JTAG Test Access Port

A TAP is the set of signals which JTAG boundary scan uses to access the functionality in a device. The TAP comprises 4 mandatory signals and an optional 5th signal.

The mandatory signals are:

• TDI - Test Data Input
• TDO - Test Data Output
• TCK - Test ClocK
• TMS - Test Mode Select

and the optional signal is:

• nTRST - Test ReSeT

The TAP on a device can be wired directly to a header on a PCB, in order for test software like XJTAG to connect to it, or the TAPs of a set of devices can be connected in a JTAG chain. A PCB may have multiple JTAG chains, or may have a single one which includes all of the JTAG devices.