Electronic Shelf Label Service

When specification content indicates that there are multiple alternatives to satisfy specification requirements, if one alternative is explained or illustrated in an example it is not intended to limit other alternatives that the specification requirements permit.

It is the goal of Bluetooth SIG that specifications are clear, unambiguous, and do not contain discrepancies. However, members can report any perceived discrepancy by filing an erratum and can request a test case waiver as appropriate.

An ESL may have zero or more displays, up to a maximum of 256, and the displays are addressed using index numbers from 0 to 255. The index is called Display_Index.

The display used in an ESL typically is a low-power display (e.g., an e-ink, or similar, display that does not require power to maintain a constant image).

The purpose of a display is to display an ESL image.

An ESL may have zero or more LEDs, up to a maximum of 256, and the LEDs are addressed using index numbers from 0 to 255. The index is called LED_Index.

The light is assumed to be an LED, which may be a single-color LED or an LED that can display one of multiple colors. Although this element is assumed to be an LED, equivalent light-emitting technologies may also be used.

The LED can flash to attract the attention of a human. The LED can be off, on, or flashing in a particular pattern of flashes.

Flashing the LED generally consumes less power than having the LED continuously illuminated, and different flashing patterns may be used to indicate different meanings.

An ESL may have zero or more sensors, up to a maximum of 256, and the sensors are addressed using index numbers from 0 to 255. The index is called Sensor_Index.

An ESL sensor is an element that can be used to read a physical property of the environment (e.g., the temperature of a refrigerator).

The Unassociated state is used when an ESL has not yet been configured by an AP (i.e., the ESL has not yet been provisioned with the information required for the ESL to be able to synchronize with an AP as described in Section 2.7.3.3).

In the Unassociated state, the ESL shall enter the Generic Access Profile (GAP) Undirected Connectable mode and transmit advertising packets containing a Service UUID AD type that includes the UUID of the ESL Service.

When an ESL in the Unassociated state forms a connection with a Client, the ESL shall be in Bondable mode as described in the GAP (see [Vol 3], Part C, Section 9.4.3 of [2]). If the Bonding procedure is successfully completed with the Client, then the ESL shall transition to the Configuring state described in Section 2.7.3.2.

The Configuring state shall be entered only from the Unassociated state.

The Configuring state allows an ESL that has not been configured to be configured by the bonded Client. State information is exposed by the ESL via the GATT characteristics described in Section 3.

A Client may configure the ESL by writing new values to the writable characteristics. The configuration of the ESL has been completed when the Client has successfully written values to all of the following characteristics:

The Synchronized state is a low-power and low-bandwidth communications scheme based on the Periodic Advertising with Responses feature described in the Bluetooth Core Specification [2]. This scheme enables thousands of ESLs to synchronize with a central AP, and for short messages to be sent by the AP and responded to by the ESLs. Requirements that apply in this state may be specified in a separate profile specification. The Synchronized state may be entered only from the Configuring state or the Updating state. The ESL transitions to the Synchronized state when the ESL completes the Periodic Advertising

Sync Transfer procedure as the recipient and successfully synchronizes to a periodic advertising train transmitted by the Client, as described in Volume 6, Part B of the Bluetooth Core Specification [2].

If the ESL has not received a valid ESL message in a synchronization message from the AP for 60 minutes, or has been unable to respond to synchronization messages for 60 minutes, then the ESL shall transition to the Unsynchronized state.

A relevant profile specification may specify additional requirements that apply to transitions to and from the Synchronized state.

Larger data transfers may be accomplished by transitioning from the Synchronized state to the connection-oriented Updating state described in Section 2.7.3.4.

The Updating state may be entered only from the Synchronized state or the Unsynchronized state (see Section 2.7.3.5).

If the ESL is in the Unsynchronized state, then the ESL is already in a GAP connectable mode as described in Section 2.7.3.5.

To transition to the Updating state from the Synchronized state, the AP can use the GAP Periodic Advertising Connection procedure in the specified subevent to initiate a connection to the ESL.

If the Client successfully initiates a connection, then the ESL shall transition to the Updating state only if the peer device is the Client with which the ESL is already associated. The Server shall then transition to the Updating state.

While in the Updating state, the ESL shall ignore any data present on the LE periodic advertising with responses logical transport.

