DB15 IDC Connector

https://www.digikey.com.au/product-detail/en/cnc-tech/410M0-15-1-00/1175-1813-ND/6024195

15cm 16 way ribbon cable

https://www.digikey.com.au/product-detail/en/assmann-wsw-components/AWG28-16-G-300/AE16G-5-ND/686332

16 Pin 8×2 Header

https://www.digikey.com.au/product-detail/en/on-shore-technology-inc/302-S161/ED10523-ND/2794234

Jumper select ? for Relay vs Button (GPI vs GPO) or have 4 relays, 4 buttons?

3 way Screw Terminals (Relays C, N/C, N/O) x 4

2 way Screw Terminals x 4 (Buttons)

9v ? 5v ? 500mA DC Input (5v Relays)

https://www.digikey.com.au/product-detail/en/omron-electronics-inc-emc-div/G5LE-1-ASI-DC5/Z5212-ND/414237

Transistors x4

Diodes x 4

Resistors

LEDs PSU

LED Off/On per channel?

PCB

DIN Rail enclosure