This circuit is for an interface between the parallel port of a PC and a socalled JTAG interface. This type of circuit was originally developed by a company called Altera, which referred to it as the ‘Byteblaster’.The version described here is compatible with the Byteblaster (electrically it is almost identical). This has the advantage that programs written for use with the Byteblaster can also be used with this interface. The circuit itself is very simple. K1 is connected to the parallel port of the PC, whereas signals D0, D1 and D6 are brought to the JTAG connector via IC1. The output of the JTAG connector (TDO) is in turn fed back to the PC’s BUSY signal via IC1. Pin 7 of the JTAG connector is used to provide an extra output. This signal is brought to the SELECT input of the parallel port via IC1.