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RS232 Information

Floyd Kling 

(8/17/09)

 

RS232 interface is arguably the most well known connector on a personal computer.  Mostly used as the interface to modems, it is also used for many other serial connections such has mouse and printer ports.  Well known as "Comm" (Communications) or "Serial" port.

 

While EIA RS232 defines the electrical and signal definitions, internationally the ITU-T (CCITT) equivalent are V.24 for the Signal definitions and V.28 for the electrical characteristics.  RS232 and V.24/V.28 are virtually identical.

 

The Personal computer RS232 ports are mostly Asynchronous.    It is very rare to find a PC that uses the Serial port for synchronous operation.  Special cards and firmware are needed for synchronous operation.  This paper will indicate full pin definitions for both Asynchronous and Synchronous operation, but will focus on Asynchronous operation for the signal definitions.

 

(FYI – Your internet connections are "Asynchronous" i.e. 8 data bits, one start, one stop = 10 bits per character)

 

Electrical Characteristics:

 

Receiving the signal - viewed as the receiver/input

·          DC resistance:  Must be between 3 to 7Kohms

·          Open -circuit voltage: Less than 2 volts.

·          Signals greater than +3VDC are considered the binary "1" (on) condition

·          Signals less than -3VDC are considered the binary "0" (off) condition

·          Signals of -3VDC to +3VDC are consider in transition and not considered a valid level for determining a state.

           

Generating the signal - viewed as the driver/output

·          Not to exceed +/-25VDC (no load)

·          Open-circuit must be greater than +/-5volts(no load)

·          Must be between +/-5 to15VDC with a load of 3 to 7Kohms.

·          Signals greater than +5VDC are considered the binary "1" (on) condition

·          Signals less than -5VDC are considered the binary "0" (off) condition

·          Signals range of -5VDC to +5VDC is consider in transition and not considered a valid level for determining a state.

 

Physical:


The DTE connector (On your PC) utilizes a 25pin (DB25) Male connector and the DCE connector (On the modem) utilizes a Female 25pin (DB25) connector.  Many PC's use DE9 (9pin) connectors  (Yes DE-9.  DE-9 connectors have commonly albeit incorrectly, referred to as DB-9's)

 

Noteworthy site on connectors and nomenclature: 

http://www.interfacebus.com/Connetor_D-Sub_Manufacturers.html

Signal direction is viewed as the modem sees the signal.  i.e. TXD (Transmit Data) is the data from the Terminal (PC) "To" the Modem and out the phone line to the remote modem.  RXD (Receive Data) is the data from the Modem to your PC.

 

 

Signal Definitions - listed by the DB25 (DE9) pin designations:

 

FG - Frame Ground - Pin 1 (NA but can be used for Signal Gnd)

Normally not connected but is intended for connection to "Earth" ground.  Not necessarily the same as "Signal Ground - Pin 7)

 

TXD - Transmit Data - Pin 2 (Pin 3) <Input to the modem>

Data from the PC (DTE) to the modem (DCE eventually sent out the phone line.

 

RXD - Received Data - Pin 3 (Pin 2) <Output from the modem>

Data from the Modem (DCE) to the DTE (PC).  This is the data received from the modem.

 

RTS - Request to Send - Pin 4 (Pin 7) <Input to the modem>

Signal from the DTE to the Modem indicating the DTE has data to send.  RTS is used with CTS for 'Hardware flow control'

 

In the command mode, the modem ignores RTS.  In asynchronous operation, the modem ignores RTS unless RTS/CTS flow control is selected by the &Kn command.

 

In synchronous on-line operation, the modem can be commanded by the &Rn command to ignore RTS or to respond to RTS by turning on CTS after the delay specified by Register S26.

 

 

CTS - Clear to Send - Pin 5 (Pin 8) <Output from the modem>

CTS true indicates the modem is ready to accept data from the DTE. In asynchronous operation, in error correction or normal mode, CTS is always ON (high) unless RTS/CTS flow control is selected by the &Kn command.

 

In synchronous operation, the modem also holds CTS ON during asynchronous command state.

 

The modem turns CTS OFF immediately upon going off-hook and holds CTS OFF until both DSR and DCD(Carrier Detect) are ON and the modem is ready to transmit and receive synchronous data. The modem can also be commanded by the &Rn command to turn CTS ON in response to an RTS OFF-to-ON transition.

 

DSR - Data Set Ready - Pin 6 (Pin 6) <Output from the modem>

DSR indicates modem status to the DTE. DSR OFF (low) indicates that the DTE is to disregard all signals appearing on the interchange circuits except Ring Indicator (RI - pin 22). DSR output is controlled by the AT&Sn command.

 

SG - Signal Ground - Pin 7 (Pin 5)

Common for all the interface pins.  Pin 7 may, or may not be tied to pin 1.

 

DCD - Data Carrier Detect - Pin 8 (Pin 1) (also known as 'RSDL' or "CXR') <Output from the modem>

DCD output is ON when a carrier is detected on the telephone line or OFF when carrier is not detected.  Controlled by &C0 command

 

DTR - Data Terminal Ready - Pin 20 (Pin 4) <Input to the modem>

The DTR input is turned ON (high) by the DTE when the DTE is ready to transmit or receive data. DTR ON prepares the modem to be connected to the telephone line, and maintains the connection established by the DTE (manual answering) or internally (automatic answering). DTR OFF forces disconnect under control of the &Dn and &Qn commands.

 

DTR also determines if the modem will auto answer the call based on &Dn and &Qn. 

 

RI - Ring Indicate - Pin 22 (Pin 9) <Output from the modem>

RI output ON (high) indicates the presence of a ringing signal on the telephone line.

 

 

25Pin DB25

9 Pin

DE9

Signal

EIA

Name

EIA Desig

CCITT

Desig

Direction

To-From

Modem

1

 

Shield (Fame Ground)

 FG

 AA

101

 

2

3

Transmitted Data

TxD

BA

103

To

3

2

Received Data

RxD

BB

104

From

4

7

Request To Send

RTS

CA

105

To

5

8

Clear To Send

CTS

CB

106

From

6

6

Data Set Ready

DSR

CC

107

From

7

5

Signal Ground

SG

AB

102

 

8

1

Data Carrier Detect

DCD

CF

109

From

9

 

Pos DC Test Voltage

 

 

-

From

10

 

Neg DC Test Voltage

 

 

-

From

11

 

Unassigned

 

 

-

 

12

 

Secondary Carrier Detect

SDCD

SCF

122

From

13

 

Secondary CTS

SCTS

SCB

121

From

14

 

Secondary TxD

STD

SBA

118

To

15

 

Transmit Signal Element Timing

TC

DB

114

From

16

 

Secondary RxD

SRD

SBB

119

From

17

 

Receive Signal Element Timing

RC

DD

115

From

18

 

Receiver Dibit Clock

 

 

-

 

19

 

Secondary RTs

SRTS

SCA

120

To

20

4

Data Terminal Ready

DTR

CD

108.2

To

21

 

Signal Quality Detect

SQ

CG

110

To

22

9

Ring Indicate

 RI

CE

125

From

23

 

Data Rate Select

 

CH/CI

111/112

To

24

 

EXT Transmit CLK.

(TC)

DA

113

To

25

 

BUSY/Test Mode

 

TM

-

From

 

 

END

 

 

Copyright © 1997-2011 Floyd Kling 
- All Rights Reserved -