When in the Updating state, the ESL Service may be used to reconfigure the ESL device by updating the values of one or more of the characteristics described in Section 2.7.3.2. In addition, in the Updating state, the Object Transfer Profile [5] (if supported) may be used to write new image data to the device. Support for the Object Transfer Profile is provided by associating an Image_Index value with a specific image that is stored in the ESL device, as described in Section 3.6. A 48-bit Object identifier (ID) can be derived from each Image_Index value for use in the Object Transfer Service [6] (see Section 3.6.2.1).

The Server shall reject any pairing requests that are received while the Server is in the Updating state.

When in the Unsynchronized state, the ESL is associated with an AP but is not in a connection and is not synchronized.

In the Unsynchronized state, the ESL shall enter a GAP connectable mode. If a connection is formed with a Client, then the ESL transitions to the Updating state as described in Section 2.7.3.4.

If the ESL is not moved to the Updating state for 60 minutes, then the ESL shall transition to the Unassociated state, and shall remove all bonding information with the AP and delete the value of the AP Sync Key Material in internal storage.

Each ESL is a member of a group, identified by the Group_ID part of the ESL Address.

The ESL_ID in the ESL Address characteristic shall be a value in the range 0x00 to 0xFE.

The value 0xFF is reserved for the Broadcast Address. Refer to a relevant profile specification where further requirements for the use of the Broadcast Address may be specified. For example, the Broadcast Address may be used in the profile to enable a message to be addressed simultaneously to all the ESLs that belong to the group identified by the Group_ID part of the ESL Address.

Because there is a maximum of 128 groups, and 255 ESL_IDs that are unique within each group, a total of 32,640 ESL devices can be allocated a locally unique address.

Each image storage location has an associated 48-bit Object ID value that can be calculated from the Image_Index value for that location.

When a 48-bit Object ID is used (e.g., for use with the Object Transfer Service [6]), an Object ID can be derived for a given image storage location by adding the associated Image_Index value to the following base value: 0x000000000100 (i.e., 256, in decimal notation). A profile specification may apply additional requirements.

Sensors typically provide data representing measurements. A Client can obtain the data from a sensor by using the Read Sensor Data command described in Section 3.9.2.6. In order to interpret the data that has been read from a sensor, the format of the data produced by the sensor needs to be known. The Sensor_Type value identifies the data format as described in Section 3.7.1.1.1 and Section 3.7.1.1.2.

3.7.1.1.2. 32-bit value

When the value of the Size field is 0x01, the Sensor_Type field shall contain a 32-bit value consisting of two 16-bit sub-fields as shown in Figure 3.5.

Figure 3.5: Vendor-specific sensor type sub-fields

The Company_ID sub-field shall contain a Company ID, a unique 16-bit number assigned by the Bluetooth SIG to a member company. Company ID values are defined in the Bluetooth SIG Assigned Numbers [1].

The Sensor_Code sub-field shall contain a number assigned to a vendor-specific sensor type by the member whose Company ID is in the Company_ID sub-field.

An sRGB LED is a multi-color LED that can change color. The current color of the LED is identified by the value of bits 5 to 0, which are able to represent 63 possible colors as combinations of quantities of red, green, and blue, as shown in Table 3.5. The RGB value exposed is a best approximation in the sRGB color space to the present color of the LED, in which 0b00 represents the minimum level and 0b11 represents the maximum level for a color component.

A monochrome LED is a single-color LED. The single color is identified by the value of bits 5 to 0, which are able to represent 63 possible colors as combinations of quantities of red, green, and blue, as shown in Table 3.6. The RGB value exposed is a best approximation in the sRGB color space to the fixed color of the LED, in which 0b00 represents the minimum level and 0b11 represents the maximum level for a color component.

The Tag and Length fields are each 4 bits in size. The combination of the Tag and Length field values (i.e., the value of the entire LSO) comprises an opcode. Therefore, there are 256 possible opcodes in total.

The total length of the Parameters field that follows the opcode can be calculated from value of the Length field; the total length of the Parameters field shall be equal to (Length + 1) octets. Therefore, there are 16 possible opcodes in which Length = 0b0000 that have 1 parameter octet containing additional data, 16 possible opcodes in which Length = 0b0001 that have 2 parameter octets containing additional data, 16 possible opcodes in which Length = 0b1111 that have 16 parameter octets containing additional data, etc.

The overall length of each TLV is therefore equal to (Length + 2) octets. For example, the Service Reset command described in Section 3.9.2.3 has a Length nibble of 0b0000, meaning that it has a Parameters field consisting of 1 octet and an overall length, including the opcode itself, of 2 octets. The shortest possible TLV is 2 octets in size, and the longest is 17 octets.

