Multi-PCB JTAG cascade circuit and cascade method

The invention provides a multi-PCB (Printed Circuit Board) JTAG (Joint Test Action Group) cascade circuit and a cascade method, which are used for JTAG cascade debugging of Xilinx FPGAs (Field Programmable Gate Array) among a plurality of PCBs, the circuit comprises a PC (Personal Computer), a USB to JTAG module and a plurality of JTAG connectors. The PC is connected with the USB to JTAG module, the TDI pins of the USB to JTAG are connected with the input end of the first JTAG connector, with the output end being connected with the input end of the next JTAG connector, the output end of the last JTAG connector is connected with a TDO pin of the USB to JTAG module interface; the JTAG connector is used for connecting a JTAG cable; and each JTAG connector is connected with a switching circuitand is used for automatically switching off/on according to the level change of the detection signal of the JTAG connector to the JTAG cable. Whether the JTAG connector is inserted or not is judged through the detection sign