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Inertial Sense Binary (ISB) Protocol

The Inertial Sense binary protocol provides the most efficient way to communicate with the µINS, µAHRS, and µIMU because it preserved the native floating point and integer binary format used in computers. Binary protocol is not human readable like NMEA Protocol. Binary protocol uses Data Set (DID) C structures defined in SDK/src/data_sets.h of the InertialSense SDK.


Writing to and reading from InertialSense products is done using "Set" and "Get" commands. The following helper function portWrite() which assists with writing data to the serial port is used throughout this document.

static int portWrite(int port, const unsigned char* buf, int len)
    return serialPortWrite(&serialPort, buf, len);

Setting Data

The is_comm_set_data() function will encode a message used to set data or configurations.

// Set INS output Euler rotation in radians to 90 degrees roll for mounting
void setInsOutputRotation()
    float rotation[3] = { 90.0f*C_DEG2RAD_F, 0.0f, 0.0f };
    int messageSize = is_comm_set_data(portWrite, 0, comm, DID_FLASH_CONFIG, sizeof(float) * 3, offsetof(nvm_flash_cfg_t, insRotation), rotation);

Getting Data

Data broadcasting or streaming is enabled by using the Realtime Message Controller (RMC) or the get data command.

Get Data Command

The is_comm_get_data() function will encode a PKT_TYPE_GET_DATA message that enables broadcast of a given message at a multiple of the Data Source Update Rates. Set the data rate (period multiple) to zero disable message broadcast and pull a single packet of data. Set the data size and offset to zero to request the entire data set.

// Ask for INS message w/ update 40ms period (4ms source period x 10).  Set data rate to zero to disable broadcast and pull a single packet.
is_comm_get_data(portWrite, 0, comm, DID_INS_1, 0, 0, 10);

Data Source Update Rates

DID Update Rate (Period)
DID_INS_[1-4] (4ms default) Configured with DID_FLASH_CONFIG.startupNavDtMs
(4ms default) Configured with DID_FLASH_CONFIG.startupImuDtMs
(Any DID beginning with DID_GPS)
(200ms default) Configured with DID_FLASH_CONFIG. startupGPSDtMs
All other DIDs 1ms

*DID_PIMU integration period (dt) and output data rate are the same as DID_FLASH_CONFIG.startupNavDtMs and cannot be output at any other rate.  If a different output data rate is desired, DID_IMU which is derived from DID_PIMU can be used instead.

Realtime Message Controller (RMC)

The RMC is used to enable message broadcasting and provides updates from onboard data as soon as it becomes available with minimal latency. All RMC messages can be enabled using the Get Data Command, which is the preferred method, or by directly setting the RMC bits. The RMC bits are listed below. Message data rates are listed in the Data Source Update Rates table.

RMC Message

The following is an example of how to use the RMC. The rmc.options field controls whether RMC commands are applied to other serial ports. rmc.options = 0 will apply the command to the current serial port.

    rmc_t rmc;
    // Enable broadcasts of DID_INS_1 and DID_GPS_NAV
    rmc.bits = RMC_BITS_INS1 | RMC_BITS_GPS1_POS;       
    // Remember configuration following reboot for automatic data streaming.
    rmc.options = RMC_OPTIONS_PERSISTENT;

    is_comm_set_data(portWrite, 0, comm, DID_RMC, 0, 0, &rmc);

The update rate of the EKF is set by DID_FLASH_CONFIG.startupNavDtMs (reboot is required to apply the change). Independently, the DID_INS_x broadcast period multiple can be used to set the output data rate down to 1ms.

Persistent Messages

The persistent messages option saves the current data stream configuration to flash memory for use following reboot, eliminating the need to re-enable messages following a reset or power cycle.

