What is “Boundary Length"?
The “Boundary Length” defines the number (length) of cells in the boundary scan register. This number has to match the number of cell entries in the boundary register, which describes the structure of the boundary scan register.
Example: “Boundary_Length” entry on a Boundary Scan Description Language (BSDL)
attribute BOUNDARY_LENGTH of ttl74bct8374 " entity is 18;
Note: The example shows that there is a total of 18 boundary cells in a device.