DTE and DCE Devices
Figure 1 shows
two RS232 devices connected with a "straight-through" cable. A DTE
was originally a terminal that connected to a remote computer via a modem,
which was called a Data Communications Equipment (DCE) device. Nowadays, the
DTE device is your computer. It has been a convention, since the early days of
personal computer development, to configure the computer as a DTE device. For
ease in manufacturing cables, electronics manufacturers of devices to be
connected to a personal computer normally choose to make the device conform to
the DCE standard. This means a "straight through cable" is used. If
two DTE devices are to be connected, a "null modem" or
"cross-over" cable must be used as shown in Figure 2. The K3 is
configured as a DCE device and so a straight through cable, also called an
RS232 cable or serial cable, is used to connect to a computer.
Transmitted data is an output
on a DTE device and an input on a DCE device.
Notice in
Figure 1 that the serial data pin names, TxD (Transmitted Data) and RxD
(Received Data), and their signal flow directions, are defined in
reference to the DTE device. That is, TxD is transmitted data and is an output
from the DTE device. Perversely, it seems, the TxD name is used for an input
signal on the DCE device. The has caused a great deal of confusion amongst
amateurs trying to hook up serial devices. An invaluable tool for
troubleshooting RS232 connections is a "blinky box" tester shown in
Figure 5.
The signals RTS
and DTR stand for Request to Send and Data Terminal Ready. In the
original RS232 definition they, along with other handshaking signals, helped
control the flow of serial data between the DTE and DCE device. In the K3 the
RTS and DTR signals from the computer may be used to provide push-to-talk (PTT)
and CW keying signals.
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