What is a Boundary Scan TAP Controller?

A TAP controller is a 16-state machine, programmed by the Test Mode Select (TMS) and Test Clock (TCK) inputs, which controls the flow of data bits to the Instruction Register (IR) and the Data Registers (DR). The TAP Controller can be thought of as the control center of a boundary-scan device.
The TAP Controller State Diagram shown in Figure 1 allows you to track the sequence of any boundary-scan test through the TAP Controller. This state diagram applies to all components that comply with IEEE Standard 1149.1. 

Figure 1 – TAP controller state diagram 

Each position in the Data Register and Instruction Register columns represents a state of the TAP Controller (the 16-state machine that controls each boundary-scan device). The states in the two columns vary only by the suffixes. These suffixes identify the column and the register operation that they affect.

• DR is the Data Registers column; data instructions passed through this column affect the operation and contents of the data registers. The data registers can be the Boundary Register or Bypass Register, which all boundary-scan devices must contain, or they can be the optional IDCODE or USERCODE Registers, or even a designer-specified register that complies with the IEEE standard.

• IR is the Instruction Register column; data instructions passed through this column affect the operation and contents of the Instruction Register, which is a mandatory component of every boundary-scan device.

Along the data flow lines connecting each tap state, you see a number: a 0 or a 1. These numbers are the bits states of the TMS input signal required to move from one state to the next.