MICROFORUM

Can you tell me the types of available tunnel diodes?

I think if you can't use RF material for price reasons, then grounding along the signal tracks is very important and don't use a solder mask either. PCB should be gold-plated in this case.

Why to use MCP23009 ? Texas Instruments has many expanders 8- and 16 bits. Some with open collector. All have standard addressing.