Specifications

94 Chapter 9 Pinout reference
Difference between DTE and DCE
One of the most misunderstood areas of RS-232 communications is the
difference between Data Terminal Equipment (DTE) and Data
Communications Equipment (DCE). DTE is typically either a dumb terminal
or the serial port on a computer/workstation. DCE is typically a modem,
Channel Service Unit/Data Service Unit (CSU/DSU), or another data
communications unit. This is all most people need to know about these
terms and their meanings.
When signal definitions and direction become involved, these terms can
get more complicated. For example, when a unit transmits data, it is sent
out of the unit. However, in terms of DCE, this transmitted data becomes
an input signal. This is because the specification for data transmission is
written from the perspective of the DTE end of the link. As another, the
Receive Data signal is an input to DTE, but an output from DCE.
Therefore, a straight-through one-to-one cable is all that is necessary
between a modem and a standard DTE serial port. However, if it desired to
connect two DTE ports together, it is necessary to simulate a pair of DCE
devices, typically modems that would normally be between the two DTE
devices. To perform this simulation, a null-modem device or cable is used.