Should be pretty simple, I2C/SPI wires in and the parallel I/O pins simply routed to a 50-way connector.

Pinout shall follow this standard, with the first device driving groups 0 and 1, the second device driving groups 2 and 3.

I shall omit:

• group 4
• special function pins
• -12V

But I will need:

• +12V if I want to drive my SCB11 relay board
• +5V for most signal conditioner boards

This will be a current supply problem because a USB-to-I2C/SPI board does not supply +12V at all, and the +5V rail does not have a lot of current available - I would guess 100mA or so. The LED32 indicator board will need about 400mA just to power all the LEDs on.

Some external power supply will thus be needed.