The Linker DTE cable is a DB-25 null modem cable that connects two Data Terminal Equipment (DTE) devices directly, bypassing a modem. It uses DB-25 male connectors on both ends and crosses the transmit and receive data lines in accordance with the RS-232/ITU-T V.24 circuit definitions. Unlike the most common null modem variant, this cable uses an alternative handshake crossing where RTS (pin 4) is tied to DSR (pin 6) and CTS (pin 5) is tied to DTR (pin 20) on the opposing end, rather than the more usual RTS↔CTS and DTR↔DSR arrangement. Signal Ground (pin 7) is connected straight through, and the Protective Ground (pin 1) is also linked end-to-end.
25 PIN D-SUB MALE to Computer 1
25 PIN D-SUB MALE to Computer 2
| Pin | Pin |
|---|---|
| 1 | 1 |
| 2 | 3 |
| 3 | 2 |
| 4 | 6 |
| 5 | 20 |
| 6 | 4 |
| 7 | 7 |
| 20 | 5 |
Signals
- Pin 1 — Protective Ground (PG) — chassis/shield ground, connected straight through on both ends.
- Pin 2 — Transmit Data (TxD/BA) — serial data transmitted from this DTE; crosses to pin 3 (RxD) on the far end.
- Pin 3 — Receive Data (RxD/BB) — serial data received by this DTE; crosses from pin 2 (TxD) on the far end.
- Pin 4 — Request to Send (RTS/CA) — output from this DTE; connects to pin 6 (DSR) on the far end to assert data-set-ready to the remote device.
- Pin 5 — Clear to Send (CTS/CB) — input to this DTE; driven by pin 20 (DTR) from the far end.
- Pin 6 — Data Set Ready (DSR/CC) — input to this DTE; driven by pin 4 (RTS) from the far end.
- Pin 7 — Signal Ground (SGND) — common signal reference, connected straight through.
- Pin 20 — Data Terminal Ready (DTR/CD) — output from this DTE; connects to pin 5 (CTS) on the far end.
Notes
- This is a DTE-to-DTE null modem cable; no modem or DCE device is involved.
- The handshake crossing (RTS→DSR, CTS←DTR) differs from the more common RTS↔CTS / DTR↔DSR variant; software that relies on CTS being asserted by the remote RTS will work correctly, but software expecting a DTR↔DSR pairing should be configured accordingly.
- DB-25 connectors and their signal assignments are governed by ITU-T V.24 (circuit definitions) and V.28 (electrical characteristics), internationally equivalent to TIA/EIA-232.
- Maximum recommended cable length is 15 m (50 ft) or 2500 pF total capacitance at the original 20 kbps rate; shorter cables are required at higher baud rates.
- Signal voltage levels: SPACE (logic 0) = +3 V to +15 V at the driver output; MARK (logic 1) = −3 V to −15 V; the ±3 V band is undefined.