Rainbow 100
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The Rainbow 100 was a microcomputer introduced by Digital Equipment Corporation (DEC) in 1982 to compete in the IBM PC market. This desktop unit had the video-terminal display circuitry from the VT102, a video monitor similar to the VT220 in a box with both Z80 and 8088 CPUs. The Rainbow 100 was a triple-boot machine: VT102 mode, CP/M mode (using the Z80), and CP/M-86 or MS-DOS mode using the 8088.
The Rainbow came in three models, the 100A, 100B and 100+. The 'A' model didn't allow for a hard disk controller, whereas the 'B' allowed this option and the '+' shipped with the controller and a hard drive. Versions of the 100A shipped outside the USA included a user changeable ROM chip in a special casing. The user changed the built in ROM for this one to support their keyboard layout and language of the boot screen. The 100B has this selection built into the boot up firmware.
The '100+' model is actually a marketing designation signifying that the system ships with a hard drive installed; the 100+ and 100B are identical in all other respects. The '100B' (and '100+' by definition) had the option to boot from the hard disk (or Winchester drive, as it appeared in the boot menu), while the '100A' firmware did not support booting directly from the hard disk.
The '100A' model shipped with 128 KB (128 KiB) memory on the motherboard, while the '100B' had 192 KB (192 KiB) memory on the motherboard.
The Rainbow 100 and the other two microcomputers which DEC announced at the same time (DECmate II and Pro-350) had two quirks that annoyed conservative users. The LK201 keyboard used a new layout that made some ASR33 and VT100 users unhappy. However, the VT220 style of this keyboard can clearly be seen in the layout of the enhanced 101-key keyboard adopted by IBM in 1987. The floppy disk drives took 400 KB single-sided quad-density disks when all the other microcomputers were using 360 KB double-sided double-density disks. Initially users had to buy media from DEC with a special reinforcing ring at the hub, but this became unnecessary when other manufacturers began producing diskettes that suited both types of drive.
Of note was the single motor used to drive both disk drives via a common spindle, which were arranged one on top of the other. That meant that one disk went underneath the first but inserted upside down. This confused many first time users who found that the machine would not read the disk.
The Graphics Option was a user-installable module that added graphics and color display capabilities to the Rainbow system. The Graphic module was based on a NEC 7220 Graphic Display Controller (GDC) and a 8x64 K DRAM video memory.
Various Rainbow option upgrades were available from DEC including a high resolution color graphics card (driving an RGB DEC monitor), ST-506 hard drive controller and drive (typically 5, 10 or 20 MB), a memory expansion module expanding memory up to a maximum 896 KB, an 8087 math co-processor upgrade and a DECnet network adapter.
Third party upgrades were also available including an 80286 processor upgrade (Turbow-286) and 3.5 inch disk adapter kit (IDrive), and a battery-backed clock chip (ClikClok), all from Suitable Solutions.
A benefit of the Rainbow was that it could continue to run 8-bit CP/M software as users moved into the 16-bit world of MS-DOS. The disadvantage was that little DOS software was released on Rainbow media and there were subtle differences between MS-DOS, as run on a Rainbow, and MS-DOS (or PC-DOS) running on true IBM PC compatibles. Towards the end of its life, Rainbow users were able to run IBM PC compatible MS-DOS software using an emulation application called Code Blue.
Of note is that Microsoft Windows 1.0 was ported to the Rainbow by DEC. There was also a 3rd party upgrade that included a 286 processor board and a modified version of Windows 3.0.
