pretty_j1939

python3 libs and scripts for pretty-printing J1939 candump logs.

This package can:

1. pretty-print J1939 traffic captured in candump logs AND
2. convert a J1939 Digital Annex (Excel) file into a JSON structure for use in the above. Support for both legacy .xls and modern .xlsx formats is included.

Some examples of pretty printing

Formatted content (one per line) next to candump data:

$ pretty_j1939 --candata --format example.candump.txt | head
(1543509533.000838) can0 10FDA300#FFFF07FFFFFFFFFF ; {
                                                   ;     "DA": "All(255)",
                                                   ;     "PGN": "EEC6(64931)",
                                                   ;     "SA": "Engine #1(  0)",
                                                   ;     "Engine Variable Geometry Turbocharger Actuator #1": "2.8000000000000003 [%]"
                                                   ; }
(1543509533.000915) can0 18FEE000#FFFFFFFFB05C6800 ; {
                                                   ;     "DA": "All(255)",
                                                   ;     "PGN": "VD(65248)",
                                                   ;     "SA": "Engine #1(  0)",

Single-line contents next to candump data:

$ pretty_j1939 --candata example.candump.txt | head
(1543509533.000838) can0 10FDA300#FFFF07FFFFFFFFFF ; {"SA":"Engine #1(  0)","DA":"All(255)","PGN":"EEC6(64931)","Engine Variable Geometry Turbocharger Actuator #1":"2.8000000000000003 [%]"}
(1543509533.000915) can0 18FEE000#FFFFFFFFB05C6800 ; {"SA":"Engine #1(  0)","DA":"All(255)","PGN":"VD(65248)","Total Vehicle Distance":"854934.0 [m]"}
(1543509533.000991) can0 08FE6E0B#0000000000000000 ; {"SA":"Brakes - System Controller( 11)","DA":"All(255)","PGN":"HRW(65134)","Front Axle, Left Wheel Speed":"0.0 [kph]","Front axle, right wheel speed":"0.0 [kph]","Rear axle, left wheel speed":"0.0 [kph]","Rear axle, right wheel speed":"0.0 [kph]"}
(1543509533.001070) can0 18FDB255#FFFFFFFF0100FFFF ; {"SA":"Diesel Particulate Filter Controller( 85)","DA":"All(255)","PGN":"AT1IMG(64946)","Aftertreatment 1 Diesel Particulate Filter Differential Pressure":"0.1 [kPa]"}
(1543509533.001145) can0 0CF00400#207D87481400F087 ; {"SA":"Engine #1(  0)","DA":"All(255)","PGN":"EEC1(61444)","Engine Torque Mode":"2 (Unknown)","Actual Engine - Percent Torque (Fractional)":"0.0 [%]","Driver's Demand Engine - Percent Torque":"0 [%]","Actual Engine - Percent Torque":"10 [%]","Engine Speed":"649.0 [rpm]","Source Address of Controlling Device for Engine Control":"0 [SA]","Engine Demand - Percent Torque":"10 [%]"}
(1543509533.001220) can0 18FF4500#6D00FA00FF00006A ; {"SA":"Engine #1(  0)","DA":"All(255)","PGN":"PropB_45(65349)","Manufacturer Defined Usage (PropB_PDU2)":"0x6d00fa00ff00006a"}
(1543509533.001297) can0 18FEDF00#82FFFFFF7DE70300 ; {"SA":"Engine #1(  0)","DA":"All(255)","PGN":"EEC3(65247)","Nominal Friction - Percent Torque":"5 [%]","Estimated Engine Parasitic Losses - Percent Torque":"0 [%]","Aftertreatment 1 Exhaust Gas Mass Flow Rate":"199.8 [kg/h]","Aftertreatment 1 Intake Dew Point":"0 (00 - Not exceeded the dew point)","Aftertreatment 1 Exhaust Dew Point":"0 (00 - Not exceeded the dew point)","Aftertreatment 2 Intake Dew Point":"0 (00 - Not exceeded the dew point)","Aftertreatment 2 Exhaust Dew Point":"0 (00 - Not exceeded the dew point)"}
(1543509533.001372) can0 1CFE9200#FFFFFFFFFFFFFFFF ; {"SA":"Engine #1(  0)","DA":"All(255)","PGN":"EI1(65170)"}
(1543509533.001447) can0 18F00131#FFFFFF3F00FFFFFF ; {"SA":"Cab Controller - Primary( 49)","DA":"All(255)","PGN":"EBC1(61441)","Accelerator Interlock Switch":"0 (00 - Off)","Engine Retarder Selection":"0.0 [%]"}
(1543509533.001528) can0 18FEF131#F7FFFF07CCFFFFFF ; {"SA":"Cab Controller - Primary( 49)","DA":"All(255)","PGN":"CCVS1(65265)","Cruise Control Pause Switch":"1 (01 - On)","Cruise Control Active":"0 (00 - Cruise control switched off)","Cruise Control Enable Switch":"0 (00 - Cruise control disabled)","Brake Switch":"1 (01 - Brake pedal depressed)","Cruise Control Coast (Decelerate) Switch":"0 (00 - Cruise control activator not in the position \"coast\")","Cruise Control Accelerate Switch":"0 (00 - Cruise control activator not in the position \"accelerate\")"}

Formatted contents of complete frames only.

