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SAE J1939-15: Physical Layer for 250 Kbps Un-Shielded Twisted Pair Networks
Overview of SAE J1939-15
SAE J1939-15 defines the physical layer for a 250 kbps CAN-based network using un-shielded twisted pair (UTP). Part of the SAE J1939 family, this standard is designed for heavy-duty vehicles in horizontally integrated environments, ensuring that ECUs from multiple suppliers can communicate reliably. The 2018 revision supersedes J1939-15 AUG2017 and introduces allowances for unjacketed cable, clarifies node and stub spacing, updates ISO references, and corrects table references to J1939-11.
🔍 Design Insight: Using UTP reduces cost and weight compared to shielded twisted pair (STP), while still meeting the reliability requirements for on- and off-highway trucks, construction, and agricultural equipment.
Electrical Specifications and Bus Topology
The standard specifies differential voltage levels for dominant and recessive states, common mode voltage range, internal resistance, capacitance, and bit timing. The bus must be terminated at both ends with 120 Ω resistors (with tolerance that keeps the total system impedance near 60 Ω). The network topology must adhere to maximum stub lengths and minimum node spacing to avoid signal degradation.
| Parameter | Requirement |
|---|---|
| Bit Rate | 250 kbps |
| Bus Length (maximum) | 40 m |
| Termination Resistor | 120 Ω ± 1% |
| Differential Output (Dominant) | 1.5 V to 3.0 V |
| Differential Output (Recessive) | -50 mV to +50 mV |
| Common Mode Voltage | 1.5 V to 3.5 V |
| Maximum Stub Length (Type I) | 0.3 m |
| Maximum Stub Length (Type II) | 1.0 m (with lower capacitance) |
The standard also defines Type I and Type II ECU markings, which indicate the allowable stub length and internal capacitance. Compliance with these parameters is critical for reliable CAN communication.
Design, Compliance, and Common Pitfalls
Designers must carefully implement the conformance tests outlined in Section 6 of the standard, including recessive and dominant output levels, internal resistance, differential resistance, threshold levels, and internal delay time. Proper electrostatic discharge (ESD) protection and connector performance are also required.
⚠️ Common Mistake: Exceeding the maximum stub length or ignoring node spacing rules can cause reflections that disrupt network timing. Always verify that the bus topology meets J1939-15 requirements.
Additional issues arise from incorrect termination resistor values (must be 120 Ω typical) or neglecting common mode voltage range, leading to communication errors. It is also important to distinguish J1939-15 from other physical layers like J1939-11 (shielded) or J1939-14 (500 kbps).
Frequently Asked Questions
Q: How does J1939-15 differ from J1939-11?
A: J1939-15 uses un-shielded twisted pair (UTP) while J1939-11 uses shielded twisted pair (STP). Both operate at 250 kbps, but UTP offers lower cost and weight, making it suitable for many heavy-duty applications.
Q: What are the termination requirements?
A: Two 120 Ω termination resistors are placed at each end of the bus. ECUs must have internal differential resistance between 2 kΩ and 10 kΩ when powered on, and >20 kΩ when off.
Q: What is the maximum stub length allowed?
A: For Type I ECUs, maximum stub length is 0.3 m. Type II ECUs can have up to 1.0 m if the internal capacitance allows it. Always consult the standard for exact values.
Q: Can I use shielded cable with J1939-15?
A: The standard is designed for UTP, but if shielded cable is used, the shield should be grounded appropriately to avoid violating the un-shielded specification.
