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Monday 19 March 2018 at 11:58
Solved. All pins need to be connected, especially Com A.
The C220 program expects certain pins to be present, otherwise all coins will be rejected by default.
Special thanks to Rob at https://www.highway.net.au/ for helping me to solve this.
05A62444-29A1-4813-BD7C-8C45A80F2E2B (resized).png
Source: http://pinside.com/pinball/forum/topic/c220-coin-controls-wpc-a-14689-1-interface-help#post-4288506