Abstract: An apparatus that provides two different sources for generating the bytes to be inserted into frames of data being communicated through a switching unit. This apparatus allows for the insertion of variable, programmable data patterns in multiple frames. Under program control of a controller, the first source can selectively write data into designated time slots within designated rows and designated frames of multiple frames. Different data can be inserted into different rows of the same column of each frame. In addition, different data for different frames can be inserted into different rows of the same column. Also, the same data can be inserted into designated a row and column of a designated frame. The second source can write data into every row of a designated column of every frame.