Rainbow 100
| Industry | Computer hardware |
|---|---|
| Founded | November 8, 1957 in Maynard, Massachusetts, United States |
| Headquarters | Maynard , United States |
| Products | Rainbow 100 microcomputer, minicomputers, computer hardware |
The Rainbow 100 microcomputer was launched by Digital Equipment Corporation (DEC) in 1982. This desktop system incorporated a monitor resembling the VT220 and utilized a dual-processor configuration, combining a Zilog Z80 with an Intel 8088 CPU.
Designed to operate in three distinct modes, the Rainbow 100 could function as a VT100 terminal for DEC's VAX systems, run 8-bit CP/M software via the Z80 processor, or execute CP/M-86 and MS-DOS applications on the 8088. Despite its multifunctional capabilities, it struggled to gain traction as the industry rapidly embraced IBM PC compatibility as the prevailing standard. Author David Ahl characterized its market entry as a significant failure.
Introduced alongside the visually similar DEC Professional and DECmate II, the Rainbow’s poor market performance underscored DEC’s challenges in breaking into the mainstream PC sector. This failure also played a role in the downturn of New England’s computer hardware industry, which was predominantly centered on minicomputers for enterprise clients and included firms such as DEC, Data General, Wang, and Prime.
Models
The Rainbow was available in three variants: 100A, 100B, and 100+. The initial release was the "A" model, succeeded by the "B" version that incorporated firmware enhancements and minor hardware revisions. These were labeled PC-100A and PC-100B respectively; some later "B" units carried the PC-100B2 designation. A unique feature was a user-replaceable ROM chip that allowed customization of keyboard layouts and boot screen languages. The 100A’s ROM supported only three languages. Unlike conventional PCs, the Rainbow did not utilize an ISA bus, eliminating the standard 640 KB RAM limitation—both models could accommodate more than 800 KB of memory.
PC-100A
As Digital’s first model, the 100A was primarily limited by firmware that lacked hard disk boot support. It contained three 8 KB ROM chips holding system firmware and featured specific combinations of case fans and power supplies. This model did not support remapping hardware interrupt vectors, which led to conflicts with MS-DOS soft interrupts requiring special workarounds. Initially equipped with just 256 KB of RAM, memory capacity was later expanded to 828 KB through an adapter card.
PC-100B
The 100B model introduced firmware upgrades enabling booting from hard drives and replaced system firmware with two larger 16 KB ROM chips. Improvements included enhanced fan and power supply designs, plus the ability to select boot screen language and keyboard layout without physically changing ROMs. Hardware interrupt remapping was added to improve MS-DOS compatibility. Its memory expansion slot supported up to 892 KB.
The "100+" variant denoted units shipped with an installed hard drive but was otherwise identical to the "B" model. Machines equipped with a hard disk bore a 100+ badge on their casing.
CPUs
The Rainbow employed two independent data buses managed by the Zilog Z80 and Intel 8088 processors, which communicated through a shared memory space of 62 KB. The Z80 primarily controlled floppy disk operations when not running 8-bit programs, while the 8088 handled graphics processing, hard disk management, and communications tasks. Although it was theoretically possible for both CPUs to execute code simultaneously, this capability was never demonstrated in practice.
Users could upgrade the 8088 CPU to an NEC V20 processor, which provided a performance increase of approximately 10–15%, but this upgrade necessitated modifications to ROM code to accommodate timing adjustments.
Memory
The 100A variant came equipped with 64 KB of built-in memory, whereas the 100B featured 128 KB. Digital Equipment Corporation provided expansion daughterboards that could increase memory by up to an extra 768 KB. This allowed a total memory capacity of 892 KB on the 100B and 828 KB on the 100A, depending on the original setup.
Floppy disk drives
The RX50 floppy drives utilized proprietary 5¼-inch diskettes with a capacity of 400 KB, single-sided and quad-density. Initial operating systems lacked support for low-level formatting, compelling users to purchase costly RX50 media directly from Digital Equipment Corporation, which led to criticisms regarding vendor "lock-in." Subsequent versions of MS-DOS and CP/M introduced disk formatting capabilities, and third-party tools eventually permitted formatting of standard diskettes.
A distinctive feature of these drives was that both shared a single motor powering a common spindle with disks stacked vertically—one disk inserted upside-down—earning the nickname "toaster." This unconventional design initially caused confusion among users. Later revisions addressed this by adding red arrows on both the slots and diskettes to indicate proper insertion orientation.
