Welcome

With the new additions to my hardware collection came a few unexpected surprises, newer "Bridge" newsletters than I thought existed! I thought Volume 5 number 1 was the last one, but I now have 5-2, 5-7 and 5-8. Unfortunately 5-3 through 5-6 are not part of the stash, so if anyone out there can provide any of those I'd really appreciate it.

I began corresponding with Richard Brandle in August of 2007. He was kind enough to provide me with images of several "Bridge" newsletters to complete the known list of volumes/issues. Thanks again Richard.

Richard built a complete system in 1975 and lots of extras, here is a clip from his original email:

"I built a very complete digital group system in 1975, added to it until the company went under and used it until 1988. My system had almost everything DG made (except the display and keyboard, I made my own) plus several custom components (real time clock module and 20meg hard disk) all in DG cases. I ran the audio, PHIMON, DSKMON and Oasis OS's but exclusively ran Oasis once I got it.

I still have all the hardware and software and have just started a project to move all the software onto a PC which I'll run via a z/80 emulator. I also have all the original documentation and many of the DG flyers. When I get all the paper organized, I'll scan it in and send you the PDF's for your web site. The biggest problem with the software media is having the hardware to read it on. After all this time in storage, I seriously doubt the hardware is working. Do you have any of the software already in some PC file format? If not, it's one of my "to-dos" to get it converted."

Unfortunately for Richard all of my software was on tape or DG 8" floppy disks. I began a project to move to an emulator, but emulating hardware, specifically the phi-deck drives took more time and effort than I anticipated, so the project was sidelined until I have more time. As I mentioned in an early post, moving the software to more modern media is an important project, a thought also shared by Richard: "The biggest hurdle I have is conversion of the old media to something that can be used on a modern PC."

I'm afraid I wasn't much help at all as Richard rapidly progressed from building a rig to read the old ROMs to building a phi-deck device emulator, cassette emulator, printer emulator, interface to old diskette controller so he could read his old diskettes to having OASIS up and running on his emulator in just a few months! Wow! Below is a screen shot of his working emulator running OASIS!



The first email arrived in August with images of the emulator running OASIS in December, impressive to say the least! I have not corresponded with Richard since December, but hope to have an update from him soon.

I haven't been active for about eight months due to work and other commitments, but it's time to get back to work.

The first project was to get rid of all of the stupid posts on this site advertising various unsavory stuff. The good and bad of the internet I guess. So unfortunately I've had to turn off visitor entries. I just returned from a trip to Florida to pick up two systems that look to be in pretty good shape, with lots of diskettes. Also looks like the original owner was a dealer of some sort with lots of copies of brochures and yes even a COLOR "Meet the digital group" bytemaster brochure! It also looks like the OS of choice was OPUS, so there is a bit of documentation as well as a set of documentation on SOS (Single user Operating System) which I've never heard of before. More as I begin to unpack...

A special thanks to Richard Brandle for providing the missing Bridge newsletters. Richard is also working on a cool Z80 Emulator to run DG software! More info in future posts!

Sitting busily cataloging diskettes from a recent find of DG related stuff, I was working at the Bytesmaster that I luckily acquired directly from Dr. Suding about three years ago when I heard the unmistakable sound of electronics frying..... I immediately (within a couple of seconds) shutdown the system (pressed the red "Power" button) but not before the "magic blue smoke" (I had an instructor at MIT who claimed that the reason that all electronic components worked was that "magic blue smoke" was captured inside each component during manufacturing and that the component would work as advertised unless you caused the blue smoke to leak out thus ruining the component.....) appeared at the back of the 8" diskette drive cabinet.

I took the cabinet apart, disconnected from the CPU cabinet and used the nose test to locate the area leaking blue smoke. Not surprisingly, the power supply. After further disassembly I located the tantalum capacitor that was indeed cooked (deflated from loss of magic blue smoke). While I'd read about many problems with DG power supplies and modification to make them less prone to fry other things should they have trouble, I've never had a power supply go bad, which got me to thinking and reading from the old newsletters. Several references were found to ruining diskette drive electronics from power supply failures, not good. I repaired and tested the power supply, put everything back together and all was well, at least with the computer......

