The HP 16501A expansion frame has been developed to extend the HP 16500C logic analysis systems beyond its five-card limit, and meet the anticipated need for systems requiring more than five slots. Note: The 16501A expansion frame also works with other 16500 mainframes. Adding the 16501A to the 16500C creates a tightly-coupled, two-frame, ten-card system, fully controlled by the 16500C. A single power switch (located on the 16500C front panel) turns on both frames. The HP 16501A offers inter-frame module arming/triggering capabilities with a 2-nanosecond time correlation between modules. The 16501A expansion frame is very similar to the mainframe. It consists of a card cage, rear panel, power supply, and fans. The interface is a buffering scheme to pass signals between the mainframe and the 16501A backplane. It contains circuitry that controls external arming selection and enables an HP 16501A card to drive an output dedicated to triggering other instruments. The expansion frame interface board is connected to the mainframe interface board through a 68-pin flat cable. In addition to the HP 16500C rear panel signals, the cable incorporates control signals, which allow the CPU to communicate with and control the expansion frame interface board. Ground lines separate signal lines within the cable to prevent cross-coupling or soft failures. The cable is externally shielded to reduce RFI emissions. The power supply of the 16501 expansion frame is controlled by the 16500 mainframe. Because the power is controlled from another instrument, the power switch of the 16501 must be on before the 16500 is turned on or the expansion frame will not be recognized by the logic analysis system software. Module-to-module arming within the 16501A follows the arming specifications for the 16500C in that any module may arm any other module. The maximum intermodule arming level is five. When the 16501A is attached to the 16500C, the resulting 16500C/16501A system operates as a single frame. The 16500C and 16501A may receive only one external arming signal from the other frame. The 16500C mainframe may use only two external arming signals any module in the 16501A expansion mainframe or the rear-panel BNC. The arming signal can come from three sources: Any module in the 16500C mainframe, any module in the 16501A expansion mainframe, or from the rear-panel PORT IN BNC.