Circuitry that may be added to an integrated circuit to provide access to on-chip Test Access Ports (TAPs) specified by IEEE Std 1149.1 is described in this standard. The circuitry uses IEEE Std 1149.1 as its foundation, providing complete backward compatibility, while aggressively adding features to support test and applications debug. It defines six classes of IEEE 1149.7 Test Access Ports (TAP.7s), T0 to T5, with each class providing incremental capability, building on that of the lower level classes. Class T0 provides the behavior specified by 1149.1 from startup when there are multiple on-chip TAPs. Class T1 adds common debug functions and features to minimize power consumption. Class T2 adds operating modes that maximize scan performance. It also provides an optional hot-connection capability to prevent system corruption when a connection is made to a powered system. Class T3 supports operation in either a four-wire Series or Star Scan Topology. Class T4 provides for communication with either a two-pin or four-pin interface. The two-pin operation serializes IEEE 1149.1 transactions and provides for higher Test Clock rates. Class T5 adds the ability to perform data transfers concurrently with scan, supports utilization of functions other than scan, and provides control of TAP.7 pins to custom debug technologies in a manner that ensures current |