ITU-TSS X.21 interface

CCITT X21 is a physical and electrical interface that uses two types of circuits: balanced (X.27N.1 1) and and unbalanced (X.26N.10). CCITT X.21 calls out the DB-15 connector.

The physical interface between the DTE and the local PTT-supplied DCE is defined in ITU-T recommendation X.21. The DCE provides a full-duplex, bit-serial, synchronous transmission path between the DTE and the local PSE. It can operate at data rates from 600bps to 64Kbps. A second standard, X.21bis has been defined for use on existing (analogue) networks. An X.21bis is a subset of EIA-232D/V.24 therefore allowing existing user equipment to be readily interfaced using this standard. It should perhaps be emphasized here that V24 defines the data terminal equipment interface to the modem and is not concerned with the interface between the modem and the line itself. The modems themselves therefore form part of the conceptual physical connection. The V24 interface is thus independent of both modulation technique and data throughput rate.

Pinout for balanced X.21 (according to ISO4903)

Pin	Name	DTE I/O	Description	Type
1	Shield		Shield	Ground
2	T (A)		Transmit (A)	Data
3	C (A)		Control (A)	Control
4	R (A)		Receive (A)	Data
5	I (A)		Indication (A)	Control
6	S (A)		Signal Timing (A)	Timing
7	B (A) / unused		Byte Timing (A) or unassigned	Timing
8	G		Ground	Ground
9	T (B)		Transmit (B)	Data
10	C (B)		Control (B)	Control
11	R (B)		Receive (B)	Data
12	I (B)		Indication (B)	Control
13	S (B)		Signal Timing (B)	Timing
14	B (B) / unused		Byte Timing (B) or unassigned	Timing
15			unused

Meaning there is always a pair (+/-) for each signal, like used in RS422. Electrally the X.21 signals are the same as RS422, so please refer to RS422 for the exact details.
As can be seen from the pinning specifications, the Signal Element Timing (clock) is provided by the DCE. This means that your provider (local telco office) is responisble for the correct clocking and that X.21 is a synchronous interface. Hardware handshaking is done by the Control and Indication lines. The Control is used by the DTE and the Indication is the DCE one.

Description of signals:

Note: Direction is DTE (Computer) relative DCE (Modem)