<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class=""><br class=""></div><div class="">I’ve been going round and round in circles on this, most of the weekend, but not getting anywhere. I’ve rewritten the Decoder code four times so far, but just can’t get it to work. So calling for help here.</div><div class=""><br class=""></div><div class="">I attach the DBC specification. You can also find an extensive python library here:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><a href="https://github.com/eerimoq/cantools.git" class="">https://github.com/eerimoq/cantools.git</a></div></blockquote><div class=""><br class=""></div><div class="">(for anyone playing with DBC, that seems an excellent library and set of command line tools)</div><div class=""><br class=""></div><div class="">The seemingly simple problem that I am trying to solve is:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">Given a CAN frame, we need to decode a particular signal. We want the integer value (signed or unsigned) out of it.</div></blockquote><div class=""><br class=""></div><div class="">The signal species the start bit, size (number of bits), endian-ness (big or little), and value type (signed or unsigned). The DBC specification says:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><i class="">The start_bit value specifies the position of the signal within the data field of the frame. For signals with byte order Intel (little endian) the position of the least-significant bit is given. For signals with byte order Motorola (big endian) the position of the most significant bit is given. The bits are counted in a saw-tooth manner.</i></div><div class=""><i class=""><br class=""></i></div><div class=""><i class="">byte_order = '0' | '1' (* 0=little endian, 1=big endian *)<br class=""><br class="">The byte_format is 0 if the signal's byte order is Intel (little endian) or 1 if the byte order is Motorola (big endian).</i></div></blockquote><div class=""><br class=""></div><div class="">There are also factor and offset values for signals, but that is irrelevant for the moment (trivial to apply once we have the actual integer raw signal value extracted) and can be ignored for the moment.</div><div class=""><br class=""></div><div class="">Wikipedia explains endian-ness:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">In big-endian format, whenever addressing memory or sending/storing words bytewise, the most significant byte—the byte containing the most significant bit—is stored first (has the lowest address) or sent first, then the following bytes are stored or sent in decreasing significance order, with the least significant byte—the one containing the least significant bit—stored last (having the highest address) or sent last.<br class=""><br class="">Little-endian format reverses this order: the sequence addresses/sends/stores the least significant byte first (lowest address) and the most significant byte last (highest address).</div></blockquote><div class=""><br class=""></div><div class="">So, we have two strange things in the DBC specification: (1) the start bit is not always the first bit in the stream, and (2) this ’saw-tooth manner’ comment. So, I create a very simple DBC, and run it through the cantools dump to visualise it:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">$ cat mark.dbc</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">BU_: WS200 MCN_Powertrain</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">BO_ 1 LittleEnd: 8 WS200</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> SG_ OneByte : 7|8@0+ (1,0) [0|0] "" Vector__XXX</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> SG_ TwoByte : 15|16@0+ (1,0) [0|0] "" Vector__XXX</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> SG_ TwelveBit : 39|12@0+ (1,0) [0|0] "" Vector__XXX</span></font></div></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">$ cantools dump mark.dbc</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">================================= Messages =================================</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> ------------------------------------------------------------------------</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Name: LittleEnd</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Id: 0x1</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Length: 8 bytes</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Cycle time: - ms</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Senders: WS200</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Layout:</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Bit</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 7 6 5 4 3 2 1 0</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 0 |<-----------------------------x|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +-- OneByte</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 1 |<------------------------------|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 2 |------------------------------x|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> B +-- TwoByte</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> y +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> t 3 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> e +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 4 |<------------------------------|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 5 |--------------x| | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +-- TwelveBit</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 6 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 7 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Signal