I know that this pin is tied to +5 on hi rom carts and to ground on low rom carts.
What function does it do, if any?
Doh!! Yeah, pin 10.
I have reviewed the NoCash docs and it doesn't say what that pin does. So what would happen if pin 10 was grounded instead of high on a HiRom cart??
The MAD-1 appears to be purely combinatorial logic (in addition to the analog portion for the battery controller), so that pin is used to select between HiROM and LoROM output logic wrt the mapped inputs. Think of it in terms of each output pin separately, you have output = some logical combination of inputs, and that combination of inputs is different for HiROM and LoROM, so you just add one more input (the mode switch) so you have LoROM output = !MODE & (LoROM input logic), and HiROM output = MODE & (HiROM input logic).