Connecting serial devices, Dte versus dce, Db9 serial cables and loopback plugs – Comtrol RocketPort Universal PCI User Manual
Page 11: Refer to
RocketPort Universal PCI User Guide: 2000284 Rev. G
Connecting Serial Devices - 11
Connecting Serial Devices
This section provides information about the RocketPort Universal PCI connectors,
in the event that you need to build cables or loopback plugs.
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DB9 Serial Cables and Loopback Plugs
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DB25 Serial Cables and Loopback Plugs
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RJ45 Serial Cables and Loopback Plugs (4J)
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RJ11 Serial Cables and Loopback Plugs (8J)
A loopback plug is a serial port plug with pins wired together that you can use
with application (for example, the diagnostic, Test Terminal, or minicom) to test
serial ports. See
Before Calling Technical Support
on Page 21 for information
about using the test application shipped with the RocketPort Universal PCI.
DTE Versus DCE
Most serial devices, except modems, are Data Terminal Equipment (DTE) devices.
Modems are Data Communication Equipment (DCE) devices.
How you build a cable depends on which device your are connecting to (DTE or
DCE). For example, you need a null-modem cable to connect the COM ports
(COM1 or COM2) on the computer or the RocketPort Universal PCI ports to
printers, terminals, bar code readers or DNC equipment. If you are connecting a
DTE device to a DCE device you need a straight-through modem cable.
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Straight-through cable (DTE to DCE) - connects TxD to TxD and RxD to RxD.
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Null-modem cable (DTE to DTE) - connects TxD to RxD and RxD to TxD.
DB9 Serial Cables and Loopback Plugs
The following figures and table illustrate the signals present on DB9 connectors if
you need to build your own null-modem or straight-through DB9 serial cables.
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