The diagram here shows a typical printed circuit assembly with points of test. Over the years, the testing of PCAs relied largely on the use of in-circuit testers, applying mechanical probes to individual component pins. This permits each component to be tested in isolation, by applying test signals to a component's inputs and monitoring test responses at the its outputs. You can also test the interconnect this way by applying test signals to the output pins of driving components and monitoring responses at the input pins of driven components. Today, such procedures are increasingly difficult due to advances in technology.
Components are more complex, and come in much smaller packages.
This is where the boundary-scan standard comes in. Back then, it was agreed there was a need for a test structure that could be used with mixed-signal ICs. Electronic Probing Back to top The basis of dot1 is that a boundary-scan cell is associated with every pin on a chip. The ends of a boundary scan register are brought out to dedicated pins on a chip. This is shown in the following diagram as TDI and TDO in an IEEE-1149.1 digital chip.