How far will RS422/485 communicate? RS422/485 can reliably communicate up to 1.2 kilometres (~4000 feet). RS422/485 Adaptors>> This FAQ Applies to the following Brainboxes Product Ranges: CC - PCI Serial Cards ES - Ethernet to Serial PM - Serial PC Card (PCMCIA) PX - PCI Express Serial Cards UC - uPCI Serial Cards UP - uPCI Powered Serial Cards US - USB to Serial VX - Serial ExpressCard