A console cable must match the device’s serial interface and the complete cable-and-adapter wiring—not just the connector shape. An RJ45 serial console port is not an Ethernet port. For the Cisco IR8340 connection documented below, a rollover cable reverses all eight positions, and the terminal adapter routes the device’s transmit signal to the PC’s receive input. Juniper EX2300 documentation uses similar data-pin assignments but different control-signal definitions. Neither pinout should be treated as universal.
Before connecting equipment, identify the exact device model, the port labeled CONSOLE or CON, the cable wiring, and the host adapter’s electrical interface. The deciding requirement is end-to-end signal compatibility.
Select the Device-Specific Reference
| Device or connection | Reference to use | Important boundary |
|---|---|---|
| Passive RJ45-to-RJ45 rollover cable | Rollover continuity table below | Describes conductor routing only; it does not establish device compatibility |
| Cisco Catalyst IR8340 RJ45 RS-232 console to a PC DB9 serial port | IR8340 cable-and-adapter table below | Based on the model-specific hardware installation guide |
| Juniper EX2300 RJ45 RS-232 console to a PC DB9 serial port | EX2300 device-pin and DB9 mapping tables below | NC and DCD assignments differ from the IR8340 |
| Another switch, router, firewall, or console server | Its exact hardware manual and adapter documentation | Do not select a pinout by brand, cable color, or connector shape alone |
The Cisco mapping comes from “Cable and Connectors,” Table 1, in the Catalyst IR8340 Hardware Installation Guide, updated January 17, 2025. The Juniper mappings come from Tables 2 and 11 of “EX2300 Management Cable Specifications and Pinouts”; that retrieved page does not display a revision identifier.
These are manual lookup tables, not an automatic model-detection or cable-certification tool.
RJ45 Rollover Cable Pinout
A rollover cable reverses the contact order from one RJ45 plug to the other. Cisco’s “Understand Cable Requirements for Console and AUX Ports,” Document ID 12223, revision 4.0, gives the following mapping.
| End A contact | End B contact |
|---|---|
| 1 | 8 |
| 2 | 7 |
| 3 | 6 |
| 4 | 5 |
| 5 | 4 |
| 6 | 3 |
| 7 | 2 |
| 8 | 1 |
This table contains no signal names because the cable itself only connects conductors. The equipment and adapters determine whether a conductor carries TxD, RxD, ground, or a control signal.
A straight-through cable instead preserves each contact number: 1→1, 2→2, and so on. An Ethernet crossover cable exchanges selected pairs; it is not the same as a full rollover. Cisco documents these as separate cable types.
Identify Contacts Without Mirroring the Connector
For an RJ45 plug, look at the broad face with the exposed gold contacts toward you, the latch on the opposite side, and the cable extending downward. Contacts run from 1 through 8, left to right. Hold both plugs in the same orientation when comparing their conductor order.
Do not transfer that visual order directly to a device jack viewed from the front: a mating plug and jack face one another. For DB9 connectors, use the molded contact numbers or a manufacturer drawing that identifies the connector sex and viewing side. A mating-face drawing and a solder-side drawing are mirror images.
Wire colors can help identify a cable, but contact-to-contact continuity is the more useful check when colors are hidden or the cable has been repaired.
Cisco Catalyst IR8340: RJ45 Console to DB9
The IR8340 has an eight-contact RJ45 RS-232 console port. Its hardware guide documents a rollover cable followed by an RJ45-to-DB9 terminal adapter. The table below preserves those two stages so that the adapter’s RJ45 contacts are not confused with the device’s RJ45 contacts.
| IR8340 console signal | Device-side RJ45 contact | RJ45 contact after rollover | PC-side DB9 contact | PC-side signal |
|---|---|---|---|---|
| RTS | 1 | 8 | 8 | CTS |
| DTR | 2 | 7 | 6 | DSR |
| TxD | 3 | 6 | 2 | RxD |
| GND | 4 | 5 | 5 | GND |
| GND | 5 | 4 | 5 | GND |
| RxD | 6 | 3 | 3 | TxD |
| DSR | 7 | 2 | 4 | DTR |
| CTS | 8 | 1 | 7 | RTS |
Source: IR8340 Hardware Installation Guide, “Cable and Connectors,” Table 1. Cisco additionally notes that device pin 1 is internally connected to pin 8. That is a device detail, not an instruction to add a bridge inside the cable.