Vendor-specific commands are supported by use of a vendor-specific tag (see Section 3.9.2.12).

Refer to Section 3.9.2 for details on the ESL commands.

As with the command opcode format, the response opcode is composed of the Length nibble (forming the most significant nibble (MSN) of the opcode) and the Tag nibble (forming the least significant nibble of the opcode). The total length of the Parameters field that follows the opcode can be calculated from value of the Length field; the total length of the Parameters field shall be equal to (Length + 1) octets.

Opcode values used in ESL responses have no connection with opcodes of the same numerical value used in ESL commands (i.e., the opcode numbers assigned in Section 3.9.2 can be re-assigned for use in an ESL response with an unrelated meaning).

The Ping command solicits an ESL response to be sent. The Ping command has no other effect.

The Unassociate from AP command allows an ESL to be disassociated from the AP.

When the Unassociate from AP command is received, and the response has been sent, the ESL shall remove all bonding information with the AP, delete the value of the AP Sync Key Material in internal storage, the ESL ID, and delete all stored commands.

Following execution of the command, the ESL shall enter the Unassociated state.

The Service Needed state may be True or False. The Service Needed state is reported via the Service Needed bit of the Basic State response described in Section 3.9.3.3.

If a condition occurs that causes the ESL to set the Service Needed state to True, then the ESL shall set the value of the Service Needed bit to True. Once set to True, the value of the Service Needed bit shall remain True until it is successfully reset by the Client as follows.

When the Service Reset command is received by an ESL:

When the Factory Reset command is received by an ESL, the ESL shall initiate disconnection of the link with the AP.

Upon disconnection, the ESL shall become unassociated from any AP and shall revert to its original state before it was associated with an AP. The ESL shall remove all bonding information with the AP; delete the values of the AP Sync Key Material, ESL Response Key Material, and ESL Address in internal storage; and delete any stored image data that was written to the ESL.

If, after receipt of the Factory Reset command and prior to disconnection from the AP, the ESL receives any other command from the AP written to the ESL Control Point characteristic, then the other command shall be rejected with the error code: ”Unspecified Error”.

When the Update Complete command is received by an ESL, the ESL shall synchronize with the AP and then disconnect the link with the AP.

When the Read Sensor Data command is received and the Sensor_Index value is valid, the ESL shall obtain data from the sensor identified by the Sensor_Index value. However, if the sensor identified in the parameter value does not exist, the ESL shall send an Error response.

In addition to sensing environmental variables such as ambient temperature, certain sensor types may report device conditions such as battery level or hardware errors.

3.9.2.6.1. Format

The Read Sensor Data command has the following parameters:

Field	Size (bits)	Description
ESL_ID	8	See Section 3.9.1.1.1
Sensor_Index	8	Index to the sensor on the ESL

Table 3.13: Read Sensor Data command parameters

Figure 3.7: Read Sensor Data command parameters

When the Refresh Display command is received, the Display_Index value is valid, and an image is being displayed on the specified display, then the ESL shall refresh the displayed image on the display specified in the command. However, if the display identified in the parameter value does not exist, or if there is no image currently being displayed on the specified display, the ESL shall send an Error response.

The Refresh Display command enables a display such as an e-ink display to be refreshed at the request of the Client. The process of refreshing a display is implementation-specific. For example, certain types of display technology need the display to be refreshed from time to time to prevent fading or ghosting of the image. The display might flash visibly while being refreshed and this process is typically performed as a night-time routine.

If the Refresh Display command is received, the Display_Index value is valid, and an image is being displayed on the specified display, but the display specified in the command does not need to be refreshed (e.g., because the display technology employed by the implementation uses a different solution), then this shall not be considered an error condition and the command shall be processed as if the display had been refreshed.

3.9.2.7.1. Format

The Refresh Display command has the following parameters:

Field	Size (bits)	Description
ESL_ID	8	See Section 3.9.1.1.1
Display_Index	8	Index to the physical display on the ESL

Table 3.14: Refresh Display command parameters

Figure 3.8: Refresh Display command parameters

The Display Image command changes the state of the display selected by a Display_Index value to display a stored image selected by an Image_Index value.

If no error condition occurs, then the ESL shall display the image on the display as specified by the values received in the command.

If the display or the image identified in the parameter values does not exist, then the ESL shall send an Error response.

