MS-II™ Serial Interface Description

MegaSquirt-II uses a baud rate of 115200 to communicate with the tuning software. Communication is established when the tuning software sends a command character - the particular character sets the mode:

The primary commands for sending and receiving data from MS II via the RS232 serial connection are all constructed using 3 initial bytes as follows:

1. one-letter command: The possibilities for the one letter command are:
   • 'a' command. Send 'a', <CANid>, <tble_idx>. This causes the 112 bytes from the outpc. structure in MS-II to be sent back to the sender of this command. What is special about this command is that the data is buffered as it is sent so that a word can not be changed between bytes. This isn't necessary with inputs, because they are not changed in the code except through the serial interface.
   • 'r' command is a generic form of the 'a' command. It is used to read inputs and the returned bytes from MS II are not buffered.

     Send 'r', <CANid>, followed by 2 bytes specifying the offset from the array or structure corresponding to tble_idx, followed by 2 bytes specifying the total number of data bytes desired. The offset word and the number of bytes word must each be sent msb (most significant byte) first, then lsb (least significant byte). After all 7 bytes are received, MS-II™ will start sending the requested number of bytes. Thus to duplicate the 'a' command, you could send 'r', 0, 6, 0, 0, 0 (msb of 112), 112.

   • 'w' command is the reverse of the 'r' command - it writes to MS-II™. The format of the command is identical to 'r', except send 'w' and follow the seventh byte with the nbytes of data to be written to MS-II™ RAM.
   • 'e' (echo) command is the same as 'w', except that after MS-II has received all the bytes from the sender, it echoes them back for verification. Thus a sender who has sent n bytes to be written should expect n bytes back from MS-II™ starting after he has sent the last byte to be written.
   • 'y' command has format 'y', <CANid>, <tble_idx> and is used to verify that input data in a RAM block matches the corresponding flash block. This is typically used after a flash burn command to verify the data were correctly burned. MS-II sends back a 2 byte word, lsb first, that holds the number of times a ram and flash byte pair failed to match. So if it returns a zero, all is good.
   • 'b' command has format 'b', <CANid>, <tble_idx> and is used to tell MS II to burn the RAM copy of <tbl_idx> into the flash copy of it.

   There are also some specialized commands for MS-II. These are:

   • 't' command. This command is for re-flashing new coolant, MAT, EGO, and MAF tables. These tables are all 1024 word (2byte) tables. The 't' command is followed by <tble_idx> of 0 - 3, to identify the table of interest. At that point the MS-II™ erases the table flash sector entirely (that is why you do NOT want to be driving while this is going on). So you want to delay about 50 milliseconds to allow the flash to be erased before sending the the rest of the data, which consists of the table words. These are sent in order of ascending index, msb (most significant byte) byte first, then lsb (least significant byte) byte for each of the 1024 16-bit words.
   • 'C' command, no other characters, causes the current value of the 16-bit seconds counter to be sent back. It is only used as a check that comms are working.
   • 'Q' and 'S' commands (both caps) are used to retrieve two indicators of the MS-II code version. The first is the 20-byte ASCII string for the Rev Number of the code version, the second is for a 32-byte Signature string. The latter is changed in the code whenever a new feature is added, the first is changed in the code whenever there has been an input parameter or output variable added.
   • There are also re-initialization, reboot and code reloading commands, all starting with '!'. The complete commands are:
     • '!', 'x' causes MS-II™ to re-initialize various program values, for example the ncylinder-dependent coefficient for computing RPM, to be re-calculated. It should be used only when the following specific variables are changed:
       • spark polarity,
       • tach input polarity,
       • ignition options. including dual spark options,
       • warmup enrichment parameters or tables,
       • baud rate,
       • can_id.
     • '!', '!', 'x' If rpm < crank_rpm, this causes the program to reboot itself and start running - not currently used.
     • '!', '!', '!' If rpm < crank_rpm, this is the same as reboot, except the program comes up ready to load a new code version - not currently used.

2. <CANid> - a binary integer single byte from 0-15. Use 0 for MS-II™, for other (GPIO) devices it is arbitrary depending on how a user has configured the devices in his network. Just set to 0 for communication between the PC and MS-II.

3. <tble_idx> - a binary integer byte from 0-15 that specifies the block of data from which to get/send data. The variable blocks in MS-II™ corresponding to <tble_idx> 0-6 (only ones at present) are as follows:
   • 0 = array of coolant temperatures vs ADC counts, 1024 signed 16 bit words; units are degx10, so a value of 1801 represents 180.1 degrees. These are in flash only and are downloaded from the 't' command described later.
   • 1 = array of manifold air temperatures. It has the same format as the coolant temp table.
   • 2 = EGO reading vs ADC counts array, 1024 signed 16 bit words (147 = 14.7 afr)
   • 3 = MAF reading vs ADC counts array, 1024 unsigned 16 bit words, mg/secx10.
   • 4 = first input ram structure (see the inpram. structure in the MS-II main.c source code and also see the .INI file for offset address and size)
   • 5 = second input ram structure (see the in2ram. structure in the MS-II main.c source code and also see the .INI file for offset address and size)
   • 6 = output ram structure (see the outpc. structure in MS-II and see the .INI file for offset address and size)