This is assembled onto a protoboard, rather than a PCB. The "board" is simply to help with visualization of the placement on the protoboard, and the various connections.

The result is 24 channels of simple I/O. The relay board uses optoisolators, so we are driving a simple LED (and the relay board provides the current-limiting resistor). Each '688 provides 8 channels, and all 3 are wired onto the same I2C bus via their RX/TX pins.

Screw terminals are provided for power/ground and RX/TX. The channel control wires will solder directly to the DIP socket, and terminate in a female header that can be plugged into a relay board.