The initial development of Unix occurred on DEC equipment, notably their PDP-7 and 11 (Programmable Data Processor). Later DEC computers, such as their VAX systems were also popular platforms to run Unix on, the first port to VAX was in 1978 (the VAX was only released in October 1977). However DEC pushed their own proprietary VMS operating system for a long time before they acknowledged Unix.
DEC released Ultrix in 1982 for their VAX and DEC Station machines, it was based on the BSD 4.2 release. It was an attempt to bridge the void between Unix and VMS, providing support for DECnet and other proprietary DEC equipment. However it was not really Unix nor was it POSIX compliant, that did not arrive until 1988.
Also in 1988 IBM, DEC, HP, and others formed the Open Software Foundation to develop a version of Unix to compete with AT&T and Sun Microsystems. The group's initial release, based on the Carnegie-Mellon Mach kernel was called OSF/1.
DEC refined OSF/1 as Digital Unix to become the main operating system for the company's Alpha processors. It was 64-bit and retained the basis on the Mach kernel but with components from BSD, System V and other sources.