Tune SNP Protocol Throughput

The protocol supports the ability to tune certain protocol level parameters that significantly impact performance. These parameters, along with the baud rate, are dominant factors in polling throughput.

SNP data size.

Time between SNP messages.

Time before SNP attach messages.

SNP Data Size

This parameter controls the amount of data that can be sent between the host and the Series 90 controller. For CIMPLICITY projects, CIMPLICITY software will attempt to negotiate a data size of 4000 bytes. This value was selected in order to optimize the I/O performance in the computer where CIMPLICITY software is running. If a lower value is configured on the Series 90 controller, the lower value will be used.

Time between SNP Messages

This parameter controls the time between SNP messages. By default, CIMPLICITY software attempts to negotiate a value of 5 milliseconds. You can configure a higher value by defining a logical named BSM$SNP_T1_TIME.

To define this logical, add a line to the project's data\log_names.cfg file with the following format:

BSM$SNP_T1_TIME|S|default|3|<time>

Where <time> is the desired time in milliseconds between messages.

If you create or modify the logical, you must stop and restart the project for the new value to take effect.

As with the data size parameter, the time between SNP messages is a negotiated value. A higher value on the Series 90 controller will result in the higher value being used.

Time before SNP Attach Messages

This parameter controls the time delay before SNP attach messages. By default, there is a 1-millisecond delay before the SNP attach messages. If the SNP attach fails while the PLCs are running, a minimum value of 5 milliseconds is recommended to resolve the connect failure situation. You can configure a longer delay value by defining a logical named BSM$SNP_ATTACH_DELAY.

To define this logical, add a line to the project's data\log_names.cfg file with the following format:

BSM$SNP_ATTACH_DELAY|P|default|3|<time>

where <time> is the desired delay time in milliseconds before the SNP attach messages.

If you create or modify the logical, you must stop and restart the project for the new value to take effect.

More information

SNP performance notes.