The data paths explain the connection:
- Device TxD travels from device RJ45 contact 3 to PC DB9 contact 2, the PC’s receive input.
- PC TxD travels from PC DB9 contact 3 to device RJ45 contact 6, the device’s receive input.
- Device RJ45 contacts 4 and 5 reach PC DB9 contact 5, signal ground.
For a single-piece RJ45-to-DB9 cable, compare its complete continuity map against the device-side RJ45 and PC-side DB9 columns. Do not add another rollover simply because the reference connection contains one: the single-piece cable may already implement the complete mapping.
Why a Generic Cisco Table Is Not Enough
Cisco’s general console/AUX support document publishes a different RJ45-to-DB9 mapping from the IR8340 hardware guide, including different data and control routing. Those tables must not be combined into one “Cisco console pinout.” For an IR8340, use its model-specific guide; for another device, retrieve that device’s documentation rather than assuming either mapping applies.
Signal labels also need an endpoint. “TxD” at the device and “TxD” at the PC both identify transmit functions at their respective ends; the intended connection routes each transmitter to the other end’s receiver.
Juniper EX2300: Console Signals and DB9 Mapping
Juniper’s EX2300 documentation identifies the RJ45 console as an RS-232 serial interface. Its device-port assignments are:
| EX2300 RJ45 contact | Signal | Direction relative to the switch |
|---|---|---|
| 1 | NC | Not connected |
| 2 | NC | Not connected |
| 3 | TxD | Output |
| 4 | GND | Signal ground |
| 5 | GND | Signal ground |
| 6 | RxD | Input |
| 7 | DCD | Input |
| 8 | NC | Not connected |
Source: “EX2300 Management Cable Specifications and Pinouts,” Table 2. NC means the device contact is not connected; it is not an available power or general-purpose connection.
The same page’s Table 11 publishes these RJ45-to-DB9 signal pairings:
| RJ45 contact | EX2300 signal | DB9 contact | Host-side signal |
|---|---|---|---|
| 1 | NC | 8 | CTS |
| 2 | NC | 6 | DSR |
| 3 | TxD | 2 | RxD |
| 4 | GND | 5 | GND |
| 6 | RxD | 3 | TxD |
| 7 | DCD | 4 | DTR |
| 8 | NC | 7 | RTS |
Source: “EX2300 Management Cable Specifications and Pinouts,” Table 11. The published adapter table omits RJ45 contact 5, although the device-port table identifies it as ground. Do not invent a separate adapter connection for that omitted row.
The EX2300’s data pairings match the IR8340 example, but its control assignments do not: EX2300 contact 7 is DCD input, while the IR8340 table identifies contact 7 as DSR. Shared TxD and RxD positions alone do not establish complete cable interchangeability.
Check the Complete Juniper Cable Assembly
Juniper’s EX2300 cable-specification table calls the RJ45 console cable a rollover cable. Its connection procedure also describes an “Ethernet cable” used with an RJ45-to-DB9 adapter, without establishing a separate bare-adapter continuity map in that procedure. Do not interpret that wording as permission to substitute any patch cable and any modular adapter.
Use the documented assembly or verify the complete device-to-host routing. Juniper lists JNP-CBL-RJ45-DB9, JNP-CBL-RJ45-USBA, and JNP-CBL-RJ45-USBC as console connection options. These part numbers identify intended products; an unmarked adapter with the same connector shapes is not automatically equivalent.
