A ???dumb??? terminal
contains a keyboard, a display, a communications port for accessing a
remote computer, and little else. An RS-232 link connects the terminal to a
modem, which in turn accesses the phone lines that connect to the remote
computer. PCs with modems and network interfaces have made this type of terminal
connection nearly obsolete.
These days, an RS-232 port is more likely to connect a PC to an embedded system
or to connect two embedded systems. Much of the original RS-232 terminology
thus doesn??™t apply to modern applications, but the hardware interface
remains useful.
The RS-232 standard calls the terminal end of the link the data terminal equipment,
or DTE. The modem end of the link is the data circuit-terminating
equipment, or DCE.
The signals and their functions are named from the perspective of the DTE.
For example, TX (transmit data) is an output on a DTE and an input on a
DCE, while RX (receive data) is an input on a DTE and an output on a DCE.
The RS-232 ports on PCs are almost always DTEs. It doesn??™t matter which
device in a link is the DTE and which is the DCE, but every connection
between two computers must either have one of each or must emulate the
absent interface (typically DCE) with an adapter called a null modem. The null
modem swaps the lines so each output connects to its corresponding input.
Chapter 5 has more about these adapters.
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