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A JTAG interface is a special four/five-pin interface added to a chip, designed so that multiple chips on a board can have their JTAG lines daisy-chained together, and a test probe need only connect to a single "JTAG port" to have access to all chips on a circuit board. The connector pins are

  1. TDI (Test Data In)
  2. TDO (Test Data Out)
  3. TCK (Test Clock)
  4. TMS (Test Mode Select)
  5. TRST (Test ReSeT) optional.


In addition to the above 5 signals, you also need to connect VDD and Ground signals from the board. There are several interface cables that include all these pins and a few extra ones. The 14-pin (2x7) interface is known as the TI standard, while the 20-pin (2x10) interface is known as the ARM standard. The Linkstation HG uses a 16-pin (2x8) interface which is standard for Freescale/PPC boards.

See Add Jtag Port for a list of cable vendors.