Create an array control on the panel of the subVI and assign it a terminal...Drop the sub-vi on the diagramme of the caller and wire the array into the terminal.
If the 21byte array you have is on a wire somewhere the fastest way to get the control you need on the sub-VI panle is to right-click on the wire and select create control, then move that control to the panel of the sub-VI.
Whether the byte is 21 bytes long or other doe snot matter, it's the data type and dimensions (1D, 2D etc..array) that matters.