tree:</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> -- {root}</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +-- OneByte</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +-- TwoByte</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +-- TwelveBit</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> ------------------------------------------------------------------------</span></font></div></div></blockquote><div class=""><br class=""></div><div class="">I also used 'cantools generate_c_source mark.dbc’ to generate a C decoder for this, and this is what it looks like:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">int mark_little_end_unpack(</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> struct mark_little_end_t *dst_p,</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> const uint8_t *src_p,</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> size_t size)</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">{</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> if (size < 8u) {</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> return (-EINVAL);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> }</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> memset(dst_p, 0, sizeof(*dst_p));</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->one_byte |= unpack_right_shift_u8(src_p[0], 0u, 0xffu);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->two_byte |= unpack_left_shift_u16(src_p[1], 8u, 0xffu);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->two_byte |= unpack_right_shift_u16(src_p[2], 0u, 0xffu);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->twelve_bit |= unpack_left_shift_u16(src_p[4], 4u, 0xffu);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->twelve_bit |= unpack_right_shift_u16(src_p[5], 4u, 0xf0u);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> return (0);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">}</span></font></div></div></blockquote><div class=""><br class=""></div><div class="">Let’s look at that TwoByte value. This is little endian, so the least significant byte should come first in p[1] followed by the most significant in p[2]. So, why does the generator produce:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->two_byte |= unpack_left_shift_u16(src_p[1], 8u, 0xffu);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->two_byte |= unpack_right_shift_u16(src_p[2], 0u, 0xffu);</span></font></div></div></blockquote><div class=""><br class=""></div><div class="">From what I can see, that means take p[1] and shift it left 8, then add on p[2]. That seems big endian, not little endian?</div><div class=""><br class=""></div><div class="">Let’s try a big endian example:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">BO_ 3 BigEnd2: 8 WS200</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> SG_ EightBit2 : 5|8@1+ (1,0) [0|0] "" Vector__XXX</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> SG_ OneByte2 : 16|8@1+ (1,0) [0|0] "" Vector__XXX</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> SG_ TwelveBit2 : 48|11@1+ (1,0) [0|0] "” Vector__XXX</span></font></div></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Bit</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 7 6 5 4 3 2 1 0</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 0 |----------x| | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 1 | | | |<------------------|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +-- EightBit2</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 2 |<-----------------------------x|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> B +-- OneByte2</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> y +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> t 3 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> e +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 4 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 5 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 6 |------------------------------x|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 7 | | | | | |<----------|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +-- TwelveBit2</span></font></div></div></blockquote><div class=""><br class=""></div><div class="">Notice the weird alignment of EightBit2 and TwelveBit2? I presume that is the ’saw-tooth’ pattern talked about in the DBC specification.</div><div class=""><br class=""></div><div class="">A Tesla Model S encodes values like the battery temperature in CAN ID 0x102, and we decode this in C like this:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">StandardMetrics.ms_v_bat_temp->SetValue((float)((((int)d[7]&0x07)<<8)+d[6])/10);</span></font></div></blockquote><div class=""><br class=""></div><div class="">So that is little endian (d[7] is shifted right, followed by d[6] normally - so d[7] is the MSB and d[6] the LSB). But how to represent that in DBC?</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">BO_ 1 LittleEnd2: 8 WS200</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> SG_ TwoByte : 47|11@0+ (1,0) [0|0] "” Vector__XXX</span></font></div></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Bit</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 7 6 5 4 3 2 1 0</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 0 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 1 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 2 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> B 3 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> y +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> t 4 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> e +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 5 |<------------------------------|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 6 |----------x| | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +-- TwoByte</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 7 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+—+</span></font></div></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><div class=""> dst_p->two_byte |= unpack_left_shift_u16(src_p[5], 3u, 0xffu);</div><div class=""> dst_p->two_byte |= unpack_right_shift_u16(src_p[6], 5u, 0xe0u);</div></span></font></div></blockquote><div class=""><br class=""></div><div class="">That doesn’t work. How about big endian?