Help figuring out the interface on this bill validator?
The physical layer sounds like RS232 the asymetry +/-5V vs +/-8V  can be explained by the different ends having different drivers, possibly max232 one end and max3232 the other end.

While getting the physical interface connected to arduino is fairly simple for RS232 I was unable to locate ant protocol document and The only supported development envirionments are .net and Java. 

https://www.cranepi.com/en/support#/downloads
select "banknote" "validators" "sc advance" "api" in the dropdowns.
(login required to proceed)

Maybe these can be reverse engineered to reveal the line protocol, but there's a good chance that the line protocol is encrypted.

It would probably be possible to interface with an ARM based system like Raspberry-Pi (over USB or RS232)  with either the java API or even the .net one either using mono, or by running "Windows 10 for IOT" on the Raspberry Pi