Modbus - CAN Bricklet

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

API

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

Basic Functions

BrickletCAN.write_frame
Function ID:

1

Request:
  • frame_type -- uint8
  • identifier -- uint32
  • data -- uint8[8]
  • length -- uint8
Response:
  • success -- bool

Writes a data or remote frame to the write buffer to be transmitted over the CAN transceiver.

The Bricklet supports the standard 11-bit (CAN 2.0A) and the additional extended 18-bit (CAN 2.0B) identifiers. For standard frames the Bricklet uses bit 0 to 10 from the identifier parameter as standard 11-bit identifier. For extended frames the Bricklet additionally uses bit 11 to 28 from the identifier parameter as extended 18-bit identifier.

For remote frames the data parameter is ignored.

Returns true if the frame was successfully added to the write buffer. Returns false if the frame could not be added because write buffer is already full.

The write buffer can overflow if frames are written to it at a higher rate than the Bricklet can transmitted them over the CAN transceiver. This may happen if the CAN transceiver is configured as read-only or is using a low baud rate (see set_configuration). It can also happen if the CAN bus is congested and the frame cannot be transmitted because it constantly loses arbitration or because the CAN transceiver is currently disabled due to a high write error level (see get_error_log).

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

  • 0: standard data, for frame_type
  • 1: standard remote, for frame_type
  • 2: extended data, for frame_type
  • 3: extended remote, for frame_type
BrickletCAN.read_frame
Function ID:

2

Request:

empty payload

Response:
  • success -- bool
  • frame_type -- uint8
  • identifier -- uint32
  • data -- uint8[8]
  • length -- uint8

Tries to read the next data or remote frame from the read buffer and return it. If a frame was successfully read, then the success return value is set to true and the other return values contain the frame. If the read buffer is empty and no frame could be read, then the success return value is set to false and the other return values contain invalid data.

The identifier return value follows the identifier format described for write_frame.

For remote frames the data return value always contains invalid data.

A configurable read filter can be used to define which frames should be received by the CAN transceiver and put into the read buffer (see set_read_filter).

Instead of polling with this function, you can also use callbacks. See the enable_frame_read_callback function and the CALLBACK_FRAME_READ callback.

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

  • 0: standard data, for frame_type
  • 1: standard remote, for frame_type
  • 2: extended data, for frame_type
  • 3: extended remote, for frame_type
BrickletCAN.set_configuration
Function ID:

6

Request:
  • baud_rate -- uint8
  • transceiver_mode -- uint8
  • write_timeout -- int32
Response:

no response

Sets the configuration for the CAN bus communication.

The baud rate can be configured in steps between 10 and 1000 kbit/s.

The CAN transceiver has three different modes:

  • Normal: Reads from and writes to the CAN bus and performs active bus error detection and acknowledgement.
  • Loopback: All reads and writes are performed internally. The transceiver is disconnected from the actual CAN bus.
  • Read-Only: Only reads from the CAN bus, but does neither active bus error detection nor acknowledgement. Only the receiving part of the transceiver is connected to the CAN bus.

The write timeout has three different modes that define how a failed frame transmission should be handled:

  • One-Shot (< 0): Only one transmission attempt will be made. If the transmission fails then the frame is discarded.
  • Infinite (= 0): Infinite transmission attempts will be made. The frame will never be discarded.
  • Milliseconds (> 0): A limited number of transmission attempts will be made. If the frame could not be transmitted successfully after the configured number of milliseconds then the frame is discarded.

The default is: 125 kbit/s, normal transceiver mode and infinite write timeout.

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

  • 0: 10kbps, for baud_rate
  • 1: 20kbps, for baud_rate
  • 2: 50kbps, for baud_rate
  • 3: 125kbps, for baud_rate
  • 4: 250kbps, for baud_rate
  • 5: 500kbps, for baud_rate
  • 6: 800kbps, for baud_rate
  • 7: 1000kbps, for baud_rate
  • 0: normal, for transceiver_mode
  • 1: loopback, for transceiver_mode
  • 2: read only, for transceiver_mode
BrickletCAN.get_configuration
Function ID:

7

Request:

empty payload

Response:
  • baud_rate -- uint8
  • transceiver_mode -- uint8
  • write_timeout -- int32

Returns the configuration as set by set_configuration.

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

  • 0: 10kbps, for baud_rate
  • 1: 20kbps, for baud_rate
  • 2: 50kbps, for baud_rate
  • 3: 125kbps, for baud_rate
  • 4: 250kbps, for baud_rate
  • 5: 500kbps, for baud_rate
  • 6: 800kbps, for baud_rate
  • 7: 1000kbps, for baud_rate
  • 0: normal, for transceiver_mode
  • 1: loopback, for transceiver_mode
  • 2: read only, for transceiver_mode

Advanced Functions

BrickletCAN.set_read_filter
Function ID:

8

Request:
  • mode -- uint8
  • mask -- uint32
  • filter1 -- uint32
  • filter2 -- uint32
Response:

no response

Set the read filter configuration. This can be used to define which frames should be received by the CAN transceiver and put into the read buffer.

The read filter has five different modes that define if and how the mask and the two filters are applied:

  • Disabled: No filtering is applied at all. All frames are received even incomplete and defective frames. This mode should be used for debugging only.
  • Accept-All: All complete and error-free frames are received.
  • Match-Standard: Only standard frames with a matching identifier are received.
  • Match-Standard-and-Data: Only standard frames with matching identifier and data bytes are received.
  • Match-Extended: Only extended frames with a matching identifier are received.