I began thinking about the possibility of destroying one (or more) diskettes due to a bad power supply, or a CPU board, or, or, or. I also recently had a Phi-Deck ruin a couple of tapes that I had put together a couple of years ago due to a Phi-Deck board failure and had a 1702A ROM fail (never seen that before). I've decided that it's time to get serious about preserving all of this technology on more modern media, which sounds great until you try and start at the beginning with ROM images. Yes I know, I can simply write the ROM images to audio tape (which I have done), but have you tried to buy high quality cassette tapes lately? Besides, I want to store everything in files that I can publish on the web-site (other than in MP3) audio files. The easiest way would probably be to write a simple program to read the ROM data and transfer it to a PC (actually I use a MAC most of the time) via serial port. The other part of preservation is being able to restore things like a ROM image. I do not have an EPROM programmer able to write 1702A's (not many of those around since it takes a -48V pulse on each data pin to program!!!! I'm only aware of one commercial programmer that is $1,000+) so I'll also build an EPROM programmer if anyone out there needs any 1702s programmed.....

I'll keep you up to date on progress. I've started with a PIC based ROM reader that will likely become a ROM emulator at some point. Building a programmer is more problematic with the odd power requirements.

One of the complications of working with Digital Group systems was the different ROMS that could be used to startup the system (Audio, Phimon, Diskmon, OASIS, MCSO). Later in the chronology, if you had a Disk system, you could run a program called DDBOOT after booting into Diskmon to allow you to boot into the other operating systems. One solution was the ROM Switcher detailed in Bridge Vol.1 No.3 from August 1978. Another option was the UnROM outlined by Dr. Suding in this short article. This application was used to change ROMs on the TVC card (video interface). I'm certain that there are several articles about using the same method for the 1702a on the CPU card. I'll update this post and article when I locate the details.

"The UnROM"

by Dr: Robert Sudlng

Theory:

The 1702A EROM on the epu card may be remotely disabled by putting a low level signal on memory line Y. This circuit will Initially place a high level signal when turned on. An output of anything to Port 377 will disable the EROM: An output to Port 376 will return to the EROM.

Similarly, the 6S71L maybe disabled and replaced by some alternative character generator pattern. In this case, the 6571L Is removed from the D.G. TVC, Inserted on the supplemental "UnROM" board and the "UnROM's" cable is plugged Into the former 6571L on the TVC. Initial power on will enable the normal 657lL. An output of anything to Port 375 will disable the 6571L, and enable one of the 2708 EROM'S. Since the 6571L has an effective 16K bits of ROM, two 2708's are required, the first one handling the 1st 64 characters, and the second holding the last 64 characters. An output to Port 37. will enable the 6571L.

(Click on schematic at right for full size version)

The card may be inserted into any 22 pin I/O connector.

Parts:

1 - 7400
1 - 7410
1 - 7430
1 - 7442
2 - 74125
2 - 2708 (programmed with upper and lower)
1 - 6571L (removed from TVC)
1 - 22K 1/4 W resistor
3 - :01 disc capacitor
3 - 1 mfd tantalum capacitor
1 - 22 mfd tantalum capacitor
5 - 14 pin IC sockets
1 - 16 pin IC socket
3 - 24 pin IC sockets
1 - Dual 22 pin socket card
1 - Approx: 18" ribbon cable with 24 pin plug


Operation:

After correctly wiring card, remove 6571L from TVC and Insert on card. Plug Into any available I/O 22 pin connector. Insert the 24 pin plug Into the former 6571L socket on the TVC: Turn on the system. To UnROM, load a RAM version of the desired EROM; Out 377; Move the RAM version to address 000000ff; Proceed with the program. Similarly output to Port 375 to enable the APL character set ( or other desired set). Output to Port 374 to return to the standard ASCII.