I2C based communication bus
Ability to gang 4 units on one bus
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

BOM

The assembled boards are $41 (way too expensive), but the bare PCBs are $3 and maybe $5 in additional components. I assemble them myself.