CC-Link IE Field Basic Operation

This section describes how the features of CC-Link IE Field Basic operate.

Results Buffering

The Results Buffering feature allows a queue for decode results. When enabled, the feature queues a finite number of result data sets until the PLC (client) has time to read them. This smooths out data flow if the PLC slows down for short periods or if there are surges in read activity.

If the feature is enabled, the reader overlaps acquisition and decode operations. This can lead to faster over all trigger rates. When reads occur faster than results can be sent out (the queue is full), the most recent results are discarded until space becomes available in the results queue.

If the feature is disabled, the more recent results overwrite the earlier results. The most recent results are kept. Earlier results are discarded if the PLC does not read them fast enough.

If the queue overflows while the feature is disabled:

  • The difference between the TriggerID and ResultID values becomes greater than 1. This difference represents the number of reads that occurred but could not be queued because the queue was full.

  • The number of lost reads equals TriggerID - ResultID - 1.

  • After the next read, the ResultID value returns to the typical operating value of TriggerID - 1.

Result Fragmentation

The Result Fragmentation feature splits decode result strings into fragments if the strings are too large for the Results Data field. The fragments are then pushed to the buffer like normal results. Then, the PLC reads the fragments one by one.

Result Fragmentation depends on Result Buffering. This means that results are only fragmented if Result Buffering is enabled.

Note: For CC-Link IE Field Basic, you have to enable Result Fragmentation manually.

Enabling Result Fragmentation

To enable or disable Result Fragmentation, you have the following options:

  • Use the FFP.RESULT-FRAG DMCC.

  • Use SoftEvent 6 to enable the Results Buffering and Result Fragmentation features from the PLC.

Operation of Result Fragmentation

The PLC does not automatically reassemble the result string. Once the PLC reads a result fragment, you have to copy the fragment to a new user-defined variable. After this, you have to set and clear the ResultAck bit, which updates the Decode Results with the next fragment. This process continues until all fragments have been sent to the PLC.

In contrast, if Results Buffering is disabled, large decode result strings are cut off to fit into the Results Data field, which means that the end of the string gets lost.

Identifying Decode Result Chunks

Decode result fragments look like normal results. To identify result fragments, look at the Result ID:

  • The Result ID of the result fragments are the same.

  • The Result ID of the last result fragment is different from the Result ID of the next result.

Independent of whether results fragmentation is enabled, the Result Length always represents the number of characters to be read from the Results Data plus the remaining characters that did not fit into the Results Data buffer. With results fragmentation enabled, the remaining characters will be found in follow-up fragments, the remaining characters are discarded.

You can identify the first and last fragments of a result as follows:

  • The first fragment has a different Result ID from the previous Result ID.

  • The Result Length of the last fragment is lower or equal to the Result Data buffer size.

  • All fragments of a result share the same Result ID.

Maximum Result Data Length

Without results fragmentation, result data is truncated to fit into the results data buffer size. Depending on the protocol, the result buffer size can be fixed or configurable.

The Result Fragmentation feature allows processing of up to 50 times longer results, depending on the results buffering queue size, with an absolute limit of 65535 characters.

The maximum number of result characters for CC-Link IE Field Basic is 1014 bytes (fixed).

DMCC Result Feedback from PLC

The DMCC Result Feedback feature allows the reader to send DMCC responses back to the PLC.

By default, this feature is off. If enabled, the reader sends DMCC responses to the PLC using the same mechanism as for read results. To distinguish DMCC results from read results, bit 6 is flagged in the ResultCode field.

To enable the feature, use the FFP.DMCC-RESULTS command. Changing this configuration does not require you to reboot or reconnect reader.

You can use DMCC Result Feedback in combination with Results Fragmentation.