3.9.2.8.1. Format

The Display Image command has the following parameters:

Field	Size (bits)	Description
ESL_ID	8	See Section 3.9.1.1.1
Display_Index	8	Index to the physical display on the ESL
Image_Index	8	Index to a stored image (see Section 3.6)

Table 3.15: Display Image command parameters

Figure 3.9: Display Image command parameters

The Display Timed Image command changes the state of the display selected by a Display_Index value to display a stored image selected by an Image_Index value, at a given absolute time. The Display Timed Image command is acknowledged using a Display State response (see Section 3.9.3.4), but the command has no effect on the displayed image until the time specified by the Absolute_Time parameter.

If no error condition occurs, then the ESL shall display the image on the display as specified by the values received in the command, at the specified time.

If the display or the image identified in the parameter values does not exist, then the ESL shall send an Error response.

Although the Basic State response is not sent in response to the Display Timed Image command, see Section 3.9.2.9.3 and Section 3.9.3.3.3 for details of the effect of accepting a Display Timed Image command on the value of the Pending Display Update bit, which is exposed when the Basic State response is sent in response to another command.

After the specified time has been reached and the display has been updated as described above, the ESL shall set the Pending Display Update bit to False.

3.9.2.9.2. Format

The Display Timed Image command has the following parameters:

Field	Size (bits)	Description
ESL_ID	8	See Section 3.9.1.1.1
Display_Index	8	Index to the physical display on the ESL
Image_Index	8	Index to a stored image (see Section 3.6)
Absolute_Time	32	Time when the display changes state

Table 3.16: Display Timed Image command parameters

Figure 3.10: Display Timed Image command parameters

The LED Control command changes the state of an LED. If no error condition occurs, then the ESL shall change the state of the LED in accordance with the values received in the command.

If the LED identified in the parameter values does not exist, then the ESL shall send an Error response.

The LED Control command is addressed to one LED at a time. The LED Control command may be received multiple times to control multiple LEDs.

If an LED Control command is received and the LED specified in the command is already active (where ”active” has the meaning described in Section 3.9.3.3.1), then the new command shall immediately supersede the previous command, and the effect of the previous command related to controlling the same LED shall then be terminated.

3.9.2.10.1. Format

The LED Control command has the following parameters:

Field	Size (bits)	Description
ESL_ID	8	See Section 3.9.1.1.1
LED_Index	8	Index of the LED
Color_Red	2	Red component (see Section 3.8)
Color_Green	2	Green component (see Section 3.8)
Color_Blue	2	Blue component (see Section 3.8)
Brightness	2	0 = 25% 1 = 50% 2 = 75% 3 = 100%
Flashing_Pattern	56	See Section 3.9.2.10.2.2
Repeat_Type	1	0: Number of times 1: Time duration
Repeats_Duration	15	Number of times to repeat, or time duration in increments of 1 second

Table 3.17: LED Control command parameters

Figure 3.11: LED Control command parameters

3.9.2.10.2. Parameter information

3.9.2.10.2.1. Color

The values of the Color_Red, Color_Green, and Color_Blue fields shall have the same interpretation as specified for the ESL LED Information characteristic in Section 3.8.

However, if the LED selected by the Index value is a monochrome LED, the value of the Color fields shall be ignored upon receipt.

3.9.2.10.2.2. Flashing_Pattern

The Flashing_Pattern fields contain values that indicate the flashing pattern setting of the LED, as shown in Figure 3.12.

Figure 3.12: Flashing_Pattern fields

The Pattern field value is a sequence of bits that determine whether the LED is on or off at a given time. A bit value of 0 means ”Switch LED off”; a bit value of 1 means ”Switch LED on”. When the Flashing_Pattern bit determines that the LED is required to be on, the LED shall be illuminated.

The Flashing_Pattern bits shall be interpreted by starting at the most significant bit, and progressing one bit at a time through the bits toward the least significant bit, in order of descending bit number. The dwell time for each bit (i.e., the elapsed time before progressing to the next bit) shall be equal to either the Bit_Off_Period field value or the Bit_On_Period field value, depending on whether the instantaneous bit value is 0 or 1, respectively, as described below.

The Bit_Off_Period field represents a time period, consisting of an integer (uint8) value multiplied by 2 ms. The resulting value of Bit_Off_Period is the length of time per bit for which the LED is kept off. The valid range is 2 ms to 510 ms; a Bit_Off_Period value of 0 ms is invalid. This period applies only to the off state. The off duration can also be extended by setting consecutive bits to 0, as described below.

