Abstract: A computer system in a fault-tolerant configuration employs three identical CPUs executing the same instruction stream, with two identical, self-checking memory modules storing duplicates of the same data. Memory references by the three CPUs are made by three separate busses connected to three separate ports of each of the two memory modules. The three CPUs are loosely synchronized, as by detecting events such as memory references and stalling any CPU ahead of others until all execute the function simultaneously; interrupts can be synchronized by ensuring that all three CPUs implement the interrupt at the same point in their instruction stream. Memory references via the separate CPU-to-memory busses are voted at the three separate ports of each of the memory modules. I/O functions are implemented using two identical I/O busses, each of which is separately coupled to only one of the memory modules. A number of I/O processors are coupled to both I/O busses.

Abstract: A servomotor drive system for a printer escapement is provided which minimizes power supply requirements. The drive system has means for accelerating and decelerating the printhead escapement having a fixed current source applied to a servomotor which drives the escapement. In order to minimize power requirements, the system includes means for controlling the current in the motor which comprise means for sensing the actual motor current and feeding back the sensed current to a comparing means. A preselected varying current profile is applied to the comparing means which compares this profile to the actual motor current. Means are then provided responsive to these comparing means for altering the actual motor current to conform with this preselected varying motor current profile.