Hard disks
Digital Equipment Corporation developed a Winchester disk controller compatible with hard drives using the ST-506 interface. Built around the Western Digital WD1010 chipset, this controller supported one drive with capacities up to , limiting total storage accordingly.
Additionally, third-party hard-disk controllers were available, including options from CHS that supported dual Winchester drives.
Graphics
Out of the box, the Rainbow system displayed monochrome text in either 80×24 or 132×24 character formats, offering attributes such as bold and double-width characters.
An optional graphics module, installable by users, introduced color and enhanced graphics functionality. It incorporated an NEC 7220 graphic display controller (GDC) alongside 8×64 KB DRAM dedicated video memory.
This configuration allowed the Rainbow to operate two monitors concurrently—one dedicated to graphics output and the other for text display.
Display
The standard system generated a TTL-level composite video signal at 15 kHz compatible with RS-170 (NTSC) in monochrome. When equipped with the graphics option, it could output sync-on-green RGB signals at TTL levels.
Commonly paired with DEC’s VR201 monochrome 12-inch or VR241 color 13-inch monitors, the Rainbow uniquely supplied power to the monochrome monitor through its single video cable, eliminating the need for separate power connections.
Keyboard
The Rainbow 100 featured the LK201 keyboard, which was also used in DEC’s DECmate II and Pro-350 systems. The LK201 introduced a redesigned layout that unsettled some traditional users accustomed to Teletype Model 33 and VT100 keyboards. This design later influenced the enhanced VT220-style 101-key keyboard adopted by IBM in 1985.
Third-party
Several third-party enhancements were available for the Rainbow 100, including an 80286 processor upgrade known as Turbow-286, a 3.5-inch disk adapter kit called IDrive, and a battery-backed clock chip named ClikClok—all provided by Suitable Solutions.
In 1984, support for Native American languages was introduced when Rock Point Community School on the Navajo reservation commissioned a ROM chip containing Navajo language support, enabling bilingual computing applications.
Univation offered add-on memory cards, hard disk controllers, and LAN interface cards for the Rainbow. Their disk interface employed SASI/SCSI-1 technology based on Xebec designs but lacked software compatibility with DEC’s Winchester Disk controller.
C.H.S. developed a dual-Winchester controller that was sufficiently compatible with DEC’s disk controller to allow booting on models such as the 100B/100+/190. It utilized Western Digital's WD1010 chip; however, accessing the second hard drive required additional drivers.
REC of Switzerland produced a SCSI expansion card for the Rainbow platform. In Australia, an "ACT Winchester Option" was available alongside newly discovered drivers.
Duncan MacDonald, Inc. marketed a backup solution utilizing floppy tape cartridges holding 20MB capacity. This system employed rebranded Cipher Data Products drives and used 3M DC600A tapes capable of backing up a 20MB RD31 drive in approximately half an hour. Priced at $1200 in 1987, it supported MS-DOS, CP/M, and CCP/M partitions. The company also offered competitively priced MFM hard disks ranging from 20MB to 67MB along with steel enclosures and power supplies designed to accommodate larger disks without overburdening the Rainbow’s internal power source.
DEC Hardware Options
Below is a list of available hardware configurations for the Rainbow system (excluding country-specific kits):
| Option Code | Board Number | Description |
|---|---|---|
| standard | 54-15482 | RX-50 Disk Controller |
| PC-100A | 54-15486 | CPU Assembly for PC100A |
| PC-1XX-AA | 54-15490-AA | 64KB Memory Expansion (exclusive to PC-100A) |
| PC-1XX-AB | 54-15490-AB | 192KB Memory Expansion (exclusive to PC-100A) |
| PC-1XX-BA | 54-15688 | Color Graphics Adapter |
| PC-1XX-BB | 54-15703 | Extended Communications Module |
| PC-1XX-DA | 54-16019 | RD51 Controller Module |
| PC-100B, PC-100+, PC-190 | 54-16206 | PC100B System Board |
| PC-1XX-EA | 54-16535 | 8087 Math Co-Processor Board |
| PC-1XX-AC | 54-15961-AC | Socketed 64KB Memory Module for PC-100B or PC-100A with adapter |
| PC-1XX-AD | 54-15961-AD | Socketed 256KB Memory Module for PC-100B or PC-100A with adapter |
| PC-1XX-AK | Adapter to enable memory modules in PC-100A | |
| PC-1XX-AZ | Set of nine 256KB Static RAM Chips | |
| PC-1XX-FA | Technical Character Set ROM for 100A | |
| PC-1XX-FB | Technical Character Set ROM for 100B | |
| RCD31-BA | 20MB Half-height Disk Drive and Controller for 100A | |
| RCD31-BB | 20MB Half-height Disk Drive and Controller for 100B | |
| RD31-BA | Replacement Drive for RD-51 Controller | |
| PC-XXF-BA | Floor Stand Enclosure |
Software
The Rainbow operates on the CP/M-86/80 system, capable of identifying whether applications are designed for 16-bit CP/M-86 or 8-bit CP/M-80 and running them appropriately. DEC subsequently introduced an MS-DOS variant compatible with the system; however, limited IBM PC BIOS emulation restricted the range of MS-DOS commercial software usable. Toward the end of its production, certain IBM PC-compatible MS-DOS programs could be executed through the Code Blue emulator, which replicated BIOS functions and some hardware features, though software requiring direct video hardware access often encountered difficulties.
