DEC micros

Will's DEC Micro Personal Computer Page

9/24/01 Created
9/27/02 fix a few errors in paths when change ISP
1/17/12 add links to Rainbow and VT180 TM, and a MESS emulation link 3/7/12 add Dave Dunfield's Imagedisk info

Its a little gray where to draw the line between the DEC Micro's and the PDP-11. I concider the PRO series a PDP-11, but I think the newsgroup comp.sys.dec.micro conciders it a micro. There has been discussion about the status of the Decmate versus the PDP-8. You can decide for yourself, but I'm lumping the VT-180, Decmate, and Rainbow together because they were often sold as personal computers or used in offices for word processing. Most of them could also run CPM because they either came with a Z80 CPU or an option card with a Z80 was available.

In Jan of 2012 I was made aware of the MESS emulation project. Its likely this source code base can be used to generate emulators for the Rainbow and VT-180. They are already working on the Rainbow and I hope to get them interested in the VT-180. I've done some minnor updates to this page with that in mind.

My personal migration started with a VT-180. I was retraining myself to be computer literate, and this was my first system. A kindly Uncle, who was working for Digital, was able to get me a discount, but if one conciders inflation this fairly limited CPM machine cost significantly more than todays Pentium V. How times have changed! I used it for a number of years, and got my money's worth, its still alive and well in the basement, but normally its just used as a PDP-11 terminal now. Professionally I started working with the MS-DOS operating system on PC clones. Later when Rainbows became available at reasonable prices on the used equipment market and I obtained several. I like them because they will run both MS-DOS and CPM. They will also read and write 5.25" disks from my VT-180, low density MS-DOS disks, and DEC's RX50 which is often used with the PDP-11 series and Decmates. I'll say more about this later, but my main use of the Rainbow is to transfer data between these formats.
Ultimately I acquired a few Decmates, but am not particularly knowledgable about them.

For information about these systems I used to watch the newsgroup mentioned above. I believe its currently (2012) accessable via Google Groups but its not very active, the last real post I see was Jan 2011! Definately you should browse at Update and Vinatage-Computer forum for information and software. John Wilson also maintains an archive of Decmate disk images and he has been clever enough to store the raw disk image rather than a Teledisk image which can be harder to work with. If you start looking for Rainbow or Decmate software on the internet you are likely to find some Teledisk, *.td0, images. I've done a moderate amount of expairimentation with these and have made a decoding program freely available if you are interested. Dave Dunfield also has a program that will convert teledisk images to his *.img format, see link to his toolset in RX50 discussion below.

VT-180 (aka ROBIN)

This was my first personal computer, so I am fond of them. It is basically a VT100 terminal with a extra board installed which includes a Z80 processor, a floppy disk controller, and an extra serial port controller. The disks were singled sided with a capacity of 180 Kb, they came in a dual enclosure. I published some additional information on how the processor board fits in a VT100. This document also has links to generic VT100 information. I eventaully scanned the complete Technical Reference Manual for the VT-180 if you want to know all about it.

Mine came with a CPM operating system and a word processing program called Select. This system supported up to four individual disk drives (two dual drive units). I can still remember the surge of raw power I got when I upgraded my modem from 150 to 300 baud! I purchased the "VT-180 Technical Reference Manual" and still have it in my library. However this is a fairly standard CPM system. The VT-100 is the console and the software must use escape sequences for screen control of text.

CPM isn't main stream anymore, but was very common in the early days of personal computing. There are a lot of public domain program archives, and some system emulators available. If interested start with the CPM FAQ. The VT-180 disks can be read on an IBM PC clone with a 5.25" disk drive (high, 1.2Mb, or low, 360Kb, density), but you need custom software. Long ago when I was actively using the VT-180 I wrote such a program and can make it available if anyone is interested. These disks have 512 byte sectors, one head (single sided), 9 sectors per track, and 40 tracks. Note this is the same physical configuration the Rainbow and IBM PC can share, if I remember correctly under MSDOS a format /4 generates this physical configuration, but with MSDOS rather than CPM system and directory sectors. Each CPM group is two physical sectors for the VT-180 and although there is no track to track skew, the logical sectors are interleaved on each track. The logical sector ordering is represented by the following physical sector offset from the beginning of the track {0,2,4,6,8,1,3,5,7}. This is the same pattern used on RX50 disks, but there are only 9 sectors. The directory starts at the beginning of the 2nd track and consists of 2 groups, 4 logical sectors. This is followed by the data region of the diskette.

When I published the Technical Reference scans I included copies of my system software disks which are available at BitSavers. Caution, these are raw physical disk images obtained on a faster machine which was capable of track at once reads of the media. To recover the data you must deal with the sector interleave the VT-180 disk controller imposed. The VT180DSK.LST is a list of the disks by DEC part # with a very brief description. If memory serves they were all bootable, the two key disks VT18DIAG, the diagnostics disk, and VT180CPM, the CPM 2.2 boot disk, definately were bootable. I believe everything in this list with a 'BJ-" part number came in the original distribution. SELECT was the distributed editor, SUPERSPL was the associated spell checker, and SINSTTCH was a tutorial for Select. For programming I had the distributed MBASIC, Microsoft's Mbasic, and TPASCAL a version of Turbo Pascal I purchased separately. MULTIPLN is the distributed spread sheet program. VTKERMIT contains a compatible version of Kermit from Columbia University and another serial communications program, POLYGON.


