11.1. Jargon
RS-232 cables were originally intended to
link terminals to modems. The terminal is formally named a Data
Terminal Equipment, abbreviated to DTE. The modem
is formally named a Data Communications Equipment, abbreviated to
DCE.
A standard RS-232 cable has a 25-pin
D-type socket, which connects to the DTE, and a
25-pin D-type plug, which connects to the DCE.
All 25 pins are connected, with pin 1 on the plug wired to pin 1 on
the socket, pin 2 on the plug wired to pin 2 on the socket, and so
on. The shielding of the cable is attached to the metallic cover
on the socket.
RS-232 signaling is much more robust than
the signalling of many other communications standards. Pins can be
shorted, not connected or drive more than one output.
Signals are named from the point of view of the Data Terminal
Equipment. So Transmit Data on the DTE is
connected to Transmit Data on the DCE. The
Transmit Data pin on the DTE actually transmits
data, whereas Transmit Data pin on the DCE
actually recieves data.