Every electronic gadget nearby has some microcontroller or other programmable gadget in it. Even kitchen appliances or toy has some kind of electronics which controls prominent functions. In some cases this electronic gadget has predefined functionality which can not be changed. But in most cases this electronics contains a processor that uses flash memory to store code. And if a gadget is programmable then you can convert the agenda it runs at any time.
Of course, in many cases this programming is not needed since the electronics performs what is supposed to. But in some cases it makes sense to allow end users to update the gadget with the latest firmware. Therefore, it makes sense to have a universal interface that can be used in installation and at home to agenda electronic devices with uncomplicated additional hardware. This universal interface is Jtag. This is a accepted that defines a serial interface to exchange data in the middle of a gadget and computer (or in the middle of devices). The Jtag interface needs only four basic signals. This makes it easy to use Pc parallel port as Jtag interface. While this works many modern Jtag programmers use Usb port to join together to the Pc.
Another attractiveness of the Jtag interface is the possibility to join together many devices in chain. This way you can agenda all the Jtag enables devices on the board with only one interface. In any case you need a Jtag cable to join together to the device. Each gadget may have own Jtag pinout but this is not a problem since Jtag cables can be made (almost) universal--at least for few gadget types. Unfortunately, Jtag cable is not enough. You need also a suitable software that will agenda the target device.
Jtag is not used only for market electronics. It is also a accepted interface for all embedded systems along with Fpgas, microcontrolelrs and various memories. Each manufacturer has some kind of Jtag cable to agenda their devices. Unfortunately, these cables use dedicated software and can not be used for other purposes.
One of the very popular uses of the Jtag interface in consumer electronics is to reprogram some router models with new firmware. There are many cheap Jtag cables that can be used to bring a new life for an old router. Changing customary firmware is very popular not just for routers but also for other extremely popular electronic devices. The Xbox is such example where a uncomplicated Jtag hack can convert some built-in functions.
Universal Jtag Cable