Communication stack
- Torsten Bitterlich (Unlicensed)
- Tino Biehle
Cyclic Data Exchange
The Communication Controller (CC) uses the Serial Peripheral Interface (SPI) to communicate with an attached Application Controller (AC).
A cyclic transfer must always be triggered by the AC and must at least be once during the heartbeat timeout. The implementation at the CC updates the cyclic data after each transfer to make sure it doesn't interrupt a running transfer.
Clock domains and communication cycle
Operation of the device includes two clock domains, which run independently of each other:
Industrial Protocol cycle at the Fieldbus. Commonly the PLC interacts with the device in one clock domain, where the PLC controls the timing of the output data.
SPI Transfer cycle between AC and CC. This is driven by the AC through initiation of the SPI cycle, which reads the output data from the CC and updates input data for the next fieldbus cycle.
Due to the different cycles, a specific timing of the process data is set up. Following data transport occurs:
A New output data from PLC;
B Processing of output data in CC module and preloading of cyclic transfer buffer;
C finished cyclic transfer initiated by the AC executes data exchange between AC and CC;
D Preloading of new input data for the next cyclic transfer;
E A finished sequential cyclic transfer executes data exchange between AC and CC, thus providing new input data for the next fieldbus transfer.
Due to the independent cycles, mirroring process data takes up to several milliseconds.
Industrial Protocols like EtherCAT uses much smaller cycle times. However, the CC is able to communicate at this cycle times, even though the AC initiates the SPI transfer each millisecond.
Technical data
Transfer length:
Cyclic only: 73 Bytes
Cyclic + RPC data: 128 Bytes
Baudrate:
at SPI: max. 10 MHz, usually 2 MHz.
Delay between cyclic transfer:
SPI: 0.5 ms... Heartbeat Timeout (1 second)
Minimal round-trip time: 4 ... 6 ms (depending on the used protocol and setup)
Serial Peripheral Interface Timing and Settings
Following simplified diagram shows basic SPI timing which must be considered by the application controller.
Figure 3 Basic SPI timing
SPI Speed
The CC supports SPI speed in the range of 29.3 kHz to theretically 20 MHz. Default SPI speed for most of the platforms is 2 MHz.
SPI Setup Time
In Figure 3 Basic SPI timing, the communication scheme between AC and CC is shown. Following time needs to be considered by the Application Controller during communication: the SPI Setup Time - the time between activation of the module using the Slave Select signal and first data - must be at least 2-3 us, before the module accepts data over SPI.
SPI Cycle Time
This time is the distance between two consecutive SPI transfers. Between two transfers a minimum time of 250 us must be kept to secure proper processing in the module.
SPI Settings
The module provides a management inferface, where initial configuration can be done. These properties can be stored permanently within the device. Following properties are provided to configure the SPI interface:
Variable | Description | Default value |
SPI_TYPE | SPI master/slave configuration | 1 = SLAVE |
SPI_MODE | SPI timing mode | 0 = MODE0* |
SPI_UNITWIDTH | SPI single transfer size | 0 = 8 BIT |
SPI_BITORDER | SPI bit transfer direction | 0 = MSB* |
SPI_TRANSFERSIZE | SPI packet transfer size | 128 |
* These Values can be modified
Cyclic Frame Structure
The cyclic frame must contain the structure from following table to get accepted by the iRJ45 module/ SoM.
Byte Position | Description |
|---|---|
0 .. 1 | Fletcher-16 Checksum with Offset 0x0007 (little endian) |
2 | Sequence Count |
3 | Data Length |
4 .. 76 | Cyclic Data |
77 .. 127 | Remote Procedure Calls |
The same layout is send back by the device containing its local sequence counter. The sequence counter is tracked and if it doesn't change during the heartbeat timeout the communication gets stopped until the sequence is updated again.
Fletcher-16 Checksum (16 Bit)
To calculate the Fletcher-16 checksum this Wikipedia entry can be used. Start index is byte 4 and end is at 127. After the calculation the value 0x0007 needs to be added to not have false positives if the whole area is set to zeros. In the frame the value has a width of 16 bit and needs to have the little endian encoding.
Sequence Counter (8 Bit)
The sequence counter must be incremented on each sent out frame to be recognized as new data. It can start at any 8 bit value.
Data Length (8 Bit)
If only cyclic data is transferred the data length can be set from 0 to 73 bytes. When using RPC the data length needs to have a fixed value of 124.
