  An S-band linear accelerator is the source of particles and the front end of
the Advanced Photon Source injector. In addition, it supports a low-energy
undulator test line (LEUTL) and drives a free-electron laser (FEL). A
waveguide-switching and distribution system is now under construction. The
system configuration was revised to be consistent with the recent change to
electron-only operation. There are now six modulator-klystron subsystems, two
of which are being configured to act as hot spares for two S-band transmitters
each, so that no single failure will prevent injector operation. The two
subsystems are also used to support additional LEUTL capabilities and off-line
testing. Design considerations for the waveguide-switching subsystem, topology
selection, control and protection provisions, high-power test results, and
current status are described
