Digital Group computers

This page was most recently revised date Aug 28 2013.

Digital Group was a small company in the mid-1970's which produced their own design of microcomputers based on their own "bus" architecture, and across a number of single 8-bit microprocessors. It was somewhat popular early in microcomputing history, but fell out of favor by the 1980's as the architecture did not "catch on" for any number of reasons. I'll not provide a history of TDG as a company at this time; other Web sites have various histories.

We have some other old computer cards as well. Check my STDbus, VME, & Multibus page for links to boards supporting those bus standards. My S-100 home page for boards and docs for the IMSAI, MITS Altair and 100's of other vintage S-100 computers. I have Mac and SGI stuff as well.

Here is how to email @ me and ordering instructions.

- Herb Johnson

boards from 2010

[2010 DG]

During May 2010, at a hamfest, I acquired some boards which I discovered were from "the Digital Group". If you have more info on these boards, contact me. [I'll update this further based on my May 2012 acquisitions (see below).] Thanks to Cameron Cooper in 2012 for DG identifications of thes cards. - Herb

Here's an image of two Digital Group I/O boards. These boards have a 72-pin edge connector and a 44-pin connector. The board at the top of the photo, has four large 74100 chips near the edge connnector; I have two of those, they are Digital Group Parallel I/O Card: DG 1002, 4-port I/O boards. The board at the bottom of the photo, has four 8251 chips near the top of the board; those are 8251 serial I/O chips. This is a Digital Group Serial I/O Card: DG 1022.

Here's an image of two Digital Group memory boards, and an I/O board. the board pair at the top of the photo is a RAM card with a 72-pin edge connector, I have two of those. These memory cards are Digital Group "early" 32K static RAM cards. The board at the bottom of the photo is the same as the board at the top of the previous photo. These are 32k Static RAM card: DG 32K-2.

Here's an image of the CPU board. It has a 100-pin connector,b ut it's not S-100, this is the Digital Group CPU standard but not a DG brand card. Please note - this particular card came with a "daughter board" mounted atop the CPU socket and connected through the CPU pins. The daughter card is some kind of interface, probably SASI for a SASI to something hard drive controller. Here's the CPU board without the hard-disk controller. YOu can see in either photo, a ROM and a black block which is likely a battery-backed RAM, and an 8255 parallel I/O chip. A number of people have stated this is NOT a Digital Group brand board.

Here's some boards I can't yet identify, but they have 44-pin edge connectors and they came with the DG boards. Photos boards #1 boards #2 boards #3 boards #4 from General Computer Technology company in 1978.

Also, this board appears to be a ROM replacment board of some sort.

Former Digital Group tech Greg Peterson looked at my collection and had these comments:

"Several of your boards are proto boards. To the best of my knowledge DG never shipped any proto boards for any reason, only bare PC boards for user assembly. You should be able to get the CPU type from your CPU board. To the best of my knowledge, DG never shipped a hard disk interface ever. The floppy board was a stand alone concept. I don't know what to make of your daughterboard approach. The board with the 74100's may be one of my designs. DG created a separate company called "Peripheral Vision" to make some of the peripherals for DG systems as well as other systems. I did a parallel I/O board for them, which you may have an example of."

Greg has more comments on these boards, and about DG, and his own DG system, all on the linked Web page.

VCF East 7.0 and Digital Group

I was a vendor at VCF-E in May 2011, and there was a Digital Group exhibit there by Michael Hill. (Photo courtesy of Christian Liendo.) I met with Michael Hill and we discussed my DG boards. Here's a few photos of his exhibit. From examining the motherboard displayed there, I realized I may have a DG CPU board. As its connector did not match the other boards I had, I assumed it was some other kind of board. But the DG systems used a variety of boards.

