Archive for the ‘8008’ Category

Operating an Original SCELBI 8B

Tuesday, September 26th, 2017

I recently spent a few hours with Mark Arnold and his original SCELBI 8B. This is the very machine that Mark used to develop SCELBAL with, way back, in 1975. Towards the end of our debug session, we had the machine in a largely working order!

Mark Arnold with his SCELBI-8B

Mark Arnold with his SCELBI-8B

We spent a lot of time, troubleshooting what appeared to be an intermittent EPROM issue. Finally we discovered that that the edge connector on the EPROM board wasn’t making reliable contact with the connector. After examining the board in detail, we noticed tarnish on the edge connector contacts. We used a soft pencil eraser to clean the tarnish off of the contacts and the intermittent operation completely disappeared. Fixing a computer with a pencil eraser was a first for me, though I expect others have done the same. Once the repair was done, Mark was able to reliably run a variety of MEA operations without any problems.

With only 4K of SRAM installed, we were unable to load the full floating point SCELBAL, but we were able to load tiny SCELBAL using a simple bit-banged RS232 port. Once loaded, Mark successfully entered and ran some small BASIC programs. Weeks later, I understand that Mark still has the system powered on and it responds to MEA commands, without any need to reset or reboot.

In order to declare that the machine is in 100% working order, a few things still need to be checked out. We had only 4K of SRAM installed, the other 8K should be added back into the system. We used a modified version of MEA with the page 76 EPROM changed. I created a version of page 76 EPROM with a 2400 baud driver, but we were having some issue with corrupted serial output characters. We don’t know if that was a ground loop problem or a baud rate problem with the modified page 76 driver. Serial input at 2400 is working perfectly, but the timing for output is done differently, so it could be either.

Finally, the cassette interface needs to be connected and tried out. I don’t really expect any issues with that, as I have used that particular cassette interface with a reproduction SCELBI 8B, back when I was first troubleshooting my reproduction cassette interface.

Reproduction SCELBI 8B demonstrated at the Museum Of Computing History (UK)

Wednesday, September 20th, 2017

I just received this report from David Williams in the UK.

Hi Mike,

Just thought I’d drop you an email to let you know, I took my SCELBI 8B to the Retro Computer Festival at the Museum Of Computing History last weekend. Only a handful of people recognised the system initially but hopefully a few more people are now aware of the history behind the machine.

I spent a good while describing the system to one chap who I later discovered is the guy heading up the EDSAC replica build at Bletchly Park.

I was able to demonstrate the basics of the system using the monitor program to edit & display memory along with entering short programs. I couldn’t get SCELBAL to load due to a RAM fault (Only Identified when I got back home) which was a shame.

I’ve attached a photo of the setup. Terminal being used is a TI Silent 745. The box below the SCELBI is the power supply, Teletype interface to the right and a fast-load box as described in issue 1 & 2 of the SCELBI newsletters.

Cheers,
Dave

David William's 8B (reproduction)

David William’s 8B (reproduction)

SCELBI 8H Usability Reasessment

Wednesday, August 2nd, 2017

For a long time, I would say that the SCELBI 8H was the first practical computer marketed to the general public. The key word being practical, as certainly many of the other personal computers of the day, were not very useful machines, other than for educational purposes, at least without adding additional capabilities to them.

From the time that I got my reproduction SCELBI 8H working, I’ve been able to demonstrate a number of interesting, practical applications on it. That I was able to do so, was justification for the claim that a SCELBI 8H was a “practical” computer. However, I’ve always used a second computer to assemble the source code and then downloaded the application to the SCELBI through a serial port. A few months ago, when I talked to Bob Findley, I asked about how they did application development for the SCELBI 8H. It turns out that Bob and Nat used almost exactly the same approach that I used. Only thing is that they used a PDP-8 as a platform for the cross assembler and a teletype with it’s paper tape punch/reader to download the program to the SCELBI. Though the cross development platforms are different, what Bob and Nat did, was essentially the same as what I’ve done.

I’ve come to realize that though you can run some interesting, practical applications on the SCELBI 8H, it really can’t do it, without either help from a host computer, an impractical amount of time, or the addition of features like a built in monitor. After talking to Bob, and thinking about it a bit, I guess I’m ready to say that though the SCELBI 8H, as shipped, can run practical applications, it was not a very practical stand alone computer. However, that still doesn’t remove the significance of the SCELBI 8H as the first offering from one of the very first personal computer startup type companies.

