Abstract: An external clock unit for use with a DEC MICROVAX model 31 computer includes a first crystal oscillator for generating a clock signal of about 44 MHZ, a second crystal oscillator for generating a clock signal of about 63 MHZ, a 2 to 1 multiplexor for receiving 44 MHZ clock signal and the 63 MHZ clock signal and outputting one of clock signals, a first switch for selecting which of the two clock signals is to be outputted by the 2 to 1 multiplexor, a second switch for selecting which one of two logic signals is to be outputted by a system identification code generator in the computer and a power supply for providing power to the device. In use, the external clock unit is connected to jumper pins on the processor clock signal line and the system identification code generator.

Abstract: An external clock unit for use in increasing the clock cycle time in a DEC 8000 series computer without making any hardware changes inside the computer includes a first crystal oscillator for generating a first clock signal 160.2 MHZ, a second crystal oscillator for generating a second clock signal, a multiplexor for receiving said first clock signal and said second clock signal and outputting one of said clock signals, a first line driver for driving the clock signal of 157.53 MHZ outputted by the multiplexor, a second line driver for outputting a clock enable signal, a toggle switch for turning on the clock enable signal and a power supply for providing power to said first crystal oscillator, said second crystal oscillator, said multiplexor and said first and second line drivers. In use, the accelerator is connected to the clock module in the computer through pins in the backplane.