Modbus - Servo Brick

This is the description of the Modbus protocol for the Servo Brick. General information and technical specifications for the Servo Brick are summarized in its hardware description.

API

A general description of the Modbus protocol structure can be found here.

Every function of the Servo Brick API that has a servo_num parameter can address a servo with the servo number (0 to 6). If it is a setter function then multiple servos can be addressed at once with a bitmask for the servos, if the highest bit is set. For example: 1 will address servo 1, (1 << 1) | (1 << 5) | (1 << 7) will address servos 1 and 5, 0xFF will address all seven servos, etc. This allows to set configurations to several servos with one function call. It is guaranteed that the changes will take effect in the same PWM period for all servos you specified in the bitmask.

Basic Functions

BrickServo.enable
Function ID:

1

Request:
  • servo_num -- uint8
Response:

no response

Enables a servo (0 to 6). If a servo is enabled, the configured position, velocity, acceleration, etc. are applied immediately.

BrickServo.disable
Function ID:

2

Request:
  • servo_num -- uint8
Response:

no response

Disables a servo (0 to 6). Disabled servos are not driven at all, i.e. a disabled servo will not hold its position if a load is applied.

BrickServo.is_enabled
Function ID:

3

Request:
  • servo_num -- uint8
Response:
  • enabled -- bool

Returns true if the specified servo is enabled, false otherwise.

BrickServo.set_position
Function ID:

4

Request:
  • servo_num -- uint8
  • position -- int16
Response:

no response

Sets the position in °/100 for the specified servo.

The default range of the position is -9000 to 9000, but it can be specified according to your servo with set_degree.

If you want to control a linear servo or RC brushless motor controller or similar with the Servo Brick, you can also define lengths or speeds with set_degree.

BrickServo.get_position
Function ID:

5

Request:
  • servo_num -- uint8
Response:
  • position -- int16

Returns the position of the specified servo as set by set_position.

BrickServo.get_current_position
Function ID:

6

Request:
  • servo_num -- uint8
Response:
  • position -- int16

Returns the current position of the specified servo. This may not be the value of set_position if the servo is currently approaching a position goal.

BrickServo.set_velocity
Function ID:

7

Request:
  • servo_num -- uint8
  • velocity -- uint16
Response:

no response

Sets the maximum velocity of the specified servo in °/100s. The velocity is accelerated according to the value set by set_acceleration.

The minimum velocity is 0 (no movement) and the maximum velocity is 65535. With a value of 65535 the position will be set immediately (no velocity).

The default value is 65535.

BrickServo.get_velocity
Function ID:

8

Request:
  • servo_num -- uint8
Response:
  • velocity -- uint16

Returns the velocity of the specified servo as set by set_velocity.

BrickServo.get_current_velocity
Function ID:

9

Request:
  • servo_num -- uint8
Response:
  • velocity -- uint16

Returns the current velocity of the specified servo. This may not be the value of set_velocity if the servo is currently approaching a velocity goal.

BrickServo.set_acceleration
Function ID:

10

Request:
  • servo_num -- uint8
  • acceleration -- uint16
Response:

no response

Sets the acceleration of the specified servo in °/100s².

The minimum acceleration is 1 and the maximum acceleration is 65535. With a value of 65535 the velocity will be set immediately (no acceleration).

The default value is 65535.

BrickServo.get_acceleration
Function ID:

11

Request:
  • servo_num -- uint8
Response:
  • acceleration -- uint16

Returns the acceleration for the specified servo as set by set_acceleration.

BrickServo.set_output_voltage
Function ID:

12

Request:
  • voltage -- uint16
Response:

no response

Sets the output voltages with which the servos are driven in mV. The minimum output voltage is 2000mV and the maximum output voltage is 9000mV.

Note

We recommend that you set this value to the maximum voltage that is specified for your servo, most servos achieve their maximum force only with high voltages.

The default value is 5000.

BrickServo.get_output_voltage
Function ID:

13

Request:

empty payload

Response:
  • voltage -- uint16

Returns the output voltage as specified by set_output_voltage.

BrickServo.set_pulse_width
Function ID:

14

Request:
  • servo_num -- uint8
  • min -- uint16
  • max -- uint16
Response:

no response

Sets the minimum and maximum pulse width of the specified servo in µs.

Usually, servos are controlled with a PWM, whereby the length of the pulse controls the position of the servo. Every servo has different minimum and maximum pulse widths, these can be specified with this function.

If you have a datasheet for your servo that specifies the minimum and maximum pulse width, you should set the values accordingly. If your servo comes without any datasheet you have to find the values via trial and error.

