X.21 is an ITU-T standard for serial communications over synchronous
digital lines. The X.21 protocol is used primarily in Europe and Japan.
The X.21 line protocol is a state-driven protocol that
sets up a circuit-switched network using call setup. X.21 interfaces
use a 15-pin connector with the following eight signals:
Signal ground (G)—Reference signal used to evaluate
the logic states of the other signals. This signal can be connected
to the protective earth (ground).
DTE common return (Ga)—Reference ground signal for
the DCE interface. This signal is used only in unbalanced mode.
Transmit (T)—Binary signal that carries the data
from the DTE to the DCE. This signal can be used for data transfer
or in call-control phases such as Call Connect or Call Disconnect.
Receive (R)—Binary signal that carries the data
from the DCE to the DTE. This signal can be used for data transfer
or in call-control phases such as Call Connect or Call Disconnect.
Control (C)—DTE-controlled signal that controls
the transmission on an X.21 link. This signal must be on during data
transfer, and can be on or off during call-control phases.
Indication (I)—DCE-controlled signal that controls
the transmission on an X.21 link. This signal must be on during data
transfer, and can be on or off during call-control phases.
Signal Element Timing (S)—Clocking signal that is
generated by the DCE. This signal specifies when sampling on the line
must occur.
Byte Timing (B)—Binary signal that is on when data
or call-control information is being sampled. When an 8-byte transmission
is over, this signal switches to off.
Transmissions across an X.21 link require both the DCE and DTE
devices to be in a ready state, indicated by an all 1s transmission
on the T and R signals.