</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">BO_ 1 BigEnd2: 8 WS200</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> SG_ TwoByte : 48|11@1+ (1,0) [0|0] "” Vector__XXX</span></font></div></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> Bit</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 7 6 5 4 3 2 1 0</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 0 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 1 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 2 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> B 3 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> y +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> t 4 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> e +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 5 | | | | | | | | |</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 6 |------------------------------x|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> 7 | | | | | |<----------|</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +---+---+---+---+---+---+---+---+</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> +— TwoByte</span></font></div></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""><br class=""></span></font></div><div class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->two_byte |= unpack_right_shift_u16(src_p[6], 0u, 0xffu);</span></font></div><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class=""> dst_p->two_byte |= unpack_left_shift_u16(src_p[7], 8u, 0x07u);</span></font></div></div></blockquote><div class=""><br class=""></div><div class="">That seems correct.</div><div class=""><br class=""></div><div class="">Looking in the source for cantools dbc (cantools/cantools/database/can/formats/dbc.py), I see:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Andale Mono" class=""><span style="font-size: 14px;" class="">byte_order=(0 if signal.byte_order == 'big_endian' else 1),</span></font></div></blockquote><div class=""><br class=""></div><div class="">This code also says the same thing (0 = big endian):</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><a href="https://github.com/julietkilo/CANBabel/blob/master/src/main/java/com/github/canbabel/canio/dbc/DbcReader.java" class="">https://github.com/julietkilo/CANBabel/blob/master/src/main/java/com/github/canbabel/canio/dbc/DbcReader.java</a></div><div class=""><br class=""></div><div class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre;">boolean</span><span style="caret-color: rgb(36, 41, 46); color: rgb(36, 41, 46); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre; background-color: rgb(255, 255, 255);" class=""> isBigEndian </span><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre;">=</span><span style="caret-color: rgb(36, 41, 46); color: rgb(36, 41, 46); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre; background-color: rgb(255, 255, 255);" class=""> </span><span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre;"><span class="pl-pds" style="box-sizing: border-box;">"</span>0<span class="pl-pds" style="box-sizing: border-box;">"</span></span><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre;">.</span><span style="caret-color: rgb(36, 41, 46); color: rgb(36, 41, 46); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre; background-color: rgb(255, 255, 255);" class="">equals(splitted[</span><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre;">2</span><span style="caret-color: rgb(36, 41, 46); color: rgb(36, 41, 46); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier, monospace; font-size: 12px; white-space: pre; background-color: rgb(255, 255, 255);" class="">]);</span></div></blockquote><div class=""><br class=""></div><div class="">WTF?</div><div class=""><br class=""></div><div class="">There is this (from googling), which makes things clear as mud:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><a href="https://github.com/ebroecker/canmatrix/wiki/signal-Byteorder" class="">https://github.com/ebroecker/canmatrix/wiki/signal-Byteorder</a></div><div class=""><a href="https://se.mathworks.com/help/vnt/ug/canpack.html" class="">https://se.mathworks.com/help/vnt/ug/canpack.html</a></div><div class=""><a href="https://github.com/ebroecker/SocketCandecodeSignals/blob/master/datenbasis.c#L112-L122" class="">https://github.com/ebroecker/SocketCandecodeSignals/blob/master/datenbasis.c#L112-L122</a></div><div class=""><a href="https://github.com/julietkilo/CANBabel/blob/master/src/main/java/com/github/canbabel/canio/dbc/DbcReader.java" class="">https://github.com/julietkilo/CANBabel/blob/master/src/main/java/com/github/canbabel/canio/dbc/DbcReader.java</a></div></blockquote><div class=""><br class=""></div><div class="">My conclusions are that:</div><div class=""><br class=""></div><div class=""><ol class="MailOutline"><li class="">The DBC specification is incorrect, in that 0 is big endian, and 1 is little endian in the public code we see.<br class=""><br class="">and/or<br class=""><br class=""></li><li class="">The DBC specification is incorrect, in that all CAN bus byte ordering is little endian, and the byte_order setting only affects the interpretation of the bit_ordering, and saw-tooth decode/encode.</li></ol></div><div class=""><br class=""></div><div class="">But that really makes no sense to me. This is supposed to be the defacto standard for automotive CAN bus signal definition - how can it be such a mess?</div><div class=""><br class=""></div><div class="">Help?</div><div class=""><br class=""></div><div class="">Mark</div><div class=""><br class=""></div><div class=""></div></body></html>