Abstract: A data channel for a digital tester includes a random access local memory containing a main vector sequence, a subroutine vector sequence, and a test vector list. An index register is loaded with the address of the first vector in the list of vectors that is to be inserted as a variable into a vector stream. A sequence instruction selects the index register as the source of a test vector address when a variable vector is to be inserted into the vector stream at a point in a subroutine. The sequence instruction also resets the index register to a state which determines the address of the next variable to be inserted into the test vector pattern.