The Bit_On_Period field represents a time period, consisting of an integer (uint8) value multiplied by 2 ms. The resulting value of Bit_On_Period is the length of time per bit for which the LED is kept on. The valid range is 2 ms to 510 ms; a Bit_On_Period value of 0 ms is invalid. This period applies only to the on state. The on duration can also be extended by setting consecutive bits to 1, as described below.

For example:

• If Bit_Off_Period and Bit_On_Period are set to the same value, the value of 0b1010101010101010101010101010101010101010 (0xAAAAAAAAAA) means an LED flashing pattern that would be 50 percent on and 50 percent off, with the LED flashing continuously. Each on period would be Bit_On_Period in duration, and each off period would be Bit_Off_Period in duration. The duty cycle can be changed by using different values for Bit_Off_Period and Bit_On_Period.

• The value of 0b1010000010100000101000001010000010100000 (0xA0A0A0A0A0) would be on for 1 Bit_On_Period, then off for 1 Bit_Off_Period, on for 1 Bit_On_Period, off for 5 times the duration of Bit_Off_Period; on for 1 Bit_On_Period, then off for 1 Bit_Off_Period, on for 1 Bit_On_Period, off for 5 times the duration of Bit_Off_Period; etc.

• The value of 0b1010000000001010000000001010000000001111 (0xA00A00A00F) would be on for 1 Bit_On_Period, then off for 1 Bit_Off_Period, on for 1 Bit_On_Period, off for 9 times the duration of Bit_Off_Period; on for 1 Bit_On_Period, then off for 1 Bit_Off_Period, on for 1 Bit_On_Period, off for 9 times the duration of Bit_Off_Period; on for 1 Bit_On_Period, then off for 1 Bit_Off_Period, on for 1 Bit_On_Period, off for 9 times the duration of Bit_Off_Period; and then on for 4 times the duration of Bit_On_Period.

Each of the above example patterns would be repeated over and over again until the duration specified by the Repeat_Type and Repeats_Duration fields expired, as described in Section 3.9.2.10.2.4.

The Pattern field is 40 bits in size. However, the ESL shall ignore any leading zeroes in a pattern and treat only the remaining bits as meaningful (i.e., the meaningful part of the pattern begins only when the first bit set to 1 is encountered, reading from left to right). This rule enables flashing patterns whose length is not a submultiple of 40 bits to be used.

For example:

1. The value of 0b0000101100000000000000000000000000000000 (0x0B00000000) means an LED flashing pattern that starts with the LED being on for 1 Bit_On_Period, then off for 1 Bit_Off_Period, on for 2 times the duration of Bit_On_Period, and off for 32 times the duration of Bit_Off_Period. Only the 36 least significant bits are meaningful. If the duration specified by the Repeat_Type and Repeats_Duration fields has not expired, then the pattern repeats, starting from the first 1 again, with the leading zeroes being ignored.

2. The value of 0b0000000000000000000000000000101100000000 (0x0000000B00) means an LED flashing pattern that starts with the LED being on for 1 Bit_On_Period, then off for 1 Bit_Off_Period, on for 2 times the duration of Bit_On_Period, and off for 8 times the duration of Bit_Off_Period. Only the 12 least significant bits are meaningful. If the duration specified by the Repeat_Type and Repeats_Duration fields has not expired, then the pattern repeats, starting from the first 1 again, with the leading zeroes being ignored.

3. The value of 0b0000000000000000101100000000101100000000 (0x0000B00B00) would cause the LED to emit the same pattern of flashes as the example immediately above, provided that the duration specified by the Repeat_Type and Repeats_Duration fields is long enough to allow the patterns to be repeated. Both solutions are valid.

Under certain conditions, described in Section 3.9.2.10.2.4, the value of the Flashing_Pattern field is ignored and in this case it may be set to any value.

3.9.2.10.2.3. Brightness

The average brightness of the LED during the on period shall be determined by the value of the Brightness field. Notwithstanding the above requirement for the LED to be illuminated for the duration of an on period, the LED may be pulsed at an appropriate rate during the period when it is on to reduce the perceived brightness. For example, if the Brightness value is set to 25 percent, this may be achieved by illuminating the LED with a 25 percent duty cycle during the period when it is on. If the LED is pulsed on and off to adjust the perceived brightness, then the pulse rate should take into account the typical human flicker fusion threshold and any potential interactions with ambient lighting. However, the method of controlling the brightness is selected by the implementer.

