Asynchronous Processing

The MacroView Enhanced PLC Model for communication drivers works best with fully asynchronous driver implementations. An asynchronous implementation can manage multiple outstanding communication requests across multiple networks. The symax 802.3 protocol communications driver supports this asynchronous behaviour and defaults to a maximum of 3 outstanding requests per destination PLC. Depending on your configuration and circumstances, this can be increased to a maximum of 8 outstanding requests per PLC station for increased performance. The outstanding request count is the maximum number of command packets the driver will send at one time before holding back and waiting for a reply. This represents an implementation of the sliding window functionality within the driver.

Note: Tests with a Symax 450 processor has shown the ethernet NIM in the processor can get into a state where packets are delivered out of sequence if a continuous heavy transmission of command packets are sent with 8 outstanding requests. For this reason, site specific consideration is needed when specifying the higher outstanding request counts.