Encapsulated Interface Transport (FC43)
This function provides an easy access to the CANopen object dictionary. More details can be found in the following documentations:
- Modbus Application Protocol Specification V1.1b3
- CAN in Automation (CiA) 309-2, Interfacing CANopen with TCP/IP - Part 2: Modbus/TCP mapping
| Note |
|---|
| For parts of the messages of the Encapsulated Interface Transport another type of endianness applies (see General). |
Request and Response definition
| Name | Length | Example/Range |
|---|---|---|
| Slave Address | 1 Byte | |
| Function Code | 1 byte | 2Bh (43d) |
| MEI type | 1 byte | 0Dh (13d) |
| Protocol option fields | 2 to 5 bytes | |
| Address and data fields | N bytes | |
| CRC | 2 Bytes |
Protocol option fields:
| Name | Length | Example/Range |
|---|---|---|
| Protocol control | 1 to 2 bytes | Refer to description |
| Reserved field | 1 byte | Always 0 |
| (Optional) Counter byte | 1 byte | |
| (Optional) Network ID | 1 byte | |
| (Optional) Encoded data | 1 byte |
Protocol control:
The protocol control field contains the flags needed to control the message protocol. The protocol control bytes is defined as follows in case the value of the "Extend flag" is set (otherwise Byte 2 gets dropped) :
The most significant bit (MSB) is bit 0 for protocol control byte 1, and bit 8 for protocol control byte 2. The least significant bit (LSB) is bit 7 for protocol control byte 1, and bit 15 for protocol control byte 2.
| Bit | Field name | Description |
|---|---|---|
| 0 | Extend flag | This bit is used when the object dictionary data set is larger than what fits in one Modbus command. The data set is then extended over multiple Modbus messages, each message being a fragment of the data set. "0" = No multiple message transaction, or end of multiple message transaction; "1" = Part of a multiple message transaction |
| 1 | Extend protocol control | Length of protocol control, the value "0" indicates a length of 1 byte, the value "1" a length of 2 byte |
| 2 | Counter byte option | This bit is set to 1 to indicate that the counter byte field is used in this message. If this bit is set to 0 the counter byte does not exist in this message. |
| 3 and 4 | Reserved | 0 |
| 5 | Network ID option | Not supported, must be "0". |
| 6 | Encoded data option | Not supported, must be "0". |
| 7 | Access flag | This bit indicates the access method of the request command, "0" = read, "1" = write |
| 8 to 15 | Reserved | 0 |
Address and data fields
The address and data fields are defined in the following table:
| Field name | Byte size and order | Example / range |
|---|---|---|
| Node ID | 1 byte | 01h to 7Fh |
| Index | 1 byte, high | 0000h to FFFFh |
| 1 byte, low | ||
| Sub-index | 1 byte | 00h to FFh |
| Starting address | 1 byte, high | 0000h to FFFFh |
| 1 byte, low | ||
| Number of data values | 1 byte, high | 0000h to 00FDh |
| 1 byte, low | ||
| Read/write data | n byte | The data are encoded as written in chapter General |
Example
In order to read out the object 6042h:00h (16 bit value), the master needs to send out the following message(all values in hexadecimal representation, Slave-Id of the controller is "5")
- Request
- Response
As additional example, the following sequence of Modbus messages from master to slave can be used to start a motor:
Now following two examples of reading an object:
Error reaction
In case of an error, the following message will be sent:
| Name | Length | Example value |
|---|---|---|
| Slave Address | 1 Byte | |
| Function code | 1 byte | 2Bh +80h (171d = 43d + 128d) (indicates error) |
| Modbus exception code | 1 byte | FFh (extended exception) |
| Extended exception length | 2 bytes | 6 |
| MEI type | 1 byte | 0Dh |
| Exception code | 1 byte | CEh |
| Error code | 4 bytes | CANopen error code |
| CRC | 2 Bytes |
In case an not supported control option bit is set, the following error message will be sent:
| Name | Length | Example value |
|---|---|---|
| Slave Address | 1 Byte | |
| Function code | 1 byte | 2Bh +80h (171d = 43d + 128d) (indicates error) |
| Modbus exception code | 1 byte | FFh (extended exception) |
| Extended exception length | 2 bytes | 2 + length of Supported protocol control |
| MEI type | 1 byte | 0Dh |
| Exception code | 1 byte | AEh |
| Supported protocol control | 1 or 2 bytes | See following table |
| CRC | 2 Bytes |
| Bit | Field name | Description |
|---|---|---|
| 0 | Extend flag | This bit is used when the object dictionary data set is larger than what fits in one Modbus command. The data set is then extended over multiple Modbus messages, each message being a fragment of the data set. "0" = No multiple message transaction, or end of multiple message transaction; "1" = Part of a multiple message transaction |
| 1 | Extend protocol control | Length of protocol control, the value "0" indicates a length of 1 byte, the value "1" a length of 2 byte |
| 2 | Counter byte option | This bit is set to 1 to indicate that the counter byte field is used in this message. If this bit is set to 0 the counter byte does not exist in this message. |
| 3 and 4 | Reserved | 0 |
| 5 | Network ID option | Not supported, must be "0". |
| 6 | Encoded data option | Not supported, must be "0". |
| 7 | Access flag | This bit indicates the access method of the request command, "0" = read, "1" = write |
| 8 to 15 | Reserved | 0 |
The following example shows an error in case of a wrong request. The requests reads the object 6061h:00 with a length of 2 byte, but the object is 1 byte long, actually:
- Request
- Response
