JTAG interface was developed to solve the problem of testing complex electronic circuits with a lot of small semiconductor devices. Since it is very difficult to test operation with various signals and conditions the engineers started to investigate a possibility for a universal system that would allow them to access all internal signals without touching the board or soldered components. The result of this investigation is the JTAG interface.
JTAG is a standardized (IEEE 1149.1) serial interface with only few wires. It can be used to program, read and debug electronic circuits without physically connecting to each individual chip. All the JTAG enabled electronic devices on board are electrically connected in a loop which is controlled by a JTAG programmer. This is an interface that serially shifts data to and from the chain. This way every individual bit on any device can be accessed.
JTAG programmer is a simple interface between personal computer and JTAG board. Every JTAG programmer comes with a dedicated software which is responsible to send or read data. JTAG programmer takes care of sending this data serially via JTAG signals. Even if you have many JTAG devices on the circuit board, there is only one JTAG connector needed since all devices are connected in a chain.
Unfortunately, JTAG pinouts and connectors are not standardized. There are different JTAG programmers for different devices. Most of them can be powered via JTAG interface cable, while others need separate power supply. More expensive JTAG programmers use USB bus to connect to PC and as a power source. Cheaper JTAG programmers connect to parallel port and are usually slower than professional JTAG programmers with own microprocessor.
Cheap JTAG Programmers
Anyway, if you need to program new firmware into the wireless router, modified XBOX 360, FPGA or a development board, right JTAG programmer with appropriate software will do the job. Just be careful which programmer you will buy because each is compatible only with few device types. But because simple JTAG cables and programmers can be purchased for only $10 you can afford to have many different JTAG tools.