Functionality

• I2C based communication bus
• Ability to gang 4 units on one bus, or 128 IOs
• I2C Address range 0x20-0x27, with pairs of addresses being allocated on each board. Addresses are jumper selectable
• 16 IO ports available and can be programmed at bit level for input or output
• Current limit resistors are placed on each output and a shorting bus wire on inputs
• 5VDC operation

Schematic

IOX32 Schematic

BOM

NOTE: I am not installing the Schottky diodes and external supply as VCC is obtained as part of the I2C bus.

PCB Vendor

The assembled boards are $41 (way too expensive), but the bare PCBs are $3 and maybe $12 in additional components. I assemble them myself.

http://www.modelrailroadcontrolsystems.com/iox32-version-2-0-32-line-i-o-expander/ 

Build Notes

Version 2.2 of the IOX32 does not have the slave address select pins of one of the post expanders tied to the address select headers and will always be at address 0x20. The fix was simple enough.

Microchip Port Expander

I'm using the I2C variant. Datasheet is in the attachments section