Setup Tips and Techniques

• This section describes techniques to consider when setting up your system.

• Before the first writing to a variable, at least one read has to be made so that the driver can get the SNMP data type of that variable.

• Trap data types can only be used on IP networks. Trap data is treated as global and stored in a queue in this driver; i.e., trap variables could be defined under any I/O device and any two trap variables with the same address (such as T0) will have the same value. A read will get the variable value from the first elements of the queue and a write will delete that first element from the queue. The queue length is limited by available memory.

• The default value of TF is 127.0.0.1; that is, the local PC address.

• The GET-NEXT SNMP functionality is not supported in the driver.

• The target device has to support large PDU packets if PDUGroupOK is high. Some devices might become inoperable if this value is too large.

• When an error is detected in reading one item in a group, all items are flagged as errors. To establish which item is causing the error, set PDUGroupOK = 0, so that the driver sends a separate SNMP packet for each item, and set ReturnError = 0, so that the driver does not return an error to the I/O Server. Then the item which is causing the problem will be displayed with the value "Bad" if it is a string, or the value 2989 (the decimal equivalent of the hex 0xBAD) if it is numeric. Once the problem item has been located, always remember to set ReturnError = 1, so that further errors generate #COM.

• The fields of a Variable Tag correspond to the entries in the SNMPVARS.DBF file defined for the port the device is on. Note that the Address field is a combination of the INDEX and SNMPTYPE entries in the SNMPVARS.DBF. You do not have to adjust these parameters in Plant SCADA or Excel; the utility allows you to do this.

Note: From version 2.7.20.0, the SNMPII driver includes new functionality that prevents a device from going online if duplicate indexes exist. This functionality requires every index to be unique at all times. Unique indexes can be achieved by creating multiple ports and splitting the SNMPVARS.DBF into multiple databases (one for each respective port).