The instruction BYPASS is used to tell a device earlier on the chain to ignore your commands and pass them on. So you can test the various components through one port. On a PCB the chip(s) and peripherals form a "daisy chain" connected to the TAP (test access port). They used to be connected to the parallel port of your machine, but these days are more often connected via USB and based on one of the FTDI chips. The devices used to do the boundary scan according to JTAG are called JTAG probes. after development of the hardware and subsequent production of it. The idea was to define an interface that could be used to test hardware (micro controllers and connected peripherals after manufacturing). The process is called boundary scanning and JTAG was named after the working group: Joint Test Action Group, some time in the 1980s. JTAG was initially created to test/verify hardware devices.
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