Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Understanding the SAE J1939-11 Physical Layer: 250 Kbps Twisted Shielded Pair
The SAE J1939-11-2016 standard defines the physical layer requirements for a high-speed CAN-based communication network operating at 250 kbps over a twisted shielded pair. This specification is a critical component of the SAE J1939 family, designed for heavy-duty vehicles and equipment such as trucks, buses, construction, and agricultural machinery. It ensures interoperability among Electronic Control Units (ECUs) from various manufacturers by specifying network topology, electrical parameters, connector configurations, and conformance tests.
Network Architecture and Physical Media
The network is based on a backbone topology with stubs, using a twisted shielded pair cable. The characteristic impedance of the cable is 120 ohms, and each end of the backbone must be terminated with a 120-ohm resistor to minimize reflections. For improved EMI performance, a split termination using two 60-ohm resistors and a capacitor to ground can be implemented. The bus length and stub lengths are limited to ensure signal integrity at 250 kbps; typical maximum backbone length is up to 40 meters, and stub lengths should not exceed 1 meter. The network supports up to 30 nodes, with careful attention to node capacitance.
Design Insight: Implementing split termination helps reduce RF emissions and improve common-mode noise immunity. Proper shield grounding is essential: ground the shield at one point only to avoid ground loops. 🛠️
The standard defines two types of ECUs: Type I and Type II, with different connector options to accommodate various integration needs. Color coding of the network cables is specified to prevent miswiring: CAN_H is yellow, CAN_L is green, and ground and shield have designated colors.
| Parameter | Specification |
|---|---|
| Data Rate | 250 kbps |
| Medium | Twisted shielded pair |
| Cable Impedance | 120 ohms |
| Termination | 120 ohms at both ends (or split: 2x 60 ohms + capacitor) |
| Maximum Backbone Length | 40 m (dependent on cable quality) |
| Maximum Stub Length | 1 m |
| Maximum Nodes | 30 (limited by bus load and timing) |
| Connector Types | Type I and Type II (different pinouts) |
| Color Code | CAN_H: Yellow, CAN_L: Green, GND: Black, Shield: Braid |
Electrical Specifications and Design Considerations
The physical layer must comply with the electrical parameters defined in ISO 11898-1:2015 and this standard. The differential bus voltage levels are specified for recessive and dominant states. The standard also defines common-mode voltage range, internal resistance and capacitance of ECUs, and bit timing requirements.
- Differential Voltage: Recessive state: less than 0.5 V; Dominant state: greater than 0.9 V (typically 1.5 V to 3.0 V).
- Common-Mode Voltage Range: -2 V to +7 V relative to ground.
- Internal Resistance (ECU): Between CAN_H and CAN_L, and to ground.
- Internal Capacitance: Limits number of nodes and influences signal timing.
- Bit Timing: As per ISO 11898-1, with specific internal delay times.
Common Mistake ⚠️: Using a termination resistor only at one end or using values other than 120 ohms can cause signal reflections and communication errors. Always place one 120-ohm resistor at each end of the backbone.
Conformance tests are specified to verify that ECUs meet the electrical requirements. These tests include recessive and dominant output levels, input thresholds, internal resistance, and delay time. Ensuring compliance with these tests is essential for robust network operation.
Frequently Asked Questions
What is the maximum recommended cable length for a J1939-11 network?
At 250 kbps, the maximum backbone length typically is 40 meters with suitable quality twisted shielded pair cable. Longer lengths may require slower bit rates or additional devices like repeaters.
Can I use a 120-ohm resistor at only one end of the bus?
No. The standard requires a 120-ohm termination resistor at each end of the backbone to match the cable impedance and prevent signal reflections. Omitting one termination will lead to communication issues.
What is the difference between Type I and Type II ECUs?
Type I ECUs typically use a 9-pin Deutsch connector (DT04-9P), while Type II ECUs may use a larger connector with additional pins for power and other signals. Both types are defined to accommodate different vehicle integration requirements.
How does split termination benefit the network?
Split termination uses two 60-ohm resistors with a capacitor to ground. This configuration improves electromagnetic compatibility by reducing common-mode noise and controlling RF emissions, especially in harsh automotive environments.
