Not sure if I understand your setup correctly, but you might be able to use a RS232 repeater instead of converting it to RS485. Some RS232 repeaters works up to 2000 feet and will even isolate your interfaces. Or as earlier mentioned here wireless might be more relevant. A couple of wireless RF modules might do the job. This [RS232 to RS485 converter wiki i a good reference, it much easier to find out what you need inf you are sure about the basics.](http://www.rs232-to-rs485.com)