A switching control system has been designed and built to provide the capability of rapidly switching the waveguide and low-level cabling between different klystrons to operate the Advanced Photon Source storage ring in the event of a failure of a klystron system or to perform necessary repairs and preventative maintenance. The twelve possible modes of operation allow for complete redundancy of the booster synchrotron rf system and either a maximum of two storage ring rf systems to be completely off-line or one system to be used as a power source for an rf test stand. A programmable controller is used to send commands to intermediate control panels which interface to WR2300 waveguide switches and phase shifters, rf cavity interlock and low-level rf distribution systems, and klystron power supply controls for rapid reconfiguration of the rf systems in response to a modeselection command. Mode selection is a local manual operation using a keyswitch arrangement which prevents more than one mode from being selected at a time. The programmable controller also monitors for hardware malfunction and guards against "hot-switching" of the rf systems. The rf switching control system is monitored via the Experimental Physics and Industrial Control System (EPICS) [ 11 for remote system status check. RF SYSTEM SWITCHING OVERVIEWThe Advanced Photon Source has five 1-MW, 352-MHz klystron rf systems installed. The storage ring can operate with either two, three, or four rf stations online, using hybrid combiners/3-dB splitters to operate two rf stations in parallel (see Figure 1). One of the storage ring rf systems can also serve as an operational spare for the booster synchrotron rf system, and another can be used as a power source for a 1-MW rf test stand.The rf stations can be combined in a variety of ways to provide twelve distinct modes of operation that require changes in waveguide rf circuit configuration, interlock system delegation, and low-level rf signal distribution to permit proper operation of the rf systems. The rf system switching controls execute and verify the required circuit changes for each mode automatically in response to a manual operator command and provide system monitoring functions. Motor-driven WR2300 waveguide switches are used to route the rf output of specific rf stations to a designated load, or to take the systems off-line in the event of a failure or for maintenance. Motor-driven WR2300 phase shifters are used to correct for output port phase differences when the 3-dB hybrids are used as combiner/split- ters with rf stations in parallel, or as power dividers when a single rf system is used. The cavity and klystron interlock signals for each rf system are routed and delegated, respectively, by the switching control system to insure that all interlock circuits required by the accelerator hardware are routed to the rf system supplying the power.The heart of the rf system switching control is an Allen-Bradley programmable logic controller (PLC) that is interfaced to the existing rf system h...