Several functions are encoded in the 15-bit serial string to the HM port. The first 2 bits give the number of clock cycles between integrator resets (00 = 1K beam clocks, 01 = 2K, 10 = 4K, and 11 = 8K beam clocks). The next 11 bits set multiplexer channels; bit 14 enables or disables multiplexing, and bit 15 turns the correlator on or off. Note that in the hardware under test, the correlator was always off. Figure 3 shows the timing of the HM serial signals. The multiplexer channels M1A0 through M3A3 are connected to 11 LEDs on a small diagnostic board. A quick test of the system is to see if you can turn these LEDs on and off.
Figure 3: Timing of the Heap Manager serial port signals