System Overview 1-19
The system bus consists of a 40-bit command/address bus, a 128-bit plus ECC data
bus, and several control signals, clocks, and a bus arbiter. The bus requires that all
CPUs have the same high-speed oscillator providing the clock to the Alpha chip.
The 1200 system bus connects up to two CPUs, up to eight DIMM memory pairs on
two riser cards, and two I/O bus bridges.
The system bus clock is provided by an oscillator on the CPU in slot CPU0. This
oscillator has a 1:5 ratio to the Alpha chip. With 400 MHz CPUs, for example, the
system bus operates at 80 MHz.
The system bus backplane initiates memory refresh transactions.
5 volt and 3.43 volt power is provided directly to the motherboard from the power
supplies.
