Simple JTAG cable Principle

Simple JTAG cable Principle

 

One end of the simple JTAG cable is connected to the parallel port of the PC, and the other end is connected to the JTAG interface of the target board, the data and I/O pins in the PC parallel port are connected to the TMS, tck, TDI, TDO, and trst signal lines of the JTAG port of the target board through a 74xx244 unidirectional driver chip, then, the software on the PC is used to simulate the IEEE 1149.1 standard protocol that JTAG complies with, so as to access and control the I/O pin status of the processor on the target board, you can also access and control the I/O pins of Flash chips mounted to the processor bus to write data into flash chips.