$ pretty_j1939 --format --no-link example.candump.txt | head
{
    "PGN": "AT1HI1(64920)",
    "Aftertreatment 1 Total Fuel Used": "227.5 [liters]",
    "Aftertreatment 1 DPF Average Time Between Active Regenerations": "173933 [Seconds]",
    "Aftertreatment 1 DPF Average Distance Between Active Regenerations": "1460.5 [m]"
}
{
    "PGN": "AT1HI1(64920)",
    "Aftertreatment 1 Total Fuel Used": "227.5 [liters]",
    "Aftertreatment 1 DPF Average Time Between Active Regenerations": "173933 [Seconds]",

The JSON output can be used as an input to jq to filter or format the decoded data. E.g. we can show only messages from the "Brakes":

$ pretty_j1939 example.candump.txt --format | jq ". | select(.SA | contains(\"Brakes\"))"
{
  "PGN": "TSC1(0)",
  "DA": "Retarder - Engine( 15)",
  "SA": "Brakes - System Controller( 11)",
  "Engine Requested Speed/Speed Limit": "8031.875 [rpm]",
  "Engine Requested Torque/Torque Limit": "-125 [%]"
}
{
  "PGN": "TSC1(0)",
  "DA": "Retarder - Driveline( 16)",
  "SA": "Brakes - System Controller( 11)",
  "Engine Requested Speed/Speed Limit": "8031.875 [rpm]",
  "Engine Requested Torque/Torque Limit": "-125 [%]"
}
{
  "PGN": "TSC1(0)",
  "DA": "Retarder, Exhaust, Engine #1( 41)",
  "SA": "Brakes - System Controller( 11)",
  "Engine Requested Speed/Speed Limit": "8031.875 [rpm]",
  "Engine Requested Torque/Torque Limit": "-125 [%]"
}
{
  "PGN": "EBC1(61441)",
  "DA": "All(255)",
  "SA": "Brakes - System Controller( 11)",
  "ASR Brake Control Active": "0 (00 - ASR brake control passive but installed)",
  "Anti-Lock Braking (ABS) Active": "0 (00 - ABS passive but installed)",
[...]

Installing

pip install pretty_j1939

HOWTO

First, obtain a copy of the digital annex, see https://www.sae.org/standards/content/j1939da_201907/ for details.

Then, use the create_j1939db-json script to convert that Digital Annex into a JSON file. Both .xls and .xlsx files are supported:

create_j1939db-json -f tmp/J1939DA_DEC2020.xlsx -w tmp/J1939DA_DEC2020.json

Storing and Using your J1939db.json

The tool looks for J1939db.json in the following locations (in order):

1. The path provided via --da-json
2. The current working directory
3. The user's configuration directory:
   • Windows: %APPDATA%\pretty_j1939\J1939db.json
   • Linux/macOS: ~/.config/pretty_j1939/J1939db.json
4. Fallback: A default (very limited) J1939db.json bundled with the package. This .json is built from freely available information only.

To use your own database, simply place it in one of the search locations or specify it on the command line:

pretty_j1939 example.candump.txt --da-json my_full_db.json

Network Summary

The tool can generate a Mermaid flowchart summary of all captured traffic. This is useful for visualizing the network topology and message flow between Controller Applications.

pretty_j1939 example.candump.txt --summary

The output is a JSON object with a Summary key containing the Mermaid syntax:

{
    "Summary": "graph LR; N0[\"Engine #1(0)\"]; All[\"All(255)\"]; N0 -- EEC1(61444) --> All"
}

When using --format, the summary is printed in a multi-line, human-readable format:

{
    "Summary": "graph LR
                   N0[\"Engine #1(0)\"]
                   All[\"All(255)\"]
                   N0 -- EEC1(61444) --> All"
}

Advanced Filtering

In addition to support for the python-can bitmask filters, you can filter by J1939-specific fields:

• --filter-pgn: Filter by PGN (e.g., 61444 or 0xF004).
• --filter-sa: Filter by Source Address.
• --filter-da: Filter by Destination Address.
• --filter-ca: Filter by Controller Application address (matches either SA or DA).

Example: Show all traffic involving address 11 (Brakes):

pretty_j1939 example.candump.txt --filter-ca 11

Highlighting

You can highlight specific messages based on J1939 fields. Highlighting overrides the default theme colors for the entire line with a high-contrast style.

