Digital Equipment Corporation
started producing minicomputer systems, software and peripherals back in the 1960s.
Their first computer systems were the PDP-range of minicomputers, from the PDP-1 (1960) through the PDP-11 (1968+).
Their popular 32-bit VAX systems appeared in 1978.
By the 1980s, they were the second largest computer company in the United States, following IBM, and were the
leading producer of minicomputers.
The 1982 they released the Rainbow 100 - a low-cost, user installed personal business computer designed to
run applications in the Fortune 100 marketplace. It was displayed at the June 1982 NCC - National Computer Conference -
in Houston Texas.
The system provides hardware and software for both stand-alone processing, and as a terminal emulator for DEC
and other manufacturer's systems.
The Rainbow 100 includes a two-processor architecture based on the simultaneous operation of an 8088 and Z80A CPU.
In addition to running applications, each processor supports a portion of the needed functions of the computer.
The Z80A processor supports the functions required to read/write the floppy drive. The 8088 handles the video output,
keyboard I/O, printer port, and installed optionals.
Three models were eventually released:
100A - floppy disk based system - Winchester not supported.
100B - $2,750 - Winchester installation optional.
100+ - $5,475 - Winchester factory installed.
The dual-processor design means that the Rainbow 100 can run either the CP/M or MS-DOS operating system.
CP/M was quickly being replaced by MS-DOS as the preferred operating system, due to the IBM Personal Computer,
which was taking over the market.
Unfortunately, due to hardware differences, the DEC version of MS-DOS was not 100% compatible with the
IBM PC, so software written for one of the systems would probably not run on the other.
In addition to booting into either CP/M or MS-DOS, the Rainbow 100 can also boot into "terminal" mode -
a text-only mode based on their own popular VT102 format, as a "locally intelligent" satellite to a larger computer system.
The name "Rainbow" can be very misleading - the most basic Rainbow 100 has no color at all - it is monochromatic,
text-only system. Color and graphics is an optional add-on, utilizing the powerful NEC-7220 graphics chip,
which was used in the NEC APC, among others.
The Graphics Adapter enables high resolution color display:
400x240 resolution, with 16 colors from a pallette of 4096.
800x240 resolution, with 4 colors from a pallette of 4096.
The internal "RX50" floppy drive reads and writes single-sided, 5 1/4-inch floppy disks, storing 409KB on each, which was a lot
of data at the time.
This floppy drive is a very unique, proprietary design by DEC, and no other manufacturer used it in their systems.
While it does hold two floppy disks, it can only access one of them at time. This is because the read-heads for both drives
move in unison, controlled by a single stepper motor. Additionally, the two floppy disks must be inserted upside-down from each
other, confusing many a new user. The upper floppy disk is inserted face-down, the lower floppy is inserted face-up.
The optional Winchester hard drive is MFM ST-506 compatible, holding either 5MB or 10MB of data, although the controller
can handle a Winchester with up to 67MB capacity.
As seen below, the motherboard had many options, which were added on top of it.