External clock unit for a computer

Claim 1

An external clock unit for a DEC MICROVAX model 3100 computer, the DEC MICROVAX model 3100 computer having a CPU module which includes a processor, a 44.444 MHZ clock and a system identification code generator, said system identification code generator being constructed so that it can be arranged to output either a first set of logic signals or a second set of logic signals, said first set of logic signals being used to enable said DEC MICROVAX model 3100 computer to pass self test on start up when said processor is operating with clock signals of 44.444 MHZ and said second set of logic signals being used to enable said DEC MICROVAX model 3100 computer to pass self test on start up when said processor is operating with clock signals of 63.0 MHZ, the external clock unit operating independent of said processor and comprising: a. a first oscillator for generating clock signals of 44.444 MHZ, b. a second oscillator for generating clock signals of 63.0 MHZ, c. a 2 to 1 multiplexor for receiving said clock signals from said first and second oscillators and outputting one of said clock signals at a first output terminal, d. a first manually operable switch coupled directly to said multiplexor for selecting which clock signal is to be outputted by the multiplexor, e. a second manually operable switch adapted to be connected to said system identification code generator for controlling which set of logic signals is outputted by said signal indentification code generator, and f. a power supply for providing operating power for said external clock unit.