Template:FMX6 DM1 DM2: Difference between revisions

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 With {{{model}}} you can read 2 types of DTC messages based on J1939 protocol:
-*DM1 – Active codes
+*DM1 – Communicates currently present faults
-*DM2 – Previously Active codes
+*DM2 – Reports stored faults
-{{{model}}} is able to read DM codes and pass them to the server in IO element. When active DM1 or DM2 messages appear on CAN line it is broadcasted very often – {{{model}}} device saves the codes into the internal memory and does not flood the server with irrelevant information – only new DTC codes are sent to the server.
+{{{model}}} is able to read DM codes and pass them to the server in IO element. When active DM1 messages are present on the CAN line, they are broadcast frequently. The {{{model}}} device stores these codes in its internal memory and transmits only newly detected DTC codes.
+DM2 messages are not streamed, but instead, they must be requested.
 ==Functionality Description==
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 '''39443030303330313A30313A30313B''' -> (after conversion from hex to ANSCII) '''9D000301:01:01''';
+==DM1/DM2 Message Structure==
+'''DM1''' is a J1939 diagnostic message used to broadcast active faults and warning lamp status. Its structure is split into a header (lamp status) followed by one or more diagnostic trouble codes. Unlike most J1939 messages, DM1 has a variable length: if there are 0–1 active DTCs, it fits in a single CAN frame, while 2 or more DTCs require a multi-frame message with additional DTC entries added.
+[[File:DM1 and DM2 Message Structure.png]]
+The first 2 bytes form the lamp status header, which is shared across all DTCs. Each lamp uses 2-bit values to indicate status (off/on), along with flash signals defining how the lamp blinks (slow or fast). These signals are interpreted collectively across ECUs, with a central controller typically deciding the final lamp behavior based on the most severe reported condition.
+Each DTC contains four key components:
+*'''SPN(''Suspect Parameter Number)''''' - Identifies the faulty subsystem.
+*'''FMI(''Failure Mode Identifier)''''' - Describes the type of failure.
+*'''CM(''SPN Conversion Method)''''' - Indicates the encoding method.
+*'''OC(''Occurrence Count)''''' - Counts how often the fault has occurred.
+These elements provide detailed, structured information about vehicle faults while supporting both standardized and proprietary diagnostics.
+==DM1 Lamp Status and Flash Signals==
+The '''DM1 (Diagnostic Message 1)''' in the '''J1939''' protocol reports active '''Diagnostic Trouble Codes (DTCs)''' and controls vehicle warning indicators. It defines the behavior of the '''Malfunction Indicator Lamp (MIL)''' and other warning lamps, which can be off, on solid, or flashing, depending on the severity and priority of detected faults. Flashing typically signals a more urgent or severe condition, while a solid light indicates an active but less critical issue.
+The first byte represents the status of four indicator lamps:
+*'''PL (Protect Lamp)''' - DTC's indicate non-electronic subsystem issue.
+*'''AWL(Amber Warning Light)''' - DTC's indicate a non-critical issue that does not warrant stopping the vehicle.
+*'''RSL(Red Stop Lamp)''' -  DTC's indicate a critical issue that warrants stopping the vehicle immediately.
+*'''MIL(Malfunction Indicator Lamp)''' - At least one DTC indicates emissions related issue.
+Each lamp is encoded using 2 bits, allowing four possible states: '''off, on, slow flash, and fast flash'''. This compact encoding means all lamp states are conveyed within a single byte, with each pair of bits mapped to a specific lamp in a fixed order. These lamp states directly inform the operator about the severity and urgency of active faults.
+DM1 encodes warning lamp information in its first 2 bytes, combining both lamp status  and flash behavior. Each lamp is represented by two 2-bit fields—one in byte 1 (status) and one in byte 2 (flash).
+To decode, split each byte into 2-bit segments and map each pair to its corresponding lamp. The final behavior is determined by combining status and flash (e.g., ON + fast flash = rapidly blinking warning).
+==Global and Manufacturer SPN Codes==
+===Global-Level SPN codes===
+Standard codes are defined by the SAE J1939 standards and are recognized across all compliant vehicles and equipment. The SPNs for these codes fall within the range of '''1 to 24,324''' representing widely used parameters such as engine speed, coolant temperature, or oil pressure. FMI values are standardized, describing specific failure patterns such as high voltage, circuit open, or out-of-range conditions.
+Because they are standardized, these codes are universally interpretable by any compliant diagnostic tool without requiring manufacturer-specific references.
+===Manufacturer-Level SPN codes===
+Manufacturer-level or proprietary codes are reserved for OEM-specific faults that are not defined in the J1939 standard. These allow manufacturers to monitor unique components, systems, or operational conditions that are specific to their equipment.
+The SPNs for proprietary codes typically occupy the high end of the 19-bit field, ranging from '''516,096 to 524,287'''. FMI values may be standard or custom, but the meaning of the SPN is defined by the manufacturer. Accurate interpretation requires access to OEM documentation, as these codes are not universally defined or interpretable.
+==Functionality Block Diagram==
+Graphic representation of '''DM1''' and '''DM2''' functionality:
+[[File:DTC Functionality blok diagram.png]]

Latest revision as of 07:59, 1 April 2026