[en] This document describes the core software that resides in an Input/Output Controller (IOC), one of the major components of EPICS. The basic components are: (OPI) Operator Interface; this is a UNIX based workstation which can run various EPICS tools; (IOC) Input/Output Controller; this is a VME/VXI based chassis containing a Motorola 68xxx processor, various I/O modules, and VME modules that provide access to other I/O buses such as GPIB, (LAN), Local Area Network; and this is the communication network which allows the IOCs and OPIs to communicate. Epics provides a software component, Channel Access, which provides network transparent communication between a Channel Access client and an arbitrary number of Channel Access servers