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RS232

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by: Chris McAndrew
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Word Count: 18381

RS-232 standards (EIA-232) are defined by EIA/TIA (Electronic Industries Alliance /Telecommunications Industry Association). RS-232 defines both the physical and electrical characteristics of the interface. RS-232 is practically identical to ITU V.24 (signal description and names) and V.28 (electrical). RS232 is an Active LOW voltage driven interface and operates at +12V to -12V where:
 

Signal = 0 (LOW) > +3.0V

 Signal = 1 (HIGH) < -3.0V
 

DB25: View looking into male connector

 

DB25: View looking into female connector
 

DB9 Male and Female
 

 

DB9: View looking into male connector

 

 

DB9: View looking into female connector
 

RS232 on DB25 (RS-232C)
 

The RS-232 DB25 connector is capable of supporting two separate connections - each with its own optional clock when used in Synchronous mode or Bit-Synchronous mode. If you are using the interface purely for Asynchronous communications then you only need those marked with (ASYNC) below or you can use even fewer (if you understand what is happening). The column marked Dir shows the signal direction with respect to the DTE.
 

Pin No.

Name

Dir

Notes/Description

1

-

-

Protective/shielded ground

2

TD

OUT

Transmit Data (a.k.a TxD, Tx) (ASYNC)

3

RD

IN

Receive Data (a.k.a RxD, Rx) (ASYNC)

4

RTS

OUT

Request To Send (ASYNC)

5

CTS

IN

Clear To Send (ASYNC)

6

DSR

IN

Data Set Ready (ASYNC)

7

SGND

-

Signal Ground

8

CD

IN

Carrier Detect (a.k.a DCD).

9

-

-

Reserved for data set testing.

10

-

-

Reserved for data set testing.

11

-

-

Unassigned

12

SDCD

IN

Secondary Carrier Detect. Only needed if second channel being used.

13

SCTS

IN

Secondary Clear to send. Only needed if second channel being used.

14

STD

OUT

Secondary Transmit Data. Only needed if second channel being used.

15

DB

OUT

Transmit Clock (a.k.a TCLK, TxCLK). Synchronous use only.

16

SRD

IN

Secondary Receive Data. Only needed if second channel being used.

17

DD

IN

Receive Clock (a.k.a. RCLK). Synchronous use only.

18

LL

-

Local Loopback

19

SRTS

OUT

Secondary Request to Send. Only needed if second channel being used.

20

DTR

OUT

Data Terminal Ready. (ASYNC)

21

RL/SQ

-

Signal Quality Detector/Remote loopback

22

RI

IN

Ring Indicator. DCE (Modem) raises when incoming call detected used for auto answer applications.

23

CH/CI

OUT

Signal Rate selector.

24

DA

-

Auxiliary Clock (a.k.a. ACLK). Secondary Channel only.

25

-

-

Unassigned
 

RS232 on DB9 (EIA/TIA 574)
 

Pin No.

Name

Dir

Notes/Description

1

DCD

IN

Data Carrier Detect. Raised by DCE when modem synchronized.

2

RD

IN

Receive Data (a.k.a RxD, Rx). Arriving data from DCE.

3

TD

OUT

Transmit Data (a.k.a TxD, Tx). Sending data from DTE.

4

DTR

OUT

Data Terminal Ready. Raised by DTE when powered on. In auto-answer mode raised only when RI arrives from DCE.

5

SGND

-

Ground

6

DSR

IN

Data Set Ready. Raised by DCE to indicate ready.

7

RTS

OUT

Request To Send. Raised by DTE when it wishes to send. Expects CTS from DCE.

8

CTS

IN

Clear To Send. Raised by DCE in response to RTS from DTE.

9

RI

IN

Ring Indicator. Set when incoming ring detected - used for auto-answer application. DTE raised DTR to answer.
 

RS232 on RJ45 (RS-232D)
 

More properly EIA/TIA - 561. Use when connecting to or from a serial port with an 8 position Modular Jack (RJ45). If you are cross-connecting from a DB9 or a DB25 use the signal names to cross connect the appropriate connections.
 

Pin No.

Name

Notes/Description

1

DSR/RI

Data set Ready/ring indicator

2

DCD

Data Carrier Detect

3

DTR

Data Terminal Ready

4

SGND

Signal Ground

5

RD

Receive Data

6

TD

Transmit Data

7

CTS

Clear to Send

8

RTS

Request to Send
 

Note: Pin 1 is a multi-function pin sharing with DSR (Data Set Ready) and RI (Ring Indicator). This means it is impossible to differentiate between an incoming ring signal and when the modem has finally connected and synched up. With local (null modem connections) or if the modem is run in auto-answer mode this is not normally a problem. If used with a modem and the DTE (the computer end) wants to control the connection the problem is more real. DSR would normally indicate the 'connected and synched-up' state following DTR from the DTE. DCD will indicate that a carrier has been received but does not indicate synchronization of both ends. In most cases however CTS (Clear To Send) in response to RTS (Request To Send) will not normally be returned until an end-to-end connection is available.
 
Please click here to download the full document as a pdf
 


 
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