Choose the Correct Serial Adapter
The two RJ45 console interfaces above require RS-232 signaling. “USB serial” by itself does not specify the electrical interface: FTDI separately lists RS-232, TTL-level UART, RS-422, and RS-485 products. A USB-to-TTL cable is not a substitute for a USB-to-RS-232 adapter merely because both expose transmit and receive signals.
| Connection option | What to confirm before use |
|---|---|
| PC DB9 serial port plus passive console cable | PC port signal assignments, connector sex, and complete cable continuity |
| USB-to-DB9 adapter plus passive console cable | Explicit RS-232 support, correct DB9 signals, installed driver, and recognized serial port |
| Integrated USB-to-RJ45 console cable | Supported device pinout, RS-232 output, driver requirements, and internal routing |
| Passive RJ45-to-DB9 modular adapter | Its actual contact map and the cable type it expects |
| Console-server RJ45 serial port | Both endpoint pinouts and the cable or adapter specified for that pairing |
A passive modular adapter changes connections; it does not perform USB-to-serial conversion or convert TTL signaling into RS-232.
For the EX2300 USB-A and USB-C console adapters listed above, Juniper specifies an x64 virtual COM port driver. Check the current adapter and operating-system requirements before deployment.
Check Cable Continuity Before Connecting Equipment
Test passive cables and passive adapters while disconnected from the device, PC, USB converter, and console server.
- Identify both connectors and their contact numbers. Record whether any drawing shows the mating face or termination side.
- Test a standalone rollover cable against all eight rows of the rollover table.
- Test a separate passive adapter independently, recording each RJ45-to-DB9 connection.
- Assemble the passive cable and adapter, then check the complete device-side-to-host-side mapping.
- Check for unintended shorts as well as missing connections. Account for documented shared grounds; for example, the IR8340 assembly routes both device ground contacts to DB9 contact 5.
- Label the assembly with its measured map and intended equipment.
For an IR8340 assembly, device-side RJ45 contact 3 should reach DB9 contact 2. If it instead reaches DB9 contact 3, that assembly does not match the model-specific table above.
Do not use a passive continuity test to judge an integrated USB console cable. Its electronics interrupt the simple conductor path. Likewise, continuity results establish wiring only—not RS-232 voltage compatibility, driver operation, or reliable communication.
Match Terminal Settings to the Device
A correct cable can still produce no usable session when the serial settings are wrong.
For EX2300 initial configuration, Juniper specifies the following host settings. These are documented defaults, not proof that an already-configured switch still uses them.
| Parameter | EX2300 documented setting |
|---|---|
| Baud rate | 9600 baud |
| Data bits | 8 |
| Parity | None |
| Stop bits | 1 |
| Flow control | None |
| DCD state | Disregard |
Do not apply these settings to every device. Cisco’s general console document, for example, explicitly allows one or two stop bits depending on the router. Use the exact model’s connection instructions and any recorded configuration changes.
Open the serial port created by the actual adapter, not an assumed COM-port number. An RJ45 Ethernet management connection is a different path: Juniper documents separate Ethernet management and RS-232 console interfaces even though both use RJ45 connectors.
Isolate a Failed Console Connection
Use symptoms to choose the next check rather than changing several components at once.
| Symptom | Check next |
|---|---|
| USB adapter does not appear as a serial port | Adapter driver, USB connection, operating-system support, and adapter identification |
| Serial port opens, but the terminal remains blank | Correct device port, active console interface, documented cable assembly, baud rate, and flow-control settings |
| Text is garbled | Baud rate, data bits, parity, and stop bits before changing the pinout |
| Device output is readable, but keystrokes have no effect | Host transmit path, device receive path, flow control, and active input interface |
| Connection works with one assembly but not another | Compare complete continuity maps and adapter electrical interfaces rather than connector appearance |
On EX2300 models with both RJ45 and Mini-USB console interfaces, console activation and configuration can affect which path is usable. Juniper identifies the RJ45 console as active by default and documents a separate procedure for activating Mini-USB. The EX2300-24MP and EX2300-48MP have only an RJ45 console port, so do not apply the dual-port procedure to those models.
A readable login prompt demonstrates that the connected assembly and current settings can carry console traffic. It does not establish compatibility with another model, validate every control conductor, or certify the cable for other electrical interfaces.