US Patent 7321948 Highly available system test mechanism

Boards in a system are interconnected by a first set of signals including a first control signal and first function signals. Each board in the system includes a second set of signals corresponding to the first set of signals. When the first control signal and a first function signal are asserted, the corresponding second signals of are asserted in response and a function is performed on the boards. But, if any of the second signals are asserted, none of the first signals is asserted in response. Test signals on boards are thereby isolated from test signals coupled to all the boards on the system, so a fault on any signal in any second set of signals will not propagate to the first set of signals.