Both values have a range from 1 to 65535 (unsigned 16-bit integer). The minimum must be smaller than the maximum.

The default values are 1000µs (1ms) and 2000µs (2ms) for minimum and maximum pulse width.

BrickServo.get_pulse_width
Function ID:

15

Request:
  • servo_num -- uint8
Response:
  • min -- uint16
  • max -- uint16

Returns the minimum and maximum pulse width for the specified servo as set by set_pulse_width.

BrickServo.set_degree
Function ID:

16

Request:
  • servo_num -- uint8
  • min -- int16
  • max -- int16
Response:

no response

Sets the minimum and maximum degree for the specified servo (by default given as °/100).

This only specifies the abstract values between which the minimum and maximum pulse width is scaled. For example: If you specify a pulse width of 1000µs to 2000µs and a degree range of -90° to 90°, a call of set_position with 0 will result in a pulse width of 1500µs (-90° = 1000µs, 90° = 2000µs, etc.).

Possible usage:

  • The datasheet of your servo specifies a range of 200° with the middle position at 110°. In this case you can set the minimum to -9000 and the maximum to 11000.
  • You measure a range of 220° on your servo and you don't have or need a middle position. In this case you can set the minimum to 0 and the maximum to 22000.
  • You have a linear servo with a drive length of 20cm, In this case you could set the minimum to 0 and the maximum to 20000. Now you can set the Position with set_position with a resolution of cm/100. Also the velocity will have a resolution of cm/100s and the acceleration will have a resolution of cm/100s².
  • You don't care about units and just want the highest possible resolution. In this case you should set the minimum to -32767 and the maximum to 32767.
  • You have a brushless motor with a maximum speed of 10000 rpm and want to control it with a RC brushless motor controller. In this case you can set the minimum to 0 and the maximum to 10000. set_position now controls the rpm.

Both values have a possible range from -32767 to 32767 (signed 16-bit integer). The minimum must be smaller than the maximum.

The default values are -9000 and 9000 for the minimum and maximum degree.

BrickServo.get_degree
Function ID:

17

Request:
  • servo_num -- uint8
Response:
  • min -- int16
  • max -- int16

Returns the minimum and maximum degree for the specified servo as set by set_degree.

BrickServo.set_period
Function ID:

18

Request:
  • servo_num -- uint8
  • period -- uint16
Response:

no response

Sets the period of the specified servo in µs.

Usually, servos are controlled with a PWM. Different servos expect PWMs with different periods. Most servos run well with a period of about 20ms.

If your servo comes with a datasheet that specifies a period, you should set it accordingly. If you don't have a datasheet and you have no idea what the correct period is, the default value (19.5ms) will most likely work fine.

The minimum possible period is 1µs and the maximum is 65535µs.

The default value is 19.5ms (19500µs).

BrickServo.get_period
Function ID:

19

Request:
  • servo_num -- uint8
Response:
  • period -- uint16

Returns the period for the specified servo as set by set_period.

BrickServo.get_servo_current
Function ID:

20

Request:
  • servo_num -- uint8
Response:
  • current -- uint16

Returns the current consumption of the specified servo in mA.

BrickServo.get_overall_current
Function ID:

21

Request:

empty payload

Response:
  • current -- uint16

Returns the current consumption of all servos together in mA.

BrickServo.get_stack_input_voltage
Function ID:

22

Request:

empty payload

Response:
  • voltage -- uint16

Returns the stack input voltage in mV. The stack input voltage is the voltage that is supplied via the stack, i.e. it is given by a Step-Down or Step-Up Power Supply.

BrickServo.get_external_input_voltage
Function ID:

23

Request:

empty payload

Response:
  • voltage -- uint16

Returns the external input voltage in mV. The external input voltage is given via the black power input connector on the Servo Brick.

If there is an external input voltage and a stack input voltage, the motors will be driven by the external input voltage. If there is only a stack voltage present, the motors will be driven by this voltage.

Warning

This means, if you have a high stack voltage and a low external voltage, the motors will be driven with the low external voltage. If you then remove the external connection, it will immediately be driven by the high stack voltage

Advanced Functions

BrickServo.set_spitfp_baudrate_config
Function ID:

231

Request:
  • enable_dynamic_baudrate -- bool
  • minimum_dynamic_baudrate -- uint32
Response:

no response

The SPITF protocol can be used with a dynamic baudrate. If the dynamic baudrate is enabled, the Brick will try to adapt the baudrate for the communication between Bricks and Bricklets according to the amount of data that is transferred.

The baudrate will be increased exponentially if lots of data is send/received and decreased linearly if little data is send/received.