Mike looked at this Web page later and in email said: "...looks like among those cards are a couple of memory cards (32k I believe), some parallel I/O cards, and another interface card I can't identify. The CPU card is interesting. It doesn't look like the Z80 card I have, but I believe they had a later card created just before the company went under. Or perhaps it was a 3rd party creation - from what I've read some people bought the technical drawings shortly after the bankruptcy and started producing some clones of dg stuff. Not sure." [Mike may not have noticed that the CPU board has a "daughter board" on it. - Herb]

Mike continued: "On the CPU card I don't see any RAM - the CPU cards for dg had 2k of SRAM on the board. But the dg CPU cards do use a 100 pin connector, so that matches up. Looks like you do have a 4-port parallel I/O card, and a couple of memory cards, so that's most of a working system. To display anything, though, you would need a TV output/cassette card which I don't see there." MIke offered some docs and asked for brand info from the cards.

"The wire wrap boards are interesting. I think the rom replacer is basically a switch to disable the ROM to allow programming in the low page of memory. On the dg CPU card, the lowest 256 bytes of memory are read from ROM, and reads to this location generate a signal which disables the RAM. I've seen some schematics for an "un-ROM" which disables this and allows memory writing in the low page. Perhaps that's what this board is."

As of 2012, Mike is making progress on his DG Web site. He told me in May 2012: "The site I put up for DG computers is at Right now I have a few dg programs in audio format up there, as well as a dg emulator for PC and Mac. I would be happy to put up any additional information that you might have." We met briefly at VCF-E 8.0 in May 2012. He reported to me that Dr. Suding is still with us but has some medical issues.

Digital Group system acquired 2012

This section is a work in progress and so is rough.

I acquired the DG system described below in May 2012, from the original owner Doug Crompton. He ran a DG BBS, developed some software and hardware with some colleagues, and is/was an active in amateur radio (ham). I'm still working on reviewing and inventoring the system, so this description is still "rough". Here's a text file of the manuals and software.

[Crompton DG]

CPU Chassis and overview

Here's a photo of the top of the CPU chassis. It's a custom built chassis of course, but the motherboard is Digital Group. Here's a better view. See how the slots are labeled? The back two boards are 32K memory cards in memory slots, DG 32K-2. Forward of them is the DG 1010 Z80 CPU card in the CPU slot. Forward of that is the Parallel I/O Card: DG 1002; then the video/cassette card with 64x16 Character capability, the DG-1008-B. The card closest to the front, is the DG Double Density Floppy Controller, 090-078. Behind that card, is a slot where once resided a DG Phi-Deck digital cassette controller card. (Thanks to Cameron Cooper in 2012 for DG identifications.)

The scary bit, is under the chassis. Here's what the underside looks like. Because most cards have their connections through the motherboard (not on top of the card like S-100 class bussed systems), all the wiring is underneath. It also means the slots are dedicated to each card, which I don't think is a good idea. the time in 1975-6, there were no standards, this was one idea among others.

Here's a photo from the rear of the chassis.. From the back, you can see two RAM cards. The card with a flat cable, is a ROM extension cable going to a small board behind the "front panel". That board contains several ROMs which are switched into use.

Here's a photo of the front of the CPU chassis. Switches and lights provide various options. 4.0 / 2.5 controls the CPU clock speed. The rotary switch on the far right, swaps in various ROMS. The LED's below show data (left) and address (right).

The CPU chassis has various connectors which Doug used to connect to his I/O chassis, power supply, etc. Here's a better photo of the connectors on the CPU chassis.

I/O chassis

Here's a photo of the back of the I/O chassis. Only one I/O board remains. Here's a photo of a corner of that board, AEON brand... but it looks like the same layout as the DG four-port serial card. I think those are four Intel 8251 UART chips. So it's a four-port serial card.

[Crompton DG top back]


Board details

Here's a better view of one of the DG 32K-2 memory cards. The RAM chips are TMS4044's or similar, static RAM 4K by 1 I believe. So these would be two 32K RAM cards. Here's the DG 1010 CPU board, clearly a Z80. The next card is a DG 1002 I/O card, here's a good look at the I/O card with its parallel 74100 chips; the driver/receivers are 7404's. Then there's the DG DG-1008-B TV video/cassette card, apparently the 64 X 16 rev B. Finally the DG 090-078 double density (soft sectored) floppy controller card with FD1791B floppy chip. (Doug he told me he had a Votrax board also.)

floppy drive

Here's a photo of the Shugart 800/801 drive. Two were used with this system, I got one of them. No power supply, but I have power supplies around.

