 The VAX 4000 Model 500A, code-named ""Omega/N"", was introduced on 12 October 1993.It used the KA681 CPU module containing a 71.43 MHz (14 ns cycle time) NVAX microprocessor with 128 KB of external tertiary cache.The VAX 4000 Model 600A, code-named ""Omega/N+"", was introduced on 12 October 1993.