The test of electronic circuits has been a key topic in the industry since the first transistor was developed. The trend of employing non-invasive test access strategies has spawned a number of new technologies and methodologies which, combined, have given birth to the category of Embedded System Access (ESA) techniques. The essence of ESA is the provision of test pin electronics in the target system itself. Activating ESA results in a temporary transformation of the respective system allowing it to be tested in partitions by embedded test centers under control of the integrated test bus.