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Serial Port Basics (continued)

DTE and DCE

The RS-232 specification defines two classes of devices: data terminal equipment (DTE) and data communication equipment (DCE). Your computer's serial port is configured for DTE operation because the computer usually acts as a terminal. Modems and many other serial devices are configured as DCE, since they usually act as communications equipment.

What's the difference? A DTE device's TD signal means "I transmit data on this line." A DCE's TD signal can be read "You (the DTE) transmit data to me on this line." A DTE's RD signal means "I receive data on this signal line." A DCE's RD line means, "You, the DTE, will receive the data I transmit on this signal line." Sound confusing?

A look at a the DB9 connector pinouts and signal direction with respect to DTE (e.g. your computer) makes things a little more clear.

Signal Name

Pin Number

Direction

25-pin

9-pin

transmitted data

TD

2

3

DTE  DCE

received data

RD

3

2

DTE  DCE

request to send

RTS

4

7

DTE  DCE

clear to send

CTS

5

8

DTE  DCE

data terminal ready

DTR

20

4

DTE  DCE

data set ready

DSR

6

6

DTE  DCE

data carrier detect

DCD

8

1

DTE  DCE

ring indicator

RI

22

9

DTE  DCE

signal ground

GND

7

5

Pinouts for 9-pin and 25-pin serial connectors.
(Special thanks to Mark Seagoe for correcting an error in this table)

The cable that connects DTE devices (such as your computer) and DCE devices (such as your modem) is simple. It connects the TD pin to TD, pin RD to RD, etc. A cable that connects DTE to DTE or DCE to DCE must connect the TD to RD and RD to TD. This cable is referred to as a null modem cable because it can connect two terminals (DTE's) without the use of modems (DCE's). These cables are sometimes referred to as crossover cables. Note that GND pins are connected regardless of DCE/DTE configuration.

Most DCE devices have female connectors (DB9 or DB25), while most DTE devices are equipped with male connectors, but this is not always the case.

DB9M connector (DTE)
DB9 Male (DTE) Connector

DB9F connector (DCE)
DB9 Female (DCE) Connector

Determining whether a device is DTE or DCE: Connect a voltmeter's ground line to the connector's GND pin. Then, probe the voltage on the TD and RD pins. If the TD pin voltage is small (-3V to +3V), the device is configured to receive data on the TD pin, and thus is a DCE device. Likewise, if the RD pin voltage is small, the device is receiving data on the RD pin, and thus is DTE.

Continued:

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