• --highlight-pgn: Highlight by PGN.
• --highlight-sa: Highlight by Source Address.
• --highlight-da: Highlight by Destination Address.
• --highlight-ca: Highlight by Controller Application address.

Example: Highlight all engine-related traffic while showing everything:

pretty_j1939 example.candump.txt --highlight-ca 0 --color always

Note: Highlighting requires --color to be active (always or auto when outputting to a terminal).

Live Capture

pretty-j1939 supports live CAN capture using python-can. You can specify the interface, channel, and bitrate:

pretty_j1939 -i cantact -c 0 -b 500000 --candata

Additional driver-specific arguments can be passed using --key=value syntax:

pretty_j1939 -i vector -c 1 --app-name=MyCanApp

The script also supports multiple log formats when reading from files or pipes, including standard candump and python-can logger output. It also accepts stdin using -:

tail -f /var/log/can.log | pretty_j1939 -

CANdump Format

The --candata flag supports two modes:

• --candata=raw (or just --candata): Prints the input line exactly as provided.
• --candata=candump: Reformats the input into the standardized (TIMESTAMP) INTERFACE ID#DATA format, even when capturing live or reading from different log formats.

Library Usage

The pretty_j1939 library is designed for high-performance decoding and rendering in other Python projects.

Basic Usage

You can pass the CAN ID as an integer and the message data as either bytes or a bitstring.Bits object.

import pretty_j1939.describe
import pretty_j1939.render

# 1. Initialize the describer (supports JSON paths or in-memory dicts)
describer = pretty_j1939.describe.get_describer(da_json="J1939db.json")

# 2. Initialize the high-performance renderer with an optional theme and describer for label resolution
theme = pretty_j1939.render.HighPerformanceRenderer.load_theme("darcula")
renderer = pretty_j1939.render.HighPerformanceRenderer(
    theme_dict=theme, 
    color_system="truecolor",
    da_describer=describer.da_describer
)

# 3. Describe and render a frame
can_id = 0x0CF00400
can_data = b"\x00\x41\xFF\x20\x48\x14\x00\xF0"

description = describer(can_data, can_id)
output = renderer.render(description, indent=True)
print(output)

Transport Reassembly (J1939 TP & ISO-TP)

The library automatically handles multi-packet reassembly for standard J1939 Transport Protocol (BAM and RTS-CTS). ISO-TP (PGN 0xDA00) reassembly works similarly and is enabled by default.

# Feed sequential frames of a J1939 BAM session
# 1. Connection Management (BAM) - PGN 61444 (EEC1), 14 bytes, 2 packets
describer(b"\x20\x0E\x00\x02\xFF\x04\xF0\x00", 0x18ECFF00)
# 2. Data Transfer Packet 1
describer(b"\x01\x01\x02\x03\x04\x05\x06\x07", 0x18EBFF00)
# 3. Data Transfer Packet 2 (Final)
res = describer(b"\x02\x08\x09\x0A\x0B\x0C\x0D\x0E", 0x18EBFF00)

# 'res' will contain the decoded description of the reassembled PGN 61444
print(res["PGN"]) # "EEC1(61444)"

Generating a Network Summary

At the end of a session, you can generate a Mermaid flowchart representing the network activity.

# 4. Generate and print a network summary
summary_data = describer.get_summary()
summary_output = renderer.render_summary(summary_data, indent=True)
print(summary_output)

Testing

Core Unit Tests

The package includes an in-tree unit test suite based on pytest. These tests verify core reassembly logic, PGN/SPN decoding, and CLI functionality using the bundled default database.

python -m pytest

Integration Tests

The verify_all.py script runs the core pytest suite and then attempts additional "extensive" tests that require a full database at tmp\J1939db.json (although they should do a cursory job with the default database).

python verify_all.py

Notes on Digital Annex Sources

You need to obtain a J1939 Digital Annex from the SAE to create a JSON file that can be used by pretty_j1939 see https://www.sae.org/standards/content/j1939da_201907/ for details.

There are multiple releases; here are a couple notes to consider when purchasing your copy of the Digital Annex.

• the 201611 Digital Annex has fewer defined SPNs in it than the 201311 Digital Annex; at some point the owners of the DA started migrating 'technical' SPNs (e.g. DMs) to other documents and out of the DA
• the 201311 Digital Annex has a couple bugs in it that the create_j1939db-json has workarounds for
• the create_j1939db-json can also handle the XLS Export from isobus.net by supplying multiple excel sheets as input (with multiple -f arguments); however, the isobus.net definitions omit almost all of the commercial vehicle SPNs and PGNs so the resulting J1939db.json file may not be of great use in examining candump captures from commercial vehicles.

Future Work

• port this functionality to the python-j1939 and python-can projects
• integrate and/or move create_j1939-db-json.py to canmatrix