As I remember, it was done with add on boards in that series. You need to have the right cable pinouts and set the baud rate etc. You need to use the service manual, prog manual and comms manual usually to get all of the settings and connections. This is the comms manual
http://the-checkout-tech.com/manuals/cashregisters/Sharp_ER-3110_service_option_Inline_and_RS232_manual.html
The same site has the other manuals as well. I haven't played with them for a few years so I don't have the info anymore
Richard G, July 2014