Data Terminal Ready

Definition
Data Terminal Ready (DTR) is a control signal defined in the RS‑232 serial communication standard. It is used by a data terminal equipment (DTE), such as a computer or terminal, to indicate to the connected data communications equipment (DCE), typically a modem, that the terminal is powered on and prepared to communicate.

Technical Specification

• Standard: Defined in the Electronic Industries Alliance (EIA) RS‑232‑C standard, later incorporated into the TIA/EIA‑232‑F and related specifications.
• Signal Type: DTR is a handshake or control line that operates as a binary voltage level. In RS‑232 signaling, a logical “ON” (DTR asserted) corresponds to a voltage between +3 V and +25 V, while a logical “OFF” (DTR de‑asserted) corresponds to a voltage between −3 V and −25 V.
• Pin Assignment: In the 25‑pin DE‑9 (DB‑9) connector commonly used for RS‑232, DTR is carried on pin 4 of the DB‑25 connector and pin 1 of the DB‑9 connector.
• Functionality: When asserted, DTR signals that the DTE is ready to receive data and maintain the communication link. The DCE may respond by asserting its own control lines, such as Data Carrier Detect (DCD) or Clear to Send (CTS), depending on the modem’s flow‑control configuration.

Typical Use Cases

1. Modem Communication: In traditional dial‑up modem operation, the computer asserts DTR when it wishes to establish a connection. The modem monitors DTR and can drop the carrier (assert DCD = 0) if DTR is de‑asserted, thereby terminating the session.
2. Serial Peripheral Devices: Equipment such as printers, data loggers, and industrial controllers may use DTR to synchronize power‑up sequences or to indicate readiness for data exchange.
3. Software Control: Operating systems and terminal emulation programs provide APIs (e.g., Windows SetCommState, POSIX ioctl with TIOCM_DTR) that allow applications to programmatically assert or clear DTR, enabling software‑controlled flow‑control and connection management.

Historical Context
The DTR signal originated in the early 1960s with the development of asynchronous serial interfaces for connecting teletypewriters, computer terminals, and modems. Its inclusion in the RS‑232 standard provided a simple hardware method for managing the logical state of communication links before the widespread adoption of more sophisticated protocols.

Related Signals

• Data Set Ready (DSR) – Indicates that the DCE is powered and ready.
• Request to Send (RTS) – Used by DTE to request permission to transmit.
• Clear to Send (CTS) – Sent by DCE to grant transmission permission.
• Data Carrier Detect (DCD) – Indicates that a carrier signal (e.g., from a modem) is present.

Implementation Considerations

• Voltage Levels: Proper termination and level‑shifting are required to avoid damage, as RS‑232 voltage ranges exceed the tolerances of many modern low‑voltage logic devices.
• Signal Integrity: Long cable runs and electromagnetic interference can cause false DTR transitions; appropriate shielding and grounding practices mitigate these effects.
• Flow Control Interaction: DTR is often used in conjunction with hardware flow‑control schemes (RTS/CTS) but can also serve as a simple on/off indicator in “no‑handshake” configurations.

References

• Electronic Industries Alliance (EIA), RS‑232‑C Standard, 1969; later revisions TIA/EIA‑232‑F.
• International Telecommunication Union (ITU), Recommendations on Data Communication Interfaces, ITU‑T X.28.
• Microsoft Documentation, “Serial Communications – DTR Control”, Microsoft Docs.
• POSIX.1‑2008, “Terminal Interface”, IEEE Std 1003.1.

The information above reflects established technical standards and widely documented usage of the Data Terminal Ready signal.