History
ISO 7638 was first published in 1985 as the connector for trailer anti-lock brakes. In 1997 it was split into ISO 7638-1 for 24 V vehicles and ISO 7638-2 for 12 V, with housings coded so the two cannot be mated by mistake; it was revised in 2003 and the current edition dates from 2018. The standard also defines a park socket that holds the plug when the trailer is uncoupled.
When electronically controlled braking arrived, the tractor and trailer needed to exchange data, not just power. ISO 11992 (first editions 1998, revised 2003) defines that link: part 1 the physical and data-link layers, part 2 the brake and running-gear messages carried on ISO 7638 pins 6 and 7, part 3 other equipment, and part 4 diagnostics. European type-approval rules list which ISO 11992-2 messages are mandatory — such as brake demand and warning requests — and which are optional, such as brake-lining wear, tyre pressure, axle load and brake temperature.
Where you will find it
- Tractor-to-trailer ABS and EBS power and data
- Trailer brake warning lamp in the cab
- Trailer telematics reading EBS data (with an ISO 11992 interface)
- Truck and trailer workshops and brake testers
ISO 11992 is not ordinary CAN
The frames on pins 6 and 7 are standard CAN frames with J1939-style identifiers, but the physical layer is different from the ISO 11898-2 high-speed CAN on a diagnostic port:
- Point to point: each link joins exactly two nodes, tractor and trailer, so the system behaves the same whether a trailer is coupled or not.
- 125 kbit/s, with line voltages that follow the battery supply (fractions of 12 V or 24 V), not the 2.5 V levels of standard CAN.
- Termination is inside each transceiver — there are no 120 Ω end resistors, so never fit one across pins 6 and 7.
- Fault tolerant: the transceivers can carry on over a single wire if one line fails.
To read it with a normal CAN interface you need an ISO 11992 transceiver or a converter, such as the PEAK-System PCAN-B10011S, between your tool and the connector.
The 15-pin ISO 12098 lighting socket
The 24 V 15-pin lighting connector also carries ISO 11992 CAN — the part 3 network for equipment other than the brakes — so the brake data always stays on ISO 7638:
| ISO 12098 pin | Function | Wire |
|---|
| 13 | Common return for the data lines | 2.5 mm², white/red |
| 14 | CAN High (ISO 11992-3) | 1.5 mm², white/green |
| 15 | CAN Low (ISO 11992-3) | 1.5 mm², white/brown |
North America: SAE J560 and PLC4TRUCKS
US and Canadian trailers use the round 7-way SAE J560 connector instead, and it has no CAN. Pin 7 (blue) is the continuous ABS power circuit, and the trailer ABS reports faults to the cab as power-line carrier on that wire (PLC4TRUCKS, SAE J2497); in-cab trailer ABS warning lamps became mandatory there on 1 March 2001.