MS-DOS support was incorporated late in development, leading to conflicts between hardware and software interrupts. To resolve this, DEC redesigned interrupt handling in the Rainbow 100B, allowing MS-DOS version 3.10b to operate exclusively on this model.
DEC also adapted Microsoft Windows 1.0 for the Rainbow platform; additionally, third-party Turbow-286 expansion boards enabled a modified version of Windows 3.0 to run.
Pre-installed software packages included:
- Microsoft MBasic-86 BASIC interpreter tailored for Rainbow 100
- WordStar word processing software developed by MicroPro specifically for Rainbow 100
Documentation
- Rainbow 100 Technical Manual
- Getting Started with Rainbow 100
- MBasic-86 User's Guide published by Digital Equipment Corporation
- WordStar User's Guide for Rainbow 100
- License and Software Product Description
These manuals were supplied as printed booklets within two hard-shell cases bearing the DEC emblem.
Limitations and Issues
The Rainbow employed a distinct MS-DOS version that resulted in partial incompatibility with IBM PC or standard PC DOS software and hardware. While software was generally intended to interface through MS-DOS calls, many commercial titles accessed hardware directly to enhance performance. Following the release of IBM PC clones such as the Compaq Portable, full IBM compatibility became an industry norm. Microsoft eventually unified MS-DOS with retail version 5.0, discontinuing unique OEM variants. Although the Code Blue emulator effectively replicated the IBM BIOS, it was unable to fully intercept video and hardware-level instructions, restricting compatibility with some applications on the Rainbow. The FOSSIL TSR allowed certain terminal programs and text editors to function across early Intel 8088/8086 systems but saw limited widespread use. Various patches enabled popular development tools like Turbo Pascal and Turbo C to run, but these fixes were soon outpaced by software updates and largely faded after 1991.
Rainbow MS-DOS supported FAT12-formatted floppy disks using single-sided, quad-density, 80-track 5.25" drives without implementing a Master Boot Record (MBR) or BIOS Parameter Block (BPB). Instead, a fixed BPB was maintained in memory. The boot sector contained Z80 code responsible for loading the bootstrap code onto the 8088 processor. Disk formatting utilized a two-to-one sector interleaving pattern, complicating disk imaging since conventional tools expect an MBR containing a BPB.
Hardware expandability was constrained by design; the Rainbow lacked a general-purpose expansion bus and provided dedicated slots solely for specific functions such as memory modules, graphics adapters, or floppy controllers. A slot originally intended for a DMA-capable serial card was reassigned to hard disk controllers due to DEC’s misjudgment of priorities. Univation produced an Ethernet card compatible with that slot which supported network booting; however, its $750 cost limited widespread adoption. The combination of limited hardware expansion options—particularly the inability to utilize ISA cards—and persistent software incompatibilities significantly contributed to the Rainbow’s commercial decline.
Legacy
The Multi Emulator Super System supports running the Rainbow-specific version of Windows 1.0.
The Rainbow 100 played a significant role in the origin of the FidoNet network. A computer club based in St. Louis, planning to establish a BBS utilizing CBBS on CP/M, was provided with a Rainbow 100 by DEC. Since the Z80 CPU did not have direct access to serial ports, the group shifted focus to a DOS-compatible BBS solution. They selected FidoBBS and collaborated with its creator to adapt serial drivers for the Rainbow platform. This collaboration involved substantial long-distance communication costs for email and file transfers between St. Louis and San Francisco, which ultimately inspired the creation of FidoNet to automate message exchanges during low-cost phone periods.
Author Piers Anthony composed numerous books starting in the 1980s using his DEC Rainbow.
The DEC Rainbow made appearances in movies such as Ghostbusters, Beverly Hills Cop, and Firestarter. It is also visible in a television commercial segment within The Philadelphia Experiment.