A rollover cable is an 8-conductor cable wired so that every pin on one end is connected to its mirror-image pin on the other end (pin 1 to pin 8, pin 2 to pin 7, and so on). This reversal creates the null-modem effect needed to link two DTE-wired devices — typically a Cisco router or switch console port and a PC serial port — without a separate adapter. Cisco established the dominant RJ-45 console pinout in the 1990s (pin 1 = RTS, pin 2 = DTR, pin 3 = TD, pins 4–5 = SigGnd, pin 6 = RD, pin 7 = DSR, pin 8 = CTS), and the same wiring scheme has since been adopted by Juniper, Sun, Nokia, and Lantronix equipment. The electrical signalling on the cable is governed by TIA-232-F (formerly EIA RS-232C), the standard for asynchronous serial communication.
RJ45 MALE CONNECTOR
| Conn 1 | Conn 2 |
|---|---|
| 1 | 8 |
| 2 | 7 |
| 3 | 6 |
| 4 | 5 |
| 5 | 4 |
| 6 | 3 |
| 7 | 2 |
| 8 | 1 |
Notes
You can identify a rollover cable by holding the two plugs side-by-side with the locking tabs facing away from you: the wire colour on the outermost pin of the left plug will match the wire colour on the outermost pin of the right plug. On Cisco-sourced cables (e.g. the blue flat CAB-500RJ), pin 1 is white on one connector and pin 8 is white on the other.
The Cisco RJ-45 console port is wired as DTE and does not support hardware (RTS/CTS) flow control by default; the AUX port on the same device does support hardware flow control. Default terminal settings for all Cisco console ports are 9600 baud, 8 data bits, no parity, 1 stop bit, no flow control.
An RJ-45-to-DB-9 female adapter (Cisco part 74-0495-01) converts the rolled cable end for connection to a PC DB-9 COM port: RJ45 pin 1 → DB9 pin 8 (CTS), pin 2 → DB9 pin 6 (DSR), pin 3 → DB9 pin 2 (RxD), pins 4–5 → DB9 pin 5 (GND), pin 6 → DB9 pin 3 (TxD), pin 7 → DB9 pin 4 (DTR), pin 8 → DB9 pin 7 (RTS). A corresponding RJ-45-to-DB-25 adapter (part 29-0810-01) is also available.
Note that Cyclades/Avocent terminal servers use a different RJ-45 pinout in which pin 5 carries CTS (not a second ground) and pin 7 carries DCD (not DSR); a crossover adapter (ADB0039) is required when connecting a Cyclades port to a Cisco-schema device.
References
- Understand Cable Requirements for Console and AUX Ports — Cisco Systems
- RJ-45 to DB-9 Serial Port Adapter Pinout Information — Juniper Networks