DTR - Data Terminal Ready
DTR (Data Terminal Ready) is a control signal in serial communication, typically part of the RS-232 standard, used by a Data Terminal Equipment (DTE) device, such as a terminal or terminal emulator, to indicate to a Data Circuit-terminating Equipment (DCE) device (like a modem) that it is ready to establish or maintain a communication link with a mainframe system or its front-end processor. It signals the DTE's operational readiness to the DCE. Data Terminal Ready (DTR) is a control signal within the RS-232 serial communication standard, originating from the Data Terminal Equipment (DTE) side, such as a mainframe's front-end processor or a terminal. It indicates to the Data Communications Equipment (DCE), typically a modem, that the DTE is powered on and ready to establish a connection or receive data.
Key Characteristics
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- RS-232 Standard: A specific pin (pin 20 on a DB-25 connector) in the RS-232 standard, asserted by the DTE to indicate its readiness for data transmission.
- DTE-Initiated: Always asserted by the terminal or terminal emulator (the DTE) to signal its active state to the modem or other communication equipment (the DCE).
- Handshaking Role: Forms part of the hardware flow control or handshaking mechanism, ensuring that the terminal is powered on and ready to communicate before the modem proceeds with dialing or answering.
- Link Establishment: Crucial for initiating and maintaining a physical serial connection, especially in legacy dial-up or dedicated serial line scenarios for mainframe access.
- Mainframe Access Context: Historically vital for physical 3270-type terminals or early PC-based terminal emulators connecting via modems to IBM communication controllers (e.g., 37x5/37x6) that interface with VTAM on z/OS.
Use Cases
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- Dial-up Terminal Access: A physical 3270 terminal using a modem to dial into a mainframe network, where DTR signals the terminal's readiness to the modem, allowing the modem to initiate or accept a call.
- Dedicated Serial Connections: In older mainframe setups, direct serial connections between a terminal and a communication controller might use DTR for basic link integrity and terminal presence detection.
- Modem Pool Management: For mainframe environments supporting remote users via modem pools, DTR helps the modems manage active connections and release lines when terminals properly disconnect.
- Troubleshooting Legacy Connectivity: When diagnosing issues with serial connections to a mainframe, checking the DTR signal status can help identify if the terminal or emulator is properly initialized and communicating with its modem.