If you’ve dived into the world of vehicle diagnostics, you’ve probably seen the terms J1587 and J1939 pop up especially when dealing with heavy-duty vehicles. These two protocols are part of the backbone of how trucks, buses, and off-highway machinery communicate. But what's the difference, and why does it matter?
Let’s break it down in a no-nonsense, easy-to-understand way.
What is J1587?
Background and Purpose
J1587 is an older diagnostic protocol developed by the Society of Automotive Engineers (SAE). It works over the J1708 physical layer, a serial data bus, and was widely used in heavy-duty vehicles in the 1990s and early 2000s.
Key Features of J1587
- Based on 8-bit ASCII message format.
- Runs on the J1708 bus (a 2-wire, 9600 bps serial connection).
- Message IDs are limited and less structured.
- Mainly used for basic diagnostics and fault code reporting.
- Low data speed, but reliable and simple.
What is J1939?
Modern Replacement for J1587
J1939 was introduced as a more modern, flexible, and robust protocol built on the CAN (Controller Area Network) bus system. It's now the standard in most new heavy-duty vehicles, replacing J1587/J1708 in the majority of applications.
Main Characteristics of J1939
- Runs on CAN bus (250 kbps or 500 kbps).
- Structured messaging using PGNs (Parameter Group Numbers).
- Designed for real-time communication between multiple ECUs.
- High bandwidth and support for more complex data.
- Ideal for data logging, fleet monitoring, and emission control.
J1587 Protocol Breakdown
| Aspect | Details |
|---|---|
| Physical Layer | J1708 (RS485-like serial bus) |
| Baud Rate | 9600 bits per second |
| Message Structure | ASCII-based, limited length (21 bytes) |
| Device Addressing | Numerical IDs, less dynamic |
| Use Cases | Fault code reporting, basic diagnostics |
| Reliability | High, but limited performance |
| Network Type | Peer-to-peer |
J1939 Protocol Breakdown
| Aspect | Details |
|---|---|
| Physical Layer | CAN bus (ISO 11898) |
| Baud Rate | Typically 250 kbps or 500 kbps |
| Message Structure | Binary format, supports extended frame formats |
| Addressing | Supports both static and dynamic addressing |
| Use Cases | Engine, transmission, brakes, emission, sensors |
| Reliability | Very high, supports real-time communication |
| Network Type | Broadcast and peer-to-peer |
J1587 vs J1939 - Key Differences Table
| Feature | J1587 | J1939 |
|---|---|---|
| Protocol Foundation | Serial (J1708) | CAN (Controller Area Network) |
| Baud Rate | 9600 bps | 250 kbps or 500 kbps |
| Message Format | ASCII, 21-byte messages | Binary, supports longer frames |
| PGN/SPN Support | ❌ Not supported | ✔️ Core structure based on PGNs/SPNs |
| Diagnostic Capability | Basic (mainly DTCs) | Advanced (real-time data, control, monitoring) |
| Message Type | Simple messages | Complex, structured messages |
| Multinode Communication | Limited | Excellent support |
| Used In | Pre-2007 heavy-duty vehicles | 2007+ heavy-duty and modern fleet systems |
| Data Logging | Minimal | Extensive |
| Real-Time Communication | ❌ Not designed for it | ✔️ Designed for real-time response |
| Vehicle Network Size | Small scale | Large multi-ECU environments |
| Software Flashing | ❌ Not supported | ✔️ Supported in advanced systems |
Use Cases
| Use Case | J1587 | J1939 |
|---|---|---|
| Diagnostic Trouble Codes | ✔️ Basic | ✔️ Advanced |
| Real-Time Data Monitoring | ❌ Limited | ✔️ Yes |
| Fleet Management Systems | ⚠️ Basic Logs Only | ✔️ Fully Supported |
| Emissions Monitoring | ❌ Not Supported | ✔️ Mandatory in most cases |
| Engine/Brake Control Data | ⚠️ Some data only | ✔️ Comprehensive |
| ECU-to-ECU Communication | ❌ Not native | ✔️ Designed for it |
Performance Comparison: J1587 vs J1939
| Metric | J1587 | J1939 |
|---|---|---|
| Speed | Low | High |
| Bandwidth | Limited | High |
| Diagnostic Range | Basic | Deep & complex |
| Message Structuring | Simple, flat | Hierarchical, modular |
| Implementation Cost | Low | Moderate |
| Future-Proofing | ❌ Obsolete | ✔️ Industry standard |
Limitations of Each Protocol
| Limitation | J1587 | J1939 |
|---|---|---|
| Data Speed | Slow | Fast |
| Modern ECU Compatibility | ❌ Rarely supported today | ✔️ Fully compatible |
| Real-Time Data Support | ❌ Not optimized | ✔️ Primary use |
| Message Size Limitations | 21 bytes max | Up to 1785 bytes (via BAM transport) |
| PGN/SPN Usage | ❌ Not available | ✔️ Rich structure |
| Diagnostics Scope | Basic trouble codes | Full access to vehicle subsystems |
The Shift from J1587 to J1939
The transition from J1587/J1708 to J1939 happened around 2007, when emission regulations became stricter, and more real-time monitoring was needed. OEMs and fleet managers demanded better data, faster communication, and multi-ECU support—which J1939 delivers exceptionally well.
Future of Vehicle Communications
While J1587 served the industry well for decades, its time has passed. Today’s trucks and machinery need smarter diagnostics, faster communication, and better integration with telematics platforms. J1939 is expected to evolve further with extensions like J1939-22 and integrations with DoIP and Ethernet-based communication in the future.
Conclusion
When it comes to J1587 vs J1939, the choice is clear if you're working with modern vehicles. J1587 was great for its time—simple, effective, and widely used. But the future belongs to J1939, with its robust data handling, real-time capabilities, and multi-device communication. Whether you’re a developer, fleet technician, or vehicle system integrator, understanding both protocols is key—but implementing J1939 is the smart move for future-proofing your systems.
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