Referring to array variables

Each entry of an array is referred to by an index. You can extract individual variables (from the array) by specifying the tag name and index:

<Tag Name>[Index]

For example, to refer to the third variable of the array in the above example (Conveyor 3), use the following syntax:

The index of the first entry of an array is always 0 (zero). In this example, Conveyor_Speed[0] is the first entry of the array (Conveyor 1), and Conveyor_Speed[4] is the last entry (Conveyor 5).

Note the following when using arrays:

• Do not define large arrays, because each time an individual array entry is requested, CitectSCADA reads the entire array from the I/O Device. With large arrays, system performance could be reduced.
• The size of the array needs to be less than the maximum request lengthThe maximum number of data bits that can be read from the I/O device in a single request. For example, if the maximum request length is 2048 bits, the maximum number of integers that can be read is: 2048/16 = 128. of the protocolMessaging format consisting of a set of messages and guidelines used for communication between the server and an I/O device. The communication protocol determines how and the I/O device communicate; the type of data to exchange; rules governing communication initiation and termination; and error detection.. The I/O Device description help topic (for your I/O Device) displays the maximum request length of the protocol.
• declare digital arrays on a 16 bit boundary. CitectSCADA rounds each digital array down to the nearest 16 bit boundary. Consequently, digitals in the array are changed. For example, if you declare an array Test to start at bit 35, and CitectSCADA starts the array at bit 32. The index to the array also starts at bit 32; Test[0] refers to bit 32, not bit 35.
• Each individual array entry has its own data value for the field, valid and override elements, but the entries within an array share the override and control mode elements, quality and timestamps. Reading the quality or timestamp for an indexed array entry will return the quality or timestamp for the entire array. Writing to the value for an indexed array entry will change the timestamps and quality for the entire array. Changing the override mode and the control mode for an array entry will change it for the entire array.

See Also