  • To save persistent messages - (to flash memory), bitwise OR RMC_OPTIONS_PERSISTENT (0x200) with the RMC option field or set DID_CONFIG.system = 0x00000001 and DID_CONFIG.system = 0xFFFFFFFE. See the save persistent messages example in the Binary Communications example project.
  • To disable persistent messages - a stop all broadcasts packet followed by a save persistent messages command.

NMEA persistent messages are also available.

Enabling Persistent Messages - EvalTool
  • Enable the desired messages in the EvalTool "Data Sets" tab.
  • Press the "Save Persistent" button in the EvalTool "Data Logs" tab to store the current message configuration to flash memory for use following reboot.
  • Reset the system and verify the messages are automatically streaming. You can use the EvalTool->Data Logs dialog to view the streaming messages.

To disable all persistent messages using the EvalTool, click the "Stop Streaming" button and then "Save Persistent" button.

Enabling Persistent Messages - CLTool

Persistent messages are enabled using the CLTool by including the -persistent option along with the options for the desired messages in the command line.

cltool -c /dev/ttyS3 -persistent -msgINS2 -msgGPS

Example Projects

Examples on how to use the Inertial Sense SDK for binary communications are found in the Binary Communications Example Project and cltool project. NMEA communications examples are found in the NMEA Example Project.

Parsing Data

The ISComm library in the InertialSenseSDK provides a communications parser that can parse InertialSense binary protocol as well as other protocols.

One Byte (Simple Method)

The following parser code is simpler to implement. This method uses the is_comm_parse_byte() function to parse one byte at a time of a data stream. The return value is a non-zero protocol_type_t when valid data is found.

    uint8_t c;
    protocol_type_t ptype;
    // Read from serial buffer until empty
    while (serialPortReadChar(&s_serialPort, &c) > 0)
        // timeMs = current_timeMs();
        switch (is_comm_parse_byte(&comm, inByte))
        case _PTYPE_UBLOX:
        case _PTYPE_RTCM3:
        case _PTYPE_NMEA:

Set of Bytes (Fast Method)

The following parser code uses less processor time to parse data by copying multiple bytes at a time. This method uses is_comm_free() and is_comm_parse() along with a serial port read or buffer copy. The return value is a non-zero protocol_type_t when valid data is found.

    // Read a set of bytes (fast method)
    protocol_type_t ptype;

    // Get available size of comm buffer
    int n = is_comm_free(comm);

    // Read data directly into comm buffer
    if ((n = serialPortRead(comm->buf.tail, n)))
        // Update comm buffer tail pointer
        comm->buf.tail += n;

        // Search comm buffer for valid packets
        while ((ptype = is_comm_parse(comm)) != _PTYPE_NONE)
            switch (ptype)
            case _PTYPE_INERTIAL_SENSE_DATA:
            case _PTYPE_IS_V1_CMD:
            case _PTYPE_UBLOX:
            case _PTYPE_RTCM3:
            case _PTYPE_NMEA:

ISB Packet Overview

The IMX and GPX communicate using the Inertial Sense Binary (ISB) protocol. This section details the ISB protocol packet structure specific for protocol 2.x (software releases 2.x). Refer to the release 1.x ISB protocol document for a description of protocol 1.x (software releases 1.x). The Inertial-Sense-SDK ISComm provides functions to encode and decode ISB packets.

ISB Packet


The ISB packet structure is defined in the typedef packet_t found in ISComm.h.

Header Type and Flags

The packet type and flags are found in the byte at offset 2 in the ISB packet. The Type is the lower nibble and the Flags are the upper nibble. The packet and is defined in ISComm.h.