This lowers the baudrate in applications where little data is transferred (e.g. a weather station) and increases the robustness. If there is lots of data to transfer (e.g. Thermal Imaging Bricklet) it automatically increases the baudrate as needed.

In cases where some data has to transferred as fast as possible every few seconds (e.g. RS485 Bricklet with a high baudrate but small payload) you may want to turn the dynamic baudrate off to get the highest possible performance.

The maximum value of the baudrate can be set per port with the function set_spitfp_baudrate. If the dynamic baudrate is disabled, the baudrate as set by set_spitfp_baudrate will be used statically.

The minimum dynamic baudrate has a value range of 400000 to 2000000 baud.

By default dynamic baudrate is enabled and the minimum dynamic baudrate is 400000.

New in version 2.3.4 (Firmware).

BrickServo.get_spitfp_baudrate_config
Function ID:

232

Request:

empty payload

Response:
  • enable_dynamic_baudrate -- bool
  • minimum_dynamic_baudrate -- uint32

Returns the baudrate config, see set_spitfp_baudrate_config.

New in version 2.3.4 (Firmware).

BrickServo.get_send_timeout_count
Function ID:

233

Request:
  • communication_method -- uint8
Response:
  • timeout_count -- uint32

Returns the timeout count for the different communication methods.

The methods 0-2 are available for all Bricks, 3-7 only for Master Bricks.

This function is mostly used for debugging during development, in normal operation the counters should nearly always stay at 0.

New in version 2.3.2 (Firmware).

The following meanings are defined for the parameters of this function:

  • 0: none, for communication_method
  • 1: usb, for communication_method
  • 2: spi stack, for communication_method
  • 3: chibi, for communication_method
  • 4: rs485, for communication_method
  • 5: wifi, for communication_method
  • 6: ethernet, for communication_method
  • 7: wifi v2, for communication_method
BrickServo.set_spitfp_baudrate
Function ID:

234

Request:
  • bricklet_port -- char
  • baudrate -- uint32
Response:

no response

Sets the baudrate for a specific Bricklet port ('a' - 'd'). The baudrate can be in the range 400000 to 2000000.

If you want to increase the throughput of Bricklets you can increase the baudrate. If you get a high error count because of high interference (see get_spitfp_error_count) you can decrease the baudrate.

If the dynamic baudrate feature is enabled, the baudrate set by this function corresponds to the maximum baudrate (see set_spitfp_baudrate_config).

Regulatory testing is done with the default baudrate. If CE compatibility or similar is necessary in you applications we recommend to not change the baudrate.

The default baudrate for all ports is 1400000.

New in version 2.3.2 (Firmware).

BrickServo.get_spitfp_baudrate
Function ID:

235

Request:
  • bricklet_port -- char
Response:
  • baudrate -- uint32

Returns the baudrate for a given Bricklet port, see set_spitfp_baudrate.

New in version 2.3.2 (Firmware).

BrickServo.get_spitfp_error_count
Function ID:

237

Request:
  • bricklet_port -- char
Response:
  • error_count_ack_checksum -- uint32
  • error_count_message_checksum -- uint32
  • error_count_frame -- uint32
  • error_count_overflow -- uint32

Returns the error count for the communication between Brick and Bricklet.

The errors are divided into

  • ACK checksum errors,
  • message checksum errors,
  • framing errors and
  • overflow errors.

The errors counts are for errors that occur on the Brick side. All Bricklets have a similar function that returns the errors on the Bricklet side.

New in version 2.3.2 (Firmware).

BrickServo.enable_status_led
Function ID:238
Request:empty payload
Response:no response

Enables the status LED.

The status LED is the blue LED next to the USB connector. If enabled is is on and it flickers if data is transfered. If disabled it is always off.

The default state is enabled.

New in version 2.3.1 (Firmware).

BrickServo.disable_status_led
Function ID:239
Request:empty payload
Response:no response

Disables the status LED.

The status LED is the blue LED next to the USB connector. If enabled is is on and it flickers if data is transfered. If disabled it is always off.

The default state is enabled.

New in version 2.3.1 (Firmware).

BrickServo.is_status_led_enabled
Function ID:

240

Request:

empty payload

Response:
  • enabled -- bool

Returns true if the status LED is enabled, false otherwise.

New in version 2.3.1 (Firmware).

BrickServo.get_protocol1_bricklet_name
Function ID:

241

Request:
  • port -- char
Response:
  • protocol_version -- uint8
  • firmware_version -- uint8[3]
  • name -- char[40]

Returns the firmware and protocol version and the name of the Bricklet for a given port.

