RS-232 Serial Interface

Pin Assignments

D-Subminiature Connector, 25 Contacts

DB25 Connector Pin Out
D-Subminiature Connector, 9 Contacts

DE9 Connector Pin Out
PinNameFunctionSource PinNameFunctionSource
1FGFrame Ground--- 1DCDData Carrier DetectDCE
2TXDTransmitted DataDTE 2RXDReceived DataDCE
3RXDReceived DataDCE 3TXDTransmitted DataDTE
4RTSRequest To Send (= DTE ready)DTE 4DTRData Terminal ReadyDTE
5CTSClear To Send (= DCE ready)DCE 5GNDSignal Ground---
6DSRData Set ReadyDCE 6DSRData Set ReadyDCE
7GNDSignal Ground--- 7RTSRequest To Send (= DTE ready)DTE
8DCDData Carrier DetectDCE 8CTSClear To Send (= DCE ready)DCE
9---Positive Test VoltageDCE 9RIRing IndicatorDCE
10---Negative Test VoltageDCE  
11QMEqualizer ModeDCE
12SDCDSecondary Data Carrier DetectDCE
13SCTSSecondary Clear To SendDCE
14STDSecondary Transmitted DataDTE
15TCTransmitted ClockDCE
Secondary Received Clock
Divided Clock, Transmitter
17RCReceiver ClockDCE
18DCRDivided Clock, ReceiverDCE
19SRTSSecondary Request To SendDTE
20DTRData Terminal ReadyDTE
21SQSignal Quality DetectDCE
22RIRing IndicatorDCE
23DRSData Rate Selector
Data Rate Selector
24SCTESerial Clock Transmit ExternalDTE

DTE (Data Terminal Equipment): Computer or Terminal
DCE (Data Communications Equipment): Modem

Hardware Handshaking Connections

DTE <---> DCE

Null Modem Connections

DTE <---> DTE

DCE <---> DCE

Timing Diagram of Asynchronous Signal

Timing Diagram

EIA Standard
DriverLogical 0 (high) = +5 V to +15 V
Logical 1 (low) = -5 V to -15 V
into a load of 3 kohm to 7 kohm with a slew rate < 30 V/microsecond
ReceiverLogical 0 (high) = +3 V to +25 V
Logical 1 (low) = -3 V to -25 V
and presents a 3 kohm to 7 kohm load

Bi-directional Transmission

Half Duplex: Either unit can transmit while the other listens, but both cannot transmit simultaneously.
Full Duplex: Both units can transmit simultaneously.

Null Serial Adapter


(for Testing the Serial Port)

Other Resources

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Page author: Dawn Rorvik (rorvikd@evergreen.edu)
Last modified: 05/20/2003