typedef enum
    PKT_TYPE_INVALID                        = 0,    // Invalid packet id
    PKT_TYPE_ACK                            = 1,    // (ACK) received valid packet
    PKT_TYPE_NACK                           = 2,    // (NACK) received invalid packet
    PKT_TYPE_GET_DATA                       = 3,    // Request for data to be broadcast, response is PKT_TYPE_DATA. See data structures for list of possible broadcast data.
    PKT_TYPE_DATA                           = 4,    // Data sent in response to PKT_TYPE_GET_DATA (no PKT_TYPE_ACK is sent)
    PKT_TYPE_SET_DATA                       = 5,    // Data sent, such as configuration options.  PKT_TYPE_ACK is sent in response.
    PKT_TYPE_STOP_BROADCASTS_ALL_PORTS      = 6,    // Stop all data broadcasts on all ports. Responds with an ACK
    PKT_TYPE_STOP_DID_BROADCAST             = 7,    // Stop a specific broadcast
    PKT_TYPE_STOP_BROADCASTS_CURRENT_PORT   = 8,    // Stop all data broadcasts on current port. Responds with an ACK
    PKT_TYPE_COUNT                          = 9,    // The number of packet identifiers, keep this at the end!
    PKT_TYPE_MAX_COUNT                      = 16,   // The maximum count of packet identifiers, 0x1F (PACKET_INFO_ID_MASK)
    PKT_TYPE_MASK                           = 0x0F, // ISB packet type bitmask

    ISB_FLAGS_MASK                          = 0xF0, // ISB packet flags bitmask (4 bits upper nibble)
    ISB_FLAGS_EXTENDED_PAYLOAD              = 0x10, // Payload is larger than 2048 bytes and extends into next packet.
    ISB_FLAGS_PAYLOAD_W_OFFSET              = 0x20, // The first two bytes of the payload are the byte offset of the payload data into the data set.
} eISBPacketFlags;

Header DID

The data ID (DID) values are defined at the top of data_sets.h and identify which data set is requested or contained in the ISB packet.

Header Payload Size

The ISB packet payload size is a uint16 that will

The ISB packet footer contains a Fletcher-16 (16-bit integer). The following algorithm is used for this checksum and is found in ISComm.h.

uint16_t is_comm_fletcher16(uint16_t cksum_init, const void* data, uint32_t size)
    checksum16_u cksum; = cksum_init;
    for (uint32_t i=0; i<size; i++)
        cksum.a += ((uint8_t*)data)[i];
        cksum.b += cksum.a;

The ISB packet footer checksum is computed using the Fletcher-16 algorithm starting with an initial value of zero and sequencing over the entire packet (excluding the two footer checksum bytes).

ISB Packet with Data Offset

Packet Structure with data offset

*The first two bytes of the payload may be a uint16 offset for the data offset in the target data set when the ISB_FLAGS_PAYLOAD_W_OFFSET flag is set in the header flags.

ISB Packet with No Payload


ISB Get Data Packet

The Get Data packet of type PKT_TYPE_GET_DATA is used to query specific data according to data set ID, size, offset, and streaming period multiple. The payload size is 8. Setting the payload period to zero will result in a single response and a continuous stream of data for a non-zero period.

Stop Broadcasts Packets


The NEMA $STPB stop broadcasts command is recommended as the protocol version-independent method for disabling data streaming.

Two stop all broadcasts packets are special packet types that will disable all binary and NMEA data streams. The following functions calls are provided in the SDK to generate the stop all broadcasts packets.

All Ports

is_comm_stop_broadcasts_all_ports(portWrite, 0, &comm);
The hexadecimal string to stop all broadcasts on all ports is:

0xef 0x49 0x06 0x00 0x00 0x00 0x3e 0x1f

Current Port Only

is_comm_stop_broadcasts_current_port(portWrite, 0, &comm);

The hexadecimal string to stop all broadcasts on the current port is:

0xef 0x49 0x08 0x00 0x00 0x00 0x40 0x27

RMC Presets

RMC Preset Stream PPD

The hexadecimal string for the RMC Preset enable PPD is:

0xef 0x49 0x05 0x09 0x0c 0x00 0xe2 0x3c 0x35 0x01 0x90 0x00 0x00 0xc0 0x00 0x01 0x00 0x00 0xf7 0xb0