  As an accelerator control system survives over several years, it is often the
case that new subsystems are introduced into the original control system. The
control system for the KEK electron/positron injector-linac has been using Unix
workstations and VME computers since 1993. During the eight-year operation, we
extended the system by introducing a) Windows PCs, b) PLC controllers with a
network interface, and c) web servers based on modern information technology.
Although such new subsystems are essential to improve control functionalities,
they often cause communication problems with the original control system. We
discuss the experienced problems, and present our solutions for them.