The mask and filters are used as bit masks. Their usage depends on the mode:

  • Disabled: Mask and filters are ignored.
  • Accept-All: Mask and filters are ignored.
  • Match-Standard: Bit 0 to 10 (11 bits) of mask and filters are used to match the 11-bit identifier of standard frames.
  • Match-Standard-and-Data: Bit 0 to 10 (11 bits) of mask and filters are used to match the 11-bit identifier of standard frames. Bit 11 to 18 (8 bits) and bit 19 to 26 (8 bits) of mask and filters are used to match the first and second data byte (if present) of standard frames.
  • Match-Extended: Bit 0 to 10 (11 bits) of mask and filters are used to match the standard 11-bit identifier part of extended frames. Bit 11 to 28 (18 bits) of mask and filters are used to match the extended 18-bit identifier part of extended frames.

The mask and filters are applied in this way: The mask is used to select the identifier and data bits that should be compared to the corresponding filter bits. All unselected bits are automatically accepted. All selected bits have to match one of the filters to be accepted. If all bits for the selected mode are accepted then the frame is accepted and is added to the read buffer.

Mask Bit Filter Bit Identifier/Data Bit Result
0 X X Accept
1 0 0 Accept
1 0 1 Reject
1 1 0 Reject
1 1 1 Accept

For example, to receive standard frames with identifier 0x123 only the mode can be set to Match-Standard with 0x7FF as mask and 0x123 as filter 1 and filter 2. The mask of 0x7FF selects all 11 identifier bits for matching so that the identifier has to be exactly 0x123 to be accepted.

To accept identifier 0x123 and identifier 0x456 at the same time, just set filter 2 to 0x456 and keep mask and filter 1 unchanged.

The default mode is accept-all.

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

  • 0: disabled, for mode
  • 1: accept all, for mode
  • 2: match standard, for mode
  • 3: match standard and data, for mode
  • 4: match extended, for mode
BrickletCAN.get_read_filter
Function ID:

9

Request:

empty payload

Response:
  • mode -- uint8
  • mask -- uint32
  • filter1 -- uint32
  • filter2 -- uint32

Returns the read filter as set by set_read_filter.

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

  • 0: disabled, for mode
  • 1: accept all, for mode
  • 2: match standard, for mode
  • 3: match standard and data, for mode
  • 4: match extended, for mode
BrickletCAN.get_error_log
Function ID:

10

Request:

empty payload

Response:
  • write_error_level -- uint8
  • read_error_level -- uint8
  • transceiver_disabled -- bool
  • write_timeout_count -- uint32
  • read_register_overflow_count -- uint32
  • read_buffer_overflow_count -- uint32

Returns information about different kinds of errors.

The write and read error levels indicate the current level of checksum, acknowledgement, form, bit and stuffing errors during CAN bus write and read operations.

When the write error level extends 255 then the CAN transceiver gets disabled and no frames can be transmitted or received anymore. The CAN transceiver will automatically be activated again after the CAN bus is idle for a while.

The write and read error levels are not available in read-only transceiver mode (see set_configuration) and are reset to 0 as a side effect of changing the configuration or the read filter.

The write timeout, read register and buffer overflow counts represents the number of these errors:

  • A write timeout occurs if a frame could not be transmitted before the configured write timeout expired (see set_configuration).
  • A read register overflow occurs if the read register of the CAN transceiver still contains the last received frame when the next frame arrives. In this case the newly arrived frame is lost. This happens if the CAN transceiver receives more frames than the Bricklet can handle. Using the read filter (see set_read_filter) can help to reduce the amount of received frames. This count is not exact, but a lower bound, because the Bricklet might not able detect all overflows if they occur in rapid succession.
  • A read buffer overflow occurs if the read buffer of the Bricklet is already full when the next frame should be read from the read register of the CAN transceiver. In this case the frame in the read register is lost. This happens if the CAN transceiver receives more frames to be added to the read buffer than are removed from the read buffer using the read_frame function. Using the CALLBACK_FRAME_READ callback ensures that the read buffer can not overflow.
BrickletCAN.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 Bricklet is connected to, the position, the hardware and firmware version as well as the device identifier.

The position can be 'a', 'b', 'c' or 'd'.

The device identifier numbers can be found here

Callback Configuration Functions

BrickletCAN.enable_frame_read_callback
Function ID:3
Request:empty payload
Response:no response

Enables the CALLBACK_FRAME_READ callback.

By default the callback is disabled.

BrickletCAN.disable_frame_read_callback
Function ID:4
Request:empty payload
Response:no response

Disables the CALLBACK_FRAME_READ callback.

By default the callback is disabled.

BrickletCAN.is_frame_read_callback_enabled
Function ID:

5

Request:

empty payload

Response:
  • enabled -- bool

Returns true if the CALLBACK_FRAME_READ callback is enabled, false otherwise.

Callbacks

BrickletCAN.CALLBACK_FRAME_READ
Function ID:

11

Response:
  • frame_type -- uint8
  • identifier -- uint32
  • data -- uint8[8]
  • length -- uint8

This callback is triggered if a data or remote frame was received by the CAN transceiver.

The identifier return value follows the identifier format described for write_frame.

For remote frames the data return value always contains invalid values.

A configurable read filter can be used to define which frames should be received by the CAN transceiver at all (see set_read_filter).

To enable this callback, use enable_frame_read_callback.

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

  • 0: standard data, for frame_type
  • 1: standard remote, for frame_type
  • 2: extended data, for frame_type
  • 3: extended remote, for frame_type