Remote Procedure Call
The RPC protocol used by the Micro Core-To-Core (MCTC) implementation of the iRJ45 module is transferred in byte 77 – 127. RPC transfers are always acknowledged to minimize data loss on erroneous transfers as good as possible. Also the RPC calls can be larger than the available 50 bytes as the iRJ45 module internally stores each received RPC frame in a ring-buffer and waits until a partitioned transfer is completed before proceeding with the request.
RPC Frame Structure
The RPC frame must contain the structure from following table to be accepted by the iRJ45 module.
Byte Position | Description |
|---|---|
0 .. 1 | Fletcher-16 Checksum with Offset 0x0007 (little endian) |
2 | Local Sequence |
3 | Remote Sequence Acknowledge |
4 | Data Length |
5 | Flags |
6 .. 49 | Data |
Each time a new local sequence is sent the iRJ45 will respond with the corresponding acknowledge in the second transferred frame. If no acknowledge was received the AC can retransmit its frame to re-request the acknowledge.
Local Sequence (8 Bit)
The sequence counter must be incremented for each RPC frame with changed data. For each unseen incremental frame the iRJ45 module will put the transferred data into its internal ring-buffer and after the length matches it will process the RPC call.
Remote Sequence Acknowledge (8 Bit)
For each processed received RPC frame the AC needs to send out an acknowledge. If the received frame doesn't match the expected sequence the AC must leave the acknowledge on the previous sequence to trigger a resend from the iRJ45 module. If the resend fails the AC can also perform a re-sync.
Data Length (8 Bit)
Contains the length of the RPC data and must be between 0 (acknowledge-only frame) and 44.
Flags (8 Bit)
Bit Position | Description |
|---|---|
0 | Sync Request |
1 | Sync Acknowledge |
2 | Reserved |
3 | Request Acknowledge |
Sync Request
If set to 1 the iRJ45 module enters the RPC synchronization.
Sync Acknowledge
During the Sync Request both sides need to set the Sync Acknowledge flag, see RPC Synchronization for details.
Request Acknowledge
Forces an acknowledge from the partner device.
RPC Synchronization
Before the iRJ45 module is ready to operate and after a synchronization is lost the RPC must be synchronized. This is triggered by setting the Sync Request flag to 1.
AC | iRJ45 Module |
|---|---|
Sync Request = 1 | |
Sync Request = 1 | |
Sync Request = 0 | |
RPC_Receive() | |
RPC_Receive() | |
RPC_Receive() |
After the synchronization has finished the iRJ45 must initiate the generic RPC call SetupStateGet to verify if the partner device is in the same state. If one partner is already configured but the other is not, than the configured parter must reboot and the synchronization starts again.
Normal Operation
If the AC successfully passed the synchronization stage and configured the iRJ45 module it must call the RPC API SetupDone. At this point the iRJ45 module is ready for normal operation.
RPC Protocol
The GOAL RPC protocol uses a virtual push/pop stack to call remote API functions with arguments. Each call must have a return value that is usually set to GOAL_OK when the call succeeded.
RPC Request/Response Structure
An RPC request/response consists of the parts shown in Table 21 and Table 22.
Byte 0..1 | Byte 2..5 | Byte 6 .. x | Byte x+1 .. x+4 | Byte x+5..x+8 | Byte x+9 | Byte x+10 | Byte (x+11) .. (x+14) |
|---|---|---|---|---|---|---|---|
Static Identifier | Data Length | Data | Function Id | RPC Id(little endian) | CTC Id | Flags | Fletcher-16 |
Table 22 RPC Request Structure
Byte 0..1 | Byte 2..5 | Byte 6 .. x | Byte x+1 | Byte x+2 | Byte (x+3)..(x+4) |
|---|---|---|---|---|---|
Static Identifier | Data Length | Response Data | CTC Id | Flags | Fletcher-16 |
Table 23 RPC Response Structure
Static Identifier
The 2 byte static identifier is used on the iRJ45 module to detect the start of a new RPC request or response. If the identifier is also contained in the data bytes the Fletcher-16 checksum will make sure that it don't gets threaded like an RPC request/response.
Data Length
The data length contains the count of bytes starting with the "Function Id" and ending with the last data byte.
RPC Id
The RPC id is the module or group id. For example GOAL PROFINET uses the RPC id GOAL_ID_PNIO to register its calls.
Function Id
The function id is used as a sub id to map the function calls in the specific module to their handlers.
CTC Id
The CTC id is used to match requests and responses to their specific MCTC internal handle. A response must use the same CTC id as the request.
Flags
The flags define the type of the request, see Table 5: RPC Request/Response Flags for details.
Data
The data part contains the virtual stack of the RPC request/response.