Note that the limited capabilities of the 8H were recognized by Nat and Bob, who developed the follow on SCELBI 8B, with it’s extensive built in software suite. More about the 8B in a follow on post.

How Many SCELBI’s Were Really Sold?

Saturday, July 15th, 2017

According to Bob Findley, whom I spoke to earlier this year, the often quoted number of 200, is an exageration. Bob said they sold more than 40 completed systems and over 100 board sets.

One interesting thing that Bob said, is that until the Altair came to market, they felt that they had the market to themselves. The Mark-8 wasn’t considered a competitor, as it wasn’t sold as a functional system.

I also found out why there are so few Oscilloscope interfaces remaining (only one is known to exist at the CHM). Bob said that they only sold about 4 of them.

More information from Bob in a follow up post.

Early Draft SCELBI HW Construction Manual Available

Saturday, June 10th, 2017

This is a very early draft. Consider it to be full of errors, so compare to original documents that can be found at scelbi.com, before using. The main reason I’m posting this, is that I have created and added a couple of chapters for assembly of the oscilloscope interface. Any original documentation that existed about the oscilloscope interface has been lost. I still need to create a chapter with technical information on the oscilloscope interface, including a bill of materials, schematics and theory of operation, but this is a good start. I also need to add a section on PCB rework for this interface. The other chapters will also get revised, as I find time, with photographs of the boards and any notes or errata that I have discovered.

http://www.willegal.net/scelbi/8BHhardwarebook.pdf

SCELBI Front Panel redo in progress

Saturday, May 27th, 2017
SCELBI Front Panel

SCELBI Front Panel

This SCELBI front panel had some issues, so I decided to remove the existing anodizing, polish out the defects and completely redo it. Removing the anodizing took about an hour. There is a term, “hard anodizing”, and I found out why. It was clear that the anodizing clearly made the surface extra hard. I still need to go over this panel and remove any remaining imperfections before redoing the anodizing. I have a few more panels that I will need to polish out, before taking the batch back to the anodizer for the redo.

New (old) Keyboard Design

Saturday, May 20th, 2017
Keyboard Layout

Keyboard Layout

Here is the current state of my new parallel ASCII keyboard layout. It is setup to use Cherry key switches and the encoder is a 40 pin AVR micro controller. I plan on making the encoder/controller key mapping configurable through the keyboard itself. It should work nicely as an Apple II/IIplus replacement keyboard, an Apple 1 keyboard, or a more generic ASCII parallel keyboard. The strobe will be configurable as a pulse with configurable duration, requiring an ack, or simply follows state of keypresses. The later mode will only keep strobe active and data output valid while a key is pressed. Like Wendell Sander’s design, the power light doubles as a caps lock key. Note that I plan on making the power/caps lock light configurable to indicate general power/health or caps lock status.

Due to making configuration through keypresses, most of the solder jumper locations will go away, but I might need to leave the reset option in the layout. I still have a lot of work to do around the micro-controller section plus a lot of design verification. I also intend to retro-ize the traces to make them look more like a hand laid layout that could have been done back in the 70’s.

In keeping with the original Datanetics rev D., upon which this is based, I may leave out the silk screen layer on the final PCB.

New SCELBI 8B Video

Sunday, May 14th, 2017

The following link will lead you to a video showing off many of the components of a SCELBI 8B with audio tape, keyboard and oscilloscope interface. It starts with a description of each individual module. Then the components are all “hooked up”. The systems is powered up and some of the capabilities of the SCELBI system are demonstrated.

http://www.willegal.net/SCELBI-8B.mov

VCF east wrap up and Oscope improvement

Sunday, April 9th, 2017

I think my SCELBI 8B with oscilloscope display, cassette and keyboard went over pretty well at VCF east. I did have some issues with stability of the oscilloscope display. It was very sensitive to positioning of the cables and the scope output would sometimes get flakey if you just touched the cables.

One thing that has concerned me from the beginning of the scope project, is the nature of the grounding of the different power supplies, especially the the +/- 18 volt supply for the scope op-amps. As the data cables from the main chassis include a ground wire, I knew I had potential for a ground loop problem. I think that is what was troubling me at VCF. This week, I disconnected the digital ground inputs that were coming in with the data lines. I was using them as inputs on half the strobe input gates as mentioned in a previous post. Instead I connected the inputs of those gates to the local ground which means that ground now only has a single path back to the main +5/-9 supply. It seems to have helped. I need to do some more testing to be sure the problem is totally solved, but so far, this solution looks promising.

Spoiler Alert – Picture of the SCELBI Hardware Setup for VCF east

Sