Restrictions on data items in expressions

PI DataLink permits the following data items in expressions:

• PI points

• PI AF attributes that store data references to PI points

• PI AF attributes that store constant values

Furthermore, a PI DataLink function can only return data from a single Data Archive server. Therefore, any PI points in an expression, including those referred to by PI AF attributes, must be stored on the same Data Archive server. Similarly, PI points in a filter expression must be stored on the same Data Archive server as any PI points specified by the Data item or Expression fields.

• For example, this expression will return a value, since its PI Points both reside on PIDataArchiveA: '\\PIDataArchiveA\sinusoid' + '\\ PIDataArchiveA\cdt158'

• Meanwhile, this expression will fail, throwing the error message "Expressions cannot use PI Points from multiple servers," since its PI Points come from both PIDataArchiveA and PIDataArchiveB: '\\PIDataArchiveA\sinusoid' + '\\ PIDataArchiveB\cdt158'