DB-9 Serial Cables
Straight Through 9-Pin Serial Cable
A Straight Through or One-to-One cable is used to connect a DTE device (PC) to a DCE device (modem or other
communications device).
The transmit and receive lines are not cross-connected in this case, hence the name.
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Null (Crossover) 9-Pin Serial Cable
A Null Modem or Crossover cable is used to connect a pair of the same devices, DTE to DTE devices or DCE to
DCE devices, together.
For this to work, the Transmit (TXD) pin of one device needs to be connected to the
Receive (RXD) pin of the other device, and vice versa.
To enable handshaking between the two devices, the
Request to Send (RTS) pin of one device must be connected
to the Clear to Send (CTS) pin of the other device, and the Data Set Ready (DSR) pin is connected to the Data
Terminal Ready (DTR) pin of the other.
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DTE Device
STRAIGHT THROUGH CABLE
DCE Device
Function
Name
Pin
Signal Direction
Pin Name
Function
Data Carrier Detect
DCD
1
1
DCD
Data Carrier Detect
Receive Data
RXD
2
2
RXD
Receive Data
Transmit Data
TXD
3
3
TXD
Transmit Data
Data Terminal Ready
DTR
4
4
DTR
Data Terminal Ready
Ground Signal
GND
5
5
GND
Ground Signal
Data Set Ready
DSR
6
6
DSR
Data Set Ready
Request to Send
RTS
7
7
RTS
Request to Send
Clear to Send
CTS
8
8
CTS
Clear to Send
Ring Indicator
RI
9
9
RI
Ring Indicator
DCE Device
NULL / CROSSOVER CABLE
DCE Device
Function
Name
Pin
Signal Direction
Pin Name
Function
Data Carrier Detect
DCD
1
1
DCD
Data Carrier Detect
Receive Data
RXD
2
2
RXD
Receive Data
Transmit Data
TXD
3
3
TXD
Transmit Data
Data Terminal Ready
DTR
4
4
DTR
Data Terminal Ready
Ground Signal
GND
5
5
GND
Ground Signal
Data Set Ready
DSR
6
6
DSR
Data Set Ready
Request to Send
RTS
7
7
RTS
Request to Send
Clear to Send
CTS
8
8
CTS
Clear to Send
Ring Indicator
RI
9
9
RI
Ring Indicator
RLH Industries, Inc. • Tel. 866-DO-FIBER • Fax 714 532-1885 • www.fiberopticlink.com
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