If you have ever wondered how an OBD-to-RJ45 cable is wired internally, the answer lies in the specific arrangement and mapping of these wires.
Taking the most common BMW ENET cable, as well as general-purpose Ethernet diagnostic cables compliant with the ISO 13400-4 DoIP standard, as an example, the four Ethernet pins on the OBD connector correspond to the four pins on the RJ45 connector as follows:
| OBD Pin (Vehicle Side) | Signal Definition | → | RJ45 Pin (PC Side) | Signal Definition |
|---|---|---|---|---|
| Pin 3 | Ethernet Rx (+) Receive Positive | → | Pin 1 | Tx+ Transmit Positive |
| Pin 11 | Ethernet Rx (-) Receive Negative | → | Pin 2 | Tx- Transmit Negative |
| Pin 12 | Ethernet Tx (+) Transmit Positive | → | Pin 3 | Rx+ Receive Positive |
| Pin 13 | Ethernet Tx (-) Transmit Negative | → | Pin 6 | Rx- Receive Negative |
Two Additional Key Connections
| OBD Pins | Connection | Function |
|---|---|---|
| Pin 4 & Pin 5 | Shorted together | Combines chassis ground and signal ground to provide a common ground reference |
| Pin 8 - Pin 16 | Connected through a 510Ω–560Ω resistor | DoIP activation line - when the diagnostic tool is connected, the gateway detects the resistor value and recognizes that Ethernet diagnostic mode is ready, thereby activating DoIP communication |
Cable Type
Most DIY tutorials and commercially available ENET cables use CAT5e or CAT6 Ethernet cable.
You can strip a standard Ethernet cable and access its four twisted pairs. Only four wires are required for the Ethernet signal connection, typically:
Green/White
Green
Orange/White
Orange
The remaining wires can be shortened or left unused.
Important: The wire colors inside an Ethernet cable are only a reference based on standard wiring conventions. The actual pin-to-pin connections are what matter. As long as OBD Pins 3/11/12/13 are correctly connected to RJ45 Pins 1/2/3/6 respectively, the cable can operate normally.
Why Is the Wiring "Crossover" Instead of "Straight-Through"?
Looking at the pinout table above, you may notice an important detail:
The OBD receive lines (Rx) are connected to the RJ45 transmit lines (Tx), while the OBD transmit lines (Tx) are connected to the RJ45 receive lines (Rx).
This is not a wiring mistake - it is an intentional crossover connection.
To understand why, consider how a standard Ethernet cable works.
When connecting two computers with an Ethernet cable, the transmit (Tx) side of Computer A must connect to the receive (Rx) side of Computer B. Likewise, Computer B's transmit side must connect to Computer A's receive side.
If Tx is connected directly to Tx and Rx to Rx, both sides would be trying to transmit on the same channel while listening on the other, preventing proper data communication.
An OBD-to-RJ45 cable works on the same principle.
The vehicle-side ECU transmits data through OBD Pin 12 and Pin 13 (Tx+ / Tx-). These signals must reach the computer's network interface receive lines, RJ45 Pin 3 and Pin 6 (Rx+ / Rx-), so that the computer can receive data from the vehicle.
Conversely, the computer sends commands through the network adapter's transmit lines, RJ45 Pin 1 and Pin 2 (Tx+ / Tx-). These signals must reach the vehicle ECU's receive lines, OBD Pin 3 and Pin 11 (Rx+ / Rx-), allowing the vehicle to receive commands from the computer.
This is why:
OBD Pin 3 → RJ45 Pin 1 (Rx → Tx)
OBD Pin 12 → RJ45 Pin 3 (Tx → Rx)
This constitutes a standard crossover cable configuration.
If the cable were wired as a straight-through connection - for example, Pin 3 → Pin 3 and Pin 12 → Pin 12 - the transmit lines on both sides would be connected together, as would the receive lines. Neither side would be able to properly receive the other's data.
This incorrect wiring can cause connection errors such as "No ZGW Available" when using DIY ENET cables. In many cases, the underlying problem is that the cable has been wired straight-through instead of using the required crossover configuration.
How the Shielded Twisted Pair Is Constructed Internally
Another key design element of an OBD-to-RJ45 cable is the twisted pair, which is fundamental to stable high-speed Ethernet data transmission.
What Is a Twisted Pair?
If you open a standard CAT5e or CAT6 Ethernet cable, you will find four pairs of wires inside. Each pair consists of two conductors twisted tightly together, forming a spiral-like structure.
The two conductors of the Tx pair are twisted together, while the two conductors of the Rx pair are twisted together.
Why Are the Wires Twisted Together?
The inside of a vehicle is an electrically noisy environment.
Alternators, ignition coils, electric motors, relays, and other electrical components can continuously generate electromagnetic interference (EMI).
If two parallel conductors were used to carry high-speed signals, external interference could easily couple into the signal lines, resulting in data transmission errors.
The principle behind twisted-pair wiring is to tightly twist the two signal conductors together so that both experience nearly identical external electromagnetic conditions.
When interference occurs, almost the same noise is induced on both conductors.
At the receiver, the circuit detects the voltage difference between the two conductors - the differential signal - rather than the absolute voltage of either individual conductor.
As a result, common-mode noise on both conductors is largely canceled out, while the useful differential signal remains intact.
What Is the Shielding Layer?
In addition to the inherent EMI resistance of twisted-pair wiring, many high-quality ENET cables also include a metal foil or braided shielding layer around the cable assembly.
This configuration is known as Shielded Twisted Pair (STP).
The shielding layer surrounds the twisted pairs and acts much like a Faraday cage, helping prevent external electromagnetic fields from interfering with the signal conductors.
An often-overlooked detail in DIY ENET cable construction is connecting the metal shell of the OBD connector to the metal shielding shell of the RJ45 connector using a brown wire or another unused conductor.
This creates a continuous shielding path from the vehicle side all the way to the metal housing of the computer's network connector.
An improperly grounded cable shield can significantly reduce shielding effectiveness. In some situations, an improperly connected shield may even behave like an antenna and pick up or conduct interference.
Properly grounding the shield is therefore important for allowing it to perform its intended EMI protection function.
Internal Structure at a Glance
Summary
The internal structure of an OBD-to-RJ45 ENET cable essentially consists of:
4 signal wires connected in a crossover configuration
1 activation resistor
1 ground connection
Twisted-pair wiring
A shielding layer
Together, these features provide stable high-speed signal transmission in the electrically noisy automotive environment.
When you connect an ENET cable to the vehicle's OBD port, you are not simply connecting two physical interfaces. You are establishing a carefully designed high-speed differential signal path between the vehicle ECU and the computer's network interface.
