RS-232 and other serial ports and interfaces
Common names: ITU-TSS (CCITT) X.21, ISO 4903

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:

Signal Ground (G) - This provides reference for the logic states against the other circuits. This signal may be connected to the protective ground (earth).
DTE Common Return (Ga) - Used only in unbalanced-type configurations (X.26), this signal provides reference ground for receivers in the DCE interface. Provided at pin 8 of DB15 connector
Transmit (T) - This carries the binary signals which carry data from the DTE to the DCE. This circuit can be used in data-transfer phases or in call-control phases from the DTE to DCE (during Call Connect or Call Disconnect).
Receive (R) - Controlled by the DTE to indicate to the DCE the meaning of the data sent on the transmit circuit. This circuit must be ON during data-transfer phase and can be ON or OFF during call-control phases, as defined by the protocol.
Indication (I) - The DCE controls this circuit to indicate to the DTE the type of data sent on the Receive line. During data phase, this circuit must be ON and it can be ON or OFF during call control, as defined by the protocol.
Signal Element Timing (S) - This provides the DTE or DCE with timing information for sampling the Receive line or Transmit line. The DTE samples at the correct instant to determine if a binary 1 or 0 is being sent by the DCE. The DCE samples to accurately recover signals at the correct instant. This signal is always ON.
Byte Timing (B) - This circuit is normally ON and provides the DTE with 8-bit byte element timing. The circuit transitions to OFF when the Signal Element Timing circuit samples the last bit of an 8-bit byte. Call-control characters must align with the B lead during call-control phases. During data- transfer phase, the communicating devices bilaterally agree to use the B lead to define the end of each transmitted or received byte. The C and I leads then only monitor and record changes in this condition when the B lead changes from OFF to ON, although the C and I leads may be altered by the transitions on the S lead. This lead is frequently not used.



X.21 cross-cable scheme
X.21 X.21
1 1
2 4
3 5
4 2
5 3
6 7
7 6
8 8
9 11
10 12
11 9
12 10
13 14
14 13



15 pin D-SUB female connector layout
15 pin D-SUB female connector
at the DTE (Computer)
15 pin D-SUB female connector layout
15 pin D-SUB female connector
at the DCE (Modem)
According to 4 reports in our database (0 positive and 1 negative) this pinout may be incorrect.

Is this pinout
Source(s) of this and additional information: X.21 Page at Connectivity Knowledge Platform (Made IT),, from Hardware Book
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