An easy derivative of the SPIO (Serial, Parallel I/O) board is a version that has an I2C controller in place of the I/O port expansion header.   So introducing the Serial, Parallel, I2C (SPI2C) SBC-85 board.  This is a 2-layer, 100mm x 100mm so fits in the super-cheap prototype special from the PCB houses.  The board offers an Intel 8251 USART for up to 19.2K Baud RS232, an 8255 24-pin parallel I/O port, and the I2C controller.

The Philips PCF8584 I2C controller (spec sheet) requires two I/O ports so I removed the expansion port header and onboard home brew I/O port to free up two I/O addresses. This also left a little room for a 10×10 pad per hole sandbox playground to populate your favorite I2C chips like a temperature sensor, or whatever your heart desires. There is also a 4-pin header for off-board I2C projects like an I2C display, your Arduino, or who knows what you have laying around your bench that needs to be interfaced to the SBC-85 system.  Finally, a little interrupt patch area to let you wire-wrap your choice of which interrupt you would like to go where.

The PCF8584 is reasonably available for a few dollars so that should not be a show stopper and there are I2C devices for nearly any function you can imagine, so this should be a handy board to have as part of the SBC-85 ecosystem.  As a last minute addition, I tossed on a Maxim DS1307 I2C RTC with battery backup.

Build files and documentation is available here SPIO (Serial - Parallel I/O) Documentation » Bits Of The Golden Age