SNMPII Blocking

You can configure the SNMPII driver for blocking. You can only block longs and reals (N and R data-types). For every SNMPII I/O device a snmpvar.dbf file is created. In that file an index is defined for each SNMP tag. This index is used by Plant SCADA for blocking. That index in the snmpvar.dbf is also used in the tag address in the variable.dbf, with a prefix for the different data-types (N for long, R for real, DA for string).

You can implement blocking for the SNMPII driver by manually manipulating the index numbers in snmpvar.dbf file. If you create consecutive numbers in a data type, Plant SCADA can block these together in one request. This index number has to be unique for each tag (per I/O device); for example: N100, N101, N102, N103 will be a block of four LONGs, and R200, R201, R203 will be a block of three REALs.

The SNMPII driver by default only blocks 20 items (for longs and reals this equals 640 bits). This value is set via the [SNMP]PDUGroupOK parameter and the BIT_BLOCK value in protdir.dbf for the SNMPII protocol. The target device has to support large PDU packets if PDUGroupOK is high. Some devices may crash if this value is too large. The [SNMPII]Block parameter is set to 256 by default.

In the Kernel use the Probe command to view the blocking behavior. There you will all request from the client to the I/O server and the size of these requests. The address numbers there are derived from the index numbers in the snmpvar.dbf files.