCE
I'm ready to communicate (DCE)
7
4
RTS
Request To Send
DTE --> DCE
RTS/CTS flow control (DTE to DCE)
8
5
CTS
Clear To Send
DTE <-- DCE
RTS/CTS flow control (DCE to DTE)
9
22
RI
Ring Indicator
DTE <-- DCE
Telephone line ringing

 

RS232 Serial Cables

The following have been copied from http://www.bb-elec.com/Learning-Center/Serial-Connectivity.aspx

 

DTE - DCE (Standard Cable)

 

DCE - DCE or DTE - DTE (Null Modem Cable)

RS232 Listener Cable

A simple listener can be made for half duplex systems (NOTE: This listener cable will NOT work on full duplex systems).

 

The below circuit relies on pull down resistors inside the listener RS232 port.  If this is not the case then large value resistors (e.g. 4k7Ω) can be fitted in parallel with the diodes.

 

Simple listener that uses two serial ports, one to receive data transmitted from the PC and one to receive data transmitted from the device.

 

References

http://en.wikipedia.org/wiki/RS-232

http://en.wikipedia.org/wiki/Serial_port

http://www.aggsoft.com/rs232-pinout-cable/pinout-and-signal.htm

http://www.bb-elec.com/Learning-Center/Serial-Connectivity.aspx

 

 

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