I don't have any offical statistics, but believe a LOT of Decmates were sold. They frequently ended up as word processors runing WPS in office environments. There is a good FAQ which covers the various models. The Decmates are based on the PDP-8 and there is a good three part description of the differences between a PDP-8 and a Decmate, start here. The Decmates used the VR201 monitor and keyboard as does the Rainbow (and maybe some of the MicroVax?). The Decmate and Rainbow also both used RX50 floppy disk drives. There were Z80 option cards which allowed one to run CPM on the IIs and IIIs. I don't have a CPM aware Decmate, but believe they are compatible/interchangable with the Rainbow CPM disks described below.


Digital introduced the Rainbow about the same time as the Apple II and IBM's XT came out. I'm a bit hazy about this, the Rainbow may well have been introduced before the XT but for some reason it did not become the industry standard PC. There is probably a good marketing lesson here (see the note below)!

I have not found a good FAQ for the Rainbow, but there is an extensive archive which includes a documentation directory with a number of useful files. Their collection of PDF documents has grown over the years and now includes copies of the Technical reference manuals for the Rainbow itself, the RX50 disk drive, and the VR201 monitor. If you are interested in the Rainbow the compressed archive file of technical documents is something you should have in your library. The abridged listing below indicates some of the articles of interest contained in RBTECDOC.ZIP:

  • DECRBDAT.DOC - Compares DEC and IBM PC marketing strategies
  • MISC_INT.DOC - Software interrupt 40, basically the keyboard interface
  • IOCTL.REF - DOS function 44H, the serial port interface
  • COMM?.DOC - Two more documents about the serial ports
  • RBGIBM.DOC - Programming the Rainbow (rom services including disk io) and IBM PC compatibility
  • RBMEMRY.TXT - Rainbow memory expansion board description
  • RBXBIOS.DOC - XBIOS programmer's reference (extended bios)
I also highly recommend the following individual files in the rainbow documentation directory:
  • 720KDRIVE.TXT - Instructions which allow you to was a 720KB 3.5" floppy drive
  • V201REP - How to replace your VR201 monitor with a generic TV monitor
I have had several of the VR201 monitors smoke, or just get so dim they were not very useful. I recently obtained several Apple 2C monitors which make a great replacement if you follow the instructions above. You still need the original keyboard, and I was able to get the required handset connector (careful its NOT an RJ11, its the next size smaller) from Jameco.

Aside from DEC's marketing strategy, the thing which I think killed the Rainbow was that software vendors eventually wrote for the IBM PC and didn't bother with Rainbow compatible products. Both systems ran MS-DOS (lucky Bill!) but the bios differences mentioned in RBGIBM.DOC above were a problem. A very popular solution to this was the Code Blue emulator which allowed many IBM PC programs to run on the Rainbow. I always liked the fact that the Rainbow would also run CPM as it came standard with both an 8080 and a Z80 compatible CPU, but I guess the rest of the world didn't share my enthusiasm.

RX50 Diskette Format

The only technical comments I will add here have to do with the RX50 disk format. It is my understanding that the Z80 cpu controlled the disk interface. The first two tracks on the CPM and MSDOS RX50 contained code to boot up this interface. On current IBM PC clones, the first sector on the floppy is a boot sector which defines the geometry of the disk. This doesn't exist on the MSDOS RX50, but the first 20 sectors are reserved for the boot process. The remaining 78 cylinders, 780 sectors, logically look like a conventional MSDOS diskette: two three sector FAT tables, followed by a root directory consisting of 6 sectors (space for 96 file entries) and the data area. Care must be taken because all the data tracks (2-79) have an interleave such that the logical sector ordering is represented by the following physical sector offset from the beginning of the track {0,2,4,6,8,1,3,5,7,9}, however there is no skew between tracks. If one accounts for the interleave and constructs a boot block with the following parameters you can read or extract data from an MSDOS RX50 image:
bytes per sector 512
sectors per cluster 1
reserved sectors 20
number of fat tables 2
fat size 3 sectors
root dir size 96 enters, 6 sectors
disk size 800 sectors
heads 1
sectors/track 10
tracks 80
number hidden sectors 0
See the RX50DRV source code referenced below for more detailed information.

CPM RX50 disks also reserve the first two tracks for the system. Each CPM group contains 4 sectors on an RX50. There is no track to track skew, but the logical to physical sector interleave pattern is the same used for MSDOS {0,2,4,6,8,1,3,5,7,9}. The directory starts on the the first sector of Track 2. There are 2 groups of directory data (8 logical sectors) followed by the data region of the disk.

Its a bit tricky reading an RX50 on current PC's even if you still have a high density 5.25" disk drive. I've played with adjusting the "disk base table" with only moderate success. RX50DRV was a popular device driver which used this technique to allow access to an RX50 on an IBM PC clone equiped with a 5.25" high denisty drive. I have been most successful at recovering RX50 data using a physical RX50 drive in a Rainbow, Decmate, or Pro (presumably a Microvax would work, but I don't have one of them!). Note that the PC driver, RX50DRV, and the Rainbow bios floppy disk interrupt, 65, service routine handle the physical sector interleave. You only have to worry about this if reading the disk directly in an IBM PC clone or working with a physical disk image. Another method of obtaining data from an RX50 diskette that works almost as well is John Wilson's PUTR program. John tells me he bypasses the disk base table and talks to the floppy controller directly to do his magic. If you mount an RX50 as /FOREIGN with PUTR and use the COPY/DEVICE command to copy the disk to a file you get a physical disk image (one that includes the sector interleave). Conversely you can reverse this process to recreate the diskette.

Recently I became aware of Dave Dunfield's imaging toolset which can be downloaded from his site. His documentation is worth a visit by itself, he knows a lot more about disk drives than I do! He also has some archives of older systems in *.img format which include some of my favorite systems in the System / Install Disk section.