3.9.2.10.2.4. Repeat_Type and Repeats_Duration

The Repeat_Type bit enables the time for which the flashing pattern shall be displayed to be specified as either:

• If Repeat_Type = 0, an integer number of repeats of the flashing pattern specified by the Flashing_Pattern fields, or

• If Repeat_Type = 1, a time duration specified in units of seconds

If the value of the Repeats_Duration field is non-zero, then the flashing pattern shall be repeated continuously until the duration specified in the command has elapsed. After the duration specified in the command has elapsed, the LED shall be turned off.

For example, when the Repeat_Type bit is set to 1, and the Repeats_Duration field is set to its maximum value, 0x7FFF, the flashing duration is 9 hours, 6 minutes, and 7 seconds.

If the value of the Repeats_Duration field is non-zero, then the LED shall be turned off after the duration specified by the Repeat_Type and Repeats_Duration fields expires.

However, if the value of the Repeats_Duration field is set to the special value of 0, the Flashing_Pattern field shall be ignored and the Repeat_Type field shall have the following interpretation:

• If Repeat_Type = 0, then the LED shall be turned off continuously.

• If Repeat_Type = 1, then the LED shall be turned on continuously.

The LED Timed Control command changes the state of an LED at a given absolute time. The LED Timed Control command is acknowledged using the LED State response (see Section 3.9.3.2), but the command has no effect on the state of the LED until the time specified by the Absolute_Time parameter.

If no error condition occurs, then the ESL shall, at the specified time, change the state of the LED in accordance with the values received in the command.

If an LED Timed Control command is scheduled to start when the LED is already active (where ”active” has the meaning described in Section 3.9.3.3.1), then the LED Timed Control command shall supersede the older command at the time when the LED Timed Control command is scheduled to start, and the effect of the older command related to controlling the same LED shall then be terminated.

If the LED identified in the parameter values does not exist, then the ESL shall send an Error response.

Although the Basic State response is not sent in response to the LED Timed Control command, see Section 3.9.3.3.2 for details of the effect of accepting an LED Timed Control command on the value of the Pending LED Update bit, which is exposed when the Basic State response is sent in response to another command.

After the specified time has been reached and the LED state has been updated as described above, the ESL shall set the Pending LED Update bit to False.

3.9.2.11.2. Format

The LED Timed Control command has the following parameters:

Field	Size (bits)	Description
ESL_ID	8	See Section 3.9.1.1.1
LED_Index	8	Index of the LED
Color_Red	2	Red component (see Section 3.8)
Color_Green	2	Green component (see Section 3.8)
Color_Blue	2	Blue component (see Section 3.8)
Brightness	2	0 = 25% 1 = 50% 2 = 75% 3 = 100%
Flashing_Pattern	56	See Section 3.9.2.10.2.2
Repeat_Type	1	0: Number of times 1: Time duration
Repeats_Duration	15	Number of times to repeat or time duration in increments of 1 second
Absolute_Time	32	Time when the LED changes state

Table 3.18: LED Timed Control command parameters

The Absolute_Time is the time when the LED Timed Control command shall be executed. Repeat_Type and Repeats_Duration shall start at the specified absolute time. The interpretation of the fields shall otherwise be the same as specified in Section 3.9.2.10.

Figure 3.13: LED Timed Control command parameters

All opcodes containing the Tag value ”Vendor-specific” are vendor-specific.

If an opcode containing the vendor-specific tag is received by an ESL and the opcode is not recognized by the ESL, then the ESL shall respond by sending the Error response: Invalid Opcode.

Refer also to a relevant profile specification where requirements for identification of the vendor of the device may be specified.

The Error response has a single parameter that contains an error code as enumerated in Table 3.20.

The LED State response is of fixed length and has one parameter as shown in Table 3.21.

The Basic State response has a single parameter that consists of a 16-bit bitmap as shown in Table 3.22.

The Display State response is of fixed length and has two parameters as shown in Table 3.23.

The Sensor Value response is of variable length and has two parameters as shown in Table 3.24.

The Tag nibble contained in the opcode used in the response is a fixed value, but the length is variable and is indicated by the value of the Length nibble.

The total parameter size is equal to (Length + 1) octets as described in Section 3.9.1.2. Because one octet in the response parameters is used for the Sensor_Index value, the ESL must therefore set the value of the Length nibble equal to the size of the associated Sensor_Data field, in units of octets.

The format of the sensor data for a given Sensor_Index value is known from the Sensor Information characteristic described in Section 3.7.

The Vendor-specific response has one or more parameters, specified by the vendor.

The length is variable and can be calculated from value of the Length nibble contained in the opcode used in the response. The total parameter size shall be equal to (Length + 1) octets as described in Section 3.9.1.2.