DG ROMs I (may) have

I had some correspondence with DG owner Cameron Cooper in Sept 2012, about the ROMs in my (Crompton) DG system. I've not yet read them out (I do have a 1702 reader to do that). But in the documents I obtained with the system, there's hex dumps of three ROMS as follows: dg_ROM_diskmon_2D.jpg; dg_ROM_oasis_2D.jpg; dg_ROM_mcos_2D.jpg. The documents refer to use with the DG double-density (soft sectored) controller. My interpretation of these is that the "address" of the dumps is not the address of the ROMS. Disassembly of JMPs and CALLs suggests otherwise. Here's my hand disassembly of the DISKMON dump, which is reasonably informative. - Herb

Digital Group cassette methods

DG used at least two methods of recording data onto audio cassettes: the Don Tarbell method, and DG's own method. Others were in use in the era, before and during development of cheaper floppy drives and controllers. Most methods recorded either of two tones (frequencies) to represent 0's and 1's; and various data formats to support a file structure, checksums and so on. Tarbell came out with an early scheme he calls "bi-phase", based on 1500Hz and 750Hz tones (roughly, and optionally higher). Tarbell produced an early S-100 audio cassette card; also, many computing companies adopted his methods. DG also provided another tone method, tones at 2975 Hz and 2125 Hz, based on amateur radio RTTY standards. Greg Peterson created an archive of Tarbell and DG cassette .WAV files, from his collection. A Web page I have on his collection, includes links to his discussions and archive.

S-100 docs for Tarbell's Cassette Interface, are available from my site (as photocopies) or on other S-100 sites like this link to Dave Dunfield's Tarbell manual.

A look at the DG documents at (details below), shows docs for the two Digital Group TV boards, the 32 char and the 64 char. The tvc-32.pdf at this link, describes the tone-encoding and decoding hardware but not the bit format. Here's a JPEG image of the schematic for that portion of the TVC32 card. Don Tarbell's methods are discussed in his S-100 manual for his model 1001 Audio Cassette card (and I presume in DG documentation also). - Herb Johnson

Other Digital Group collections

In Sept 2012, I had some correspondence with new DG owner Cameron Cooper. He just acquired a DG and is getting started in understanding it and getting it up and running. In due course, I'll have a page about his remarks and system. He also identified some of the DG cards I have as above; and we discussed DG ROMS he has I have likely have (see above).

Greg "Pete" Peterson worked at Digital Group for years as a designer. He contacted me in May 2012 to discuss his history with the company. Then he started work on his Digital System, long in storage, to recover the audio cassette programs and the hardware. He produced a DVD of digitized audio of the cassettes. In 2013, he's produced source list files of the EROM Z80 ROM monitor/loader and the Z80 cassette OS. His DG history past and present, his discussion of my collection, and his 2013 work, are at this linked Web page.

Mike Hill's DG Web site is

For several years up to 2008, Dr. Robert Suding had a Web site. His primary interests at the time was amateur satellite radio and astronomical activities. His Web domain,, was apparently abandoned by him sometime in 2008. But I obtained a Internet Archive text-only version of his Web page "dg.html" about his version of the history of the company. I'll see about making it available on my site.

A Google search in May 2012 led me to to "Brian's Old Computers" at, a 2008 Web site for his former Digital Group collection among other old computers he had. The site was still available as of May 2012. He has excellent photos of many DG cards, and has digitized many DG manuals and archived DG software. Mike Hill also recommended this Web site. The site owner notes as of 2008-2009 he will not update the site but will keep it on line "for years". So far the site is "up".

There's a number of other Web pages about the Digital Group, which at some point I'll add to this Web page. A Google search will find more current and former DG owners and collectors. - Herb

Herb Johnson
New Jersey, USA
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Copyright © 2013 Herb Johnson