If your existing encoders are designed to plug into something like a NGC-Max (almost certainly true) - there is no way to plug them directly into a PC or 'device'.... for the reasons explained above. They need to be read and powered. A PC/device does not have the electronics to do that, nor do the encoder/mount makers include the required read electronics with them. You need another 'box' to do that.
If your PC, cell phone, tablet, etc, has WiFi..... here is a great solution, the Nexus: http://www.astrodevi...exus/Nexus.html
This is not a complete unit like the NexusDSC that can function on its own to provide catalogs, pointing, mount modeling, etc. The Nexus ( I call it a "dumb Nexus" because it can't do anything on its own - but it is a lot less expensive than NexusDSC) only reads the tics from the encoders, and puts them out in a WiFi signal - that then must be processed by a planetarium program like SkySafari, TheSkyX, Stellarium, etc, running on a tablet, cell phone, or PC. It does need a 9v battery or 12v power supply.
At Kopernik Observatory we have a Nexus hooked to an old 'PushTo' Losmandy G-11 with encoders. We have two 9-inch tablets running SkySafari 5 and connect both via WiFi. Both are Velcro'ed to the walls of the observatory - but can be picked off. That way two people can see what the scope is pointed at, get data and sound on the object, etc. Works especially nice with public outreach, where we let them view one of the tablets before getting to the scope eyepiece. The two devices do not interact with each other.
Just think..... if you have a friend show up.... and he/she has SkySafari running on their cell phone or a tablet..... they can connect up too.... and get the info on what your scope is pointed at!