Class T4
Whereas Classes T0 through T3 can be seen as extensions to the IEEE 1149.1 standard, Class T4 provides a quantum leap in functional value. The main benefit is the reduction in required pins from four to two, and the introduction of new scan formats that support optimized transactions, which maintain and improve performance in the reduced pin configuration.

When device die are stacked, it is better to have the fewest number of connectors possible because this increases the difficulty to stack the die. The fewer number of connectors, the easier the task since each connector adds additional costs.

In brief, the key to the two-pin operation is the elimination of the original data lines (TDI/TDO) and the bi-directional transmission of serialized data over the TMSC line.

Reducing pin count with glue-less star configurations is a valuable feature for system designers who use the stacked die and multichip modules because it simplifies the arrangement of debug pins on the mounting points and this, in turn, makes manufacturing, mounting, and stocking of parts easier and thus less expensive.

To implement this capability, the glue-less star configuration from Class T3 is used, this time without TDI and TDO. This process is illustrated in Figure 5 in the Star-2 or narrow-star configuration.

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In addition to reducing pin count, Class T4 defines optimized download-specific scan modes. In these modes, only useful information is downloaded. To improve performance of reduced-pin operation, the clock rate can be doubled. This, along with the optimized transactions just described, means performance is maintained and, in some use cases, improved.

Class T5
Class T5 gives the test port the ability to concurrently perform debug and instrumentation operations, which reduces the number of pins dedicated to instrumentation. This reduction is possible because instrumentation data is transferred during idle time, which is quite ample for the task. Typically two pins are dedicated for instrumentation.

Class T5 also allows for custom protocols. This feature is incorporated in the standard in large part because the working group recognized that many device vendors already had custom protocols. Class T5 standardizes the process to enable the use of custom protocols while ensuring an industry standard method of enabling them is implemented.