National Instruments ENET-232 Series, Manual

Page: 154 / 237

RS-232 DTE versus DCE
In the RS-232 specification, DTE (Data Terminal Equipment) and DCE
(Data Communications Equipment) refer to the types of equipment on
either end of a serial connection. In general, DTE and DCE refer to
computer equipment and modems, respectively. Because the RS-232
specification mainly involves connecting a DTE directly to a DCE and
vice versa, the pinouts are defined so that cabling is simple. That is, a
cable connected a computer to a modem by wiring pin 1 to pin 1, pin 2 to
pin 2, and so on. This method is known as
straight-through cabling.
The following figure shows straight-through cabling in a DTE-to-DCE
interface.
Straight-Through Cabling in a DTE-to-DCE Interface
Straight-through cabling is still the standard method to connect a modem
to your PC. However, because many applications use serial
communication to connect two or more DTEs without modems, the
cabling becomes more complicated. If two DTEs are wired together using
a straight-through cable, one transmitter is connected to the other
transmitter, and one receiver is connected to the other receiver. In this
setup, no transmissions can occur. Thus, these applications must use a
cabling scheme that connects the transmitter on one device to the
receiver on the other device and vice versa. This method is known as
null-modem cabling, because it replaces the two modems that traditional
RS-232 applications would require between the two DTEs. To
communicate from one DTE serial port to another, use a null-modem
cable.
The following figure shows null-modem cabling in a DTE-to-DTE
interface.
Null-Modem Cabling in a DTE-to-DTE Interface
PCI, PXI, USB (one port only), ENET, ExpressCard, and PCMCIA RS-