The following optimizations and considerations should be heeded when you are using parent and child tags in your application:
• When structuring a parent and child tags, it is good practice to configure the parent with only those parameters that are necessary to define what is unique for the station (i.e. the parent should have enough parameters to cover variants in the children). The child tags will then have the common parameters defined.
• In the event that you have hundreds of child tags, look to the parent tag's parameters that are related to the child tags (i.e. the parameters in the parent that define the variants in the children) when relaunching. If none have changed, you need not relaunch the children.
• In the event that you have a parent where some children have a certain parameter and some do not, you might consider adding to the parent a variable (e.g. "HasParm") that checks the child tags and only relaunches those that have the parameter in question.
• Don't launch tags that you do not need. Most stations don't use all, so only launch the ones required.
• Child tags cannot be modified or added using the Tag Browser; because their properties do not exist in the tag properties database, the Tag Browser's Properties and New buttons will be disabled when a child tag has been selected in the Tag Browser's list of tags.
• Unlike standard tags that store their properties in the tag properties database (where calculations cannot be performed internally), child tags can have calculations performed within their code. For example, you may assign a base address in the parent module, and then add an offset for each child's address.
• All child tags need not have the same parameters.
• The shortcut menu (or navigator) is not available to child tags; therefore, you cannot perform operational changes on child tags (i.e. you cannot right click a child tag's drawing method and change its questionable data or alarm disable status, nor can you enter manual data using the shortcut menu). This is because operational changes are programmed to be made to the tag properties database, and the properties of child tags are not stored in the tag properties database.
• Tool tips and the shortcut menu (or navigator) are available to parent tags (see the Common module in the example in Child Tag Sample Code).
• Child tags can be expensive in terms of RAM and startup time for your application (however, speed is not compromised). You must consider carefully and weigh the cost of hand coding script tags versus RAM and startup costs.
• Child tags do count towards the tag limit as defined by your VTS license.