What Is the Difference Between J1939 and J1708?
SAE J1939 and SAE J1708 are both communication standards used in heavy-duty vehicles, but they differ significantly in architecture, speed, message structure, and capability.
In simple terms:
-J1708 is an older, low-speed serial communication standard.
-J1939 is a modern, high-speed CAN-based protocol that largely replaced J1708.
Understanding the difference is essential for diagnostics, system integration, and vehicle network design.
Protocol Overview
What Is SAE J1708?
SAE J1708 is a legacy communication standard introduced in the 1980s for heavy-duty vehicles.
Key characteristics:
1.Based on RS-485 serial communication
2.Operates at 9.6 kbps
3.Limited message size (typically 21 bytes)
4.Commonly paired with SAE J1587, which defines higher-layer message formatting
J1708 was widely used in older trucks for engine diagnostics and basic data exchange.
What Is SAE J1939?
SAE J1939 is a higher-layer protocol built on CAN (Controller Area Network).
Key characteristics:
1.Uses CAN bus (ISO 11898)
2.Operates typically at 250 kbps or 500 kbps
3.Supports many ECUs on the same network
4.Uses PGN (Parameter Group Number) and SPN (Suspect Parameter Number) to define standardized data
In modern vehicle architectures, a CAN bus controller manages message arbitration, data acquisition, and network communication between ECUs. This controller-based architecture allows J1939 networks to scale efficiently while maintaining reliable communication across multiple modules such as engine control, transmission control, and instrument displays.
J1939 became the dominant communication standard for modern heavy-duty vehicles due to its higher bandwidth and structured data model.
Technical Differences Between J1939 and J1708
1. Physical Layer
J1708 → RS-485 serial wiring
J1939 → CAN bus differential pair (CAN_H / CAN_L)
J1939 supports faster, more reliable communication in distributed systems managed by networked controllers.
2. Data Transmission Speed
J1708 → ~9.6 kbps
J1939 → 250 kbps (standard) and 500 kbps in some implementations
This major bandwidth difference allows J1939 networks to handle real-time control data, diagnostics, and distributed modules under a centralized CAN-based architecture.
3. Message Structure
J1708/J1587 uses MID (Message Identifier) and PID (Parameter Identifier).
J1939 uses PGN and SPN, providing a more scalable and standardized framework.
J1939 also supports multi-packet transport for larger data payloads, enabling complex vehicle systems to exchange more detailed operational data.
4. Error Handling and Network Management
J1939 inherits CAN's arbitration and error detection mechanisms, providing:
1.Message prioritization
2.Built-in fault confinement
3.Robust network management
In contrast, J1708 provides basic error checking but lacks CAN's advanced arbitration and reliability mechanisms.
When Did J1939 Replace J1708?
Did J1939 Officially Replace J1708?
There was no single official replacement date.
Instead, J1939 gradually replaced J1708/J1587 beginning in the late 1990s and early 2000s as OEMs transitioned to CAN-based architectures.
As vehicle electronics became more complex, J1708's limited bandwidth could no longer support expanding system requirements.
By the early 2000s, most new heavy-duty truck platforms were designed around J1939 networks.
Why the Transition Happened
The shift occurred because modern vehicles required:
1.Higher data bandwidth
2.Support for more ECUs
3.Advanced diagnostics
4.Telematics integration
5.Better network reliability
J1939 was designed specifically to meet these needs in modern distributed control systems.
Are J1708 Systems Still Used Today?
Yes, primarily in older vehicles.
1.Many pre-2005 heavy-duty trucks may still use J1708/J1587.
2.Some fleets operate mixed environments.
3.Diagnostic adapters often support both protocols.
However, modern heavy-duty vehicles and mobile machinery platforms are predominantly J1939-based.
Practical Implications
For Diagnostics
J1939 supports more detailed diagnostic data and standardized fault reporting across multiple controllers.
J1708 systems can still be serviced but often require legacy diagnostic tools.
For System Integration
J1939 is more suitable for:
1.Multi-controller architectures
2.Advanced display systems
3.Telematics gateways
4.Distributed I/O modules
5.Modern mobile machinery platforms
A CAN-based architecture built around a reliable CAN bus controller enables scalable communication across engine systems, hydraulic modules, instrument clusters, and other vehicle subsystems.
J1708, by contrast, is generally limited to legacy support scenarios.
Comparison Table: J1939 vs J1708
| Feature | J1708 | J1939 |
| Physical Layer | RS-485 serial | CAN bus |
| Typical Speed | 9.6 kbps | 250-500 kbps |
| Message Model | MID/ PID | PGN/SPN |
| Multi-Packet Support | No | YES |
| Network Management | Basic | Advanced(CAN arbitration) |
| Current Usage | Legacy vehicles | Modern heavy-duty systems |
Final Summary
J1708 is an older, low-speed serial protocol used in legacy heavy-duty vehicle systems.
J1939 is a modern, CAN-based communication protocol that provides higher speed, structured messaging, and better scalability.
J1939 gradually replaced J1708 starting in the late 1990s and early 2000s, and today it is the dominant standard in heavy-duty vehicle networks built on CAN architecture.
English
