Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 454)

Appendix B
Number Systems, Hex Mask
B–6
Bits of the mask word that are set (1) pass data from a source to a
destination. Reset bits (0) do not. In the example below, data in bits 0–7 of
the source word is passed to the destination word. Data in bits 8–15 of the
source word is not passed to the destination word. Destination bits 8–15 are
not affected (they are left in their last state).
1 1 1 0 0 1 0 1 1 1 0 0 0 1 1 0 Source Word
0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 Mask Word
0 0 0 0 0 0 0 0 1 1 0 0 0 1 1 0 Destination Word
(all bits 0 initially)