Test Bit enables you to select a bit within a vector of registers, and store its status in an MB.
Operand A, Start of Vector, determines the start of the vector of registers.
Operand B, Offset in Vector, selects the bit within that vector.
Operand C, Target Bit, determines where the value of the selected bit will be stored.

Note that the maximum number of bits in the vector is 255, 16 MIs or 8 double registers. The function is located under the Logic menu on the Ladder toolbar.