User manual

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Appendix F J1939 Old DTC Conversion Methods
This appendix was created to assist the user in decoding the information
contained in the modbus DTC (DM1 and DM2) registers if the generator ECM
does not support the newest DTC conversion method. The diagnostic trouble
codes on J1939 are specified in a specific format. Older J1939 specifications
had three conversion methods for the SPN and it was impossible to tell them
apart from the DTC data alone. One had to consult the engine manufacturer.
Newer J1939 specifications follow one SPN method (Version 4) and the user can
determine if this method applies by looking at the CM bit. It will be set to 1 if
Version 4 applies, and set to 0 if Versions 1, 2, or 3 applies. If the CM bit is 0,
see below for the three conversion methods. Byte 1 to Byte 4 refers to the
individual bytes in the GSC400 DTC Modbus registers. See registers 40115 and
40116 on page 144, above.
DTC Conversion Method (Version) 1:
Byte 1: 8 most significant bits of 16 most significant bits of SPN
Byte 2: 8 least significant bits of 16 most significant bits of SPN
Byte 3: 3 most significant bits of byte contain the 3 least significant bits of SPN
5 least significant bits of byte contain the FMI
Byte 4: most significant bit of byte contains CM
7 least significant bits of byte contains OC
DTC Conversion Method (Version) 2:
Byte 1: 8 least significant bits of 16 most significant bits of SPN
Byte 2: 8 most significant bits of 16 most significant bits of SPN
Byte 3: 3 most significant bits of byte contain the 3 least significant bits of SPN
5 least significant bits of byte contain the FMI
Byte 4: most significant bit of byte contains CM
7 least significant bits of byte contains OC
DTC Conversion Method (Version) 3:
Byte 1: 8 least significant bits of SPN
Byte 2: 8 second byte of SPN
Byte 3: 3 most significant bits of byte contain the 3 most significant bits of SPN
5 least significant bits of byte contain the FMI
Byte 4: most significant bit of byte contains CM
7 least significant bits of byte contains OC
DTC conversion method 3 is the same as DTC conversion method 4 (the new
J1939 standard) except that the CM bit is 1 so it is impossible to tell it apart from
versions 1 and 2. Version 4 has the bit set to 0 which allows the user to know
the conversion format is Version 4 without having to consult the engine
manufacturer.