Projects Home
Back to Photo List
The schematic here began as a Kivio drawing and is far from suitable for gEDA or other schematic/pcb programs. The "U602" label is for the 8250 socket on the H89 serial card. The socket provides conveninent access to the eight data lines, the decoded select line for Port 340Q, the A0 line, and the strobes for reading and writing. In addition, pin 13 provides +5V for the 74LS125 and the 74LS138. Pin 20 provides ground to the Heathkit computer. The remaining 8250 signal lines are unused.
The USB board is a circuit board from SparkFun.com. It has sufficient circuitry to provide choices as to how the board is powered and other options. Not all of the pins of the actual FT245 chip are accessible, but then for our purposes they are not needed. The pins on the board are on .100 inch spacings with a .600 inch separation between the two rows. By connecting the VUSB and VCCIO pins together (and decoupling with a .1 uF (100 nf) capacitor, the board draws it power from the USB connector and not from the Heathkit. Only the 74LS125 and the 74LS138 are powered from the H89.
On the Heathkit side (the parallel side), there are no interrupt lines, only two status lines for the FIFO buffer, transmitter empty (TXE) and receiver full (RXF), both of which are "low true." There are pins for Read from the FIFO (RD) and write to the FIFO (WR), also both "low true."
A more complete description of the functioning of the gadget accompanies the code for the H89 device driver at the USB Projects page.