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Data Terminal Ready (DTR) is an RS-232 control signal sent from a computer or terminal to communications equipment, such as a modem. When asserted, it indicates that the terminal side is ready; depending on the equipment, it can prepare or maintain a connection. DTR does not carry serial data, and asserting it alone does not guarantee a working connection.
What does Data Terminal Ready mean?
DTR is short for Data Terminal Ready. In the conventional RS-232 arrangement, the computer or terminal is the data terminal equipment (DTE), and the modem or similar communications device is the data communications equipment (DCE). DTR travels from the DTE toward the DCE.
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DTR is a control indication, not a line that carries the data being sent. Digi describes it as an output from DTE to DCE and notes that UNIX systems typically use it to indicate that a serial port has been activated or opened: Digi’s RS-232 signal reference.
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In the standards definitions reproduced in Technical Aspects of Data Communications, Third Edition, the ON condition for circuit 108/2 prepares the DCE to connect signal-conversion equipment to the line and maintains a connection established by supplementary means. In practice, what happens when DTR changes depends on the connected device and its configuration.
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In a modem setup, DTR can take part in connection control. It is not, by itself, proof that the modem has connected to a remote system or that data transfer has succeeded.
How DTR differs from DSR, RTS, and CTS
These RS-232 control signals have different typical directions and roles. Their exact use can vary across devices, so the descriptions below are common roles rather than guarantees for every serial interface.
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| Signal | Typical direction | Practical meaning |
|---|---|---|
| DTR (Data Terminal Ready) | DTE → DCE | The terminal side indicates readiness; equipment may use the signal to control connection state. |
| DSR (Data Set Ready) | DCE → DTE | The communications equipment indicates its own readiness. |
| RTS (Request To Send) | DTE → DCE | The terminal requests permission or preparation to transmit. |
| CTS (Clear To Send) | DCE → DTE | The communications equipment indicates it can accept transmission. |
Digi’s RS-232 signal reference describes these directions and typical roles.
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No—not universally. A modem does not interpret the data bit stream to decide when a transfer is complete. The program controlling the DTE Ready line must decide when to change its state. As Technical Aspects of Data Communications, Third Edition, explains, “The modem is merely a level conversion device and does not interpret the meaning of the bit stream passing through.” Do not assume that deasserting DTR automatically confirms that every buffered byte has finished transmitting.
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What to check when connecting serial equipment
If you are troubleshooting legacy serial equipment, check the connection as a whole rather than treating DTR as a standalone fix:
- Confirm that the cable and connector match the devices.
- Check whether the equipment requires a straight-through or null-modem cable and whether the wiring matches the devices’ roles.
- If the computer has no compatible serial port, check whether the USB-to-RS-232 adapter supports the operating system and software you use.
- Consult the device documentation or software settings to learn what that specific device does when DTR is asserted or deasserted.
RS-232 adapters, cables, and operating systems do not all behave identically. The signal’s effect must be checked for the specific hardware and software involved. Digi’s signal reference also discusses null-modem devices and cables.
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