Test Access Port (TAP)
Definition
The set of four or five contact pads (TDI, TDO, TCK, TMS, and optionally TRST) on a circuit board that implement the IEEE 1149.1 boundary-scan interface. These are the connection points for JTAG acquisition hardware.
- Standard
- IEEE 1149.1 boundary-scan
- Contact pads
- TDI, TDO, TCK, TMS, optional TRST
- Pad count
- Four or five
- Use
- Connection point for JTAG acquisition hardware
Common questions
How does an examiner locate a TAP on an unfamiliar board?+
They probe unpopulated header pads or test points with a multimeter or logic analyser while comparing voltage and signal behaviour against known JTAG pinout patterns, since TAPs are rarely labelled on a device intended for consumer use.
What can go wrong when connecting to a TAP?+
Misidentifying a pad or applying the wrong voltage can damage the target chip or corrupt data, so examiners verify pinout with datasheets or boundary-scan description files before attaching acquisition hardware.
Related terms
- Chip-Off Acquisition
- A physical extraction method in which the flash memory chip is desoldered from the device's circuit board and read directly with specialised...
- eMMC (Embedded MultiMediaCard)
- A flash storage standard that packages NAND memory chips and a controller into one soldered module using a parallel interface. Common in...
- ISP (In-System Programming)
- A variant of direct chip access that connects to the eMMC command and data pins while the chip is still on the...
- JTAG (Joint Test Action Group)
- The industry group that produced IEEE standard 1149.1, which defines the test access port and boundary-scan architecture built into most modern integrated...
- Pinout
- A diagram or database entry that maps each physical contact on a device's circuit board to its signal function. For JTAG acquisition,...