This functions sole purpose is to allow automatic flashing of v1.x.y Bricklet plugins.

BrickServo.get_chip_temperature
Function ID:

242

Request:

empty payload

Response:
  • temperature -- int16

Returns the temperature in °C/10 as measured inside the microcontroller. The value returned is not the ambient temperature!

The temperature is only proportional to the real temperature and it has an accuracy of +-15%. Practically it is only useful as an indicator for temperature changes.

BrickServo.reset
Function ID:243
Request:empty payload
Response:no response

Calling this function will reset the Brick. Calling this function on a Brick inside of a stack will reset the whole stack.

After a reset you have to create new device objects, calling functions on the existing ones will result in undefined behavior!

BrickServo.get_identity
Function ID:

255

Request:

empty payload

Response:
  • uid -- char[8]
  • connected_uid -- char[8]
  • position -- char
  • hardware_version -- uint8[3]
  • firmware_version -- uint8[3]
  • device_identifier -- uint16

Returns the UID, the UID where the Brick is connected to, the position, the hardware and firmware version as well as the device identifier.

The position can be '0'-'8' (stack position).

The device identifier numbers can be found here

Callback Configuration Functions

BrickServo.set_minimum_voltage
Function ID:

24

Request:
  • voltage -- uint16
Response:

no response

Sets the minimum voltage in mV, below which the CALLBACK_UNDER_VOLTAGE callback is triggered. The minimum possible value that works with the Servo Brick is 5V. You can use this function to detect the discharge of a battery that is used to drive the stepper motor. If you have a fixed power supply, you likely do not need this functionality.

The default value is 5V (5000mV).

BrickServo.get_minimum_voltage
Function ID:

25

Request:

empty payload

Response:
  • voltage -- uint16

Returns the minimum voltage as set by set_minimum_voltage

BrickServo.enable_position_reached_callback
Function ID:29
Request:empty payload
Response:no response

Enables the CALLBACK_POSITION_REACHED callback.

Default is disabled.

New in version 2.0.1 (Firmware).

BrickServo.disable_position_reached_callback
Function ID:30
Request:empty payload
Response:no response

Disables the CALLBACK_POSITION_REACHED callback.

Default is disabled.

New in version 2.0.1 (Firmware).

BrickServo.is_position_reached_callback_enabled
Function ID:

31

Request:

empty payload

Response:
  • enabled -- bool

Returns true if CALLBACK_POSITION_REACHED callback is enabled, false otherwise.

New in version 2.0.1 (Firmware).

BrickServo.enable_velocity_reached_callback
Function ID:32
Request:empty payload
Response:no response

Enables the CALLBACK_VELOCITY_REACHED callback.

Default is disabled.

New in version 2.0.1 (Firmware).

BrickServo.disable_velocity_reached_callback
Function ID:33
Request:empty payload
Response:no response

Disables the CALLBACK_VELOCITY_REACHED callback.

Default is disabled.

New in version 2.0.1 (Firmware).

BrickServo.is_velocity_reached_callback_enabled
Function ID:

34

Request:

empty payload

Response:
  • enabled -- bool

Returns true if CALLBACK_VELOCITY_REACHED callback is enabled, false otherwise.

New in version 2.0.1 (Firmware).

Callbacks

BrickServo.CALLBACK_UNDER_VOLTAGE
Function ID:

26

Response:
  • voltage -- uint16

This callback is triggered when the input voltage drops below the value set by set_minimum_voltage. The response value is the current voltage given in mV.

BrickServo.CALLBACK_POSITION_REACHED
Function ID:

27

Response:
  • servo_num -- uint8
  • position -- int16

This callback is triggered when a position set by set_position is reached. If the new position matches the current position then the callback is not triggered, because the servo didn't move. The response values are the servo and the position that is reached.

You can enable this callback with enable_position_reached_callback.

Note

Since we can't get any feedback from the servo, this only works if the velocity (see set_velocity) is set smaller or equal to the maximum velocity of the servo. Otherwise the servo will lag behind the control value and the callback will be triggered too early.

BrickServo.CALLBACK_VELOCITY_REACHED
Function ID:

28

Response:
  • servo_num -- uint8
  • velocity -- int16

This callback is triggered when a velocity set by set_velocity is reached. The response values are the servo and the velocity that is reached.

You can enable this callback with enable_velocity_reached_callback.

Note

Since we can't get any feedback from the servo, this only works if the acceleration (see set_acceleration) is set smaller or equal to the maximum acceleration of the servo. Otherwise the servo will lag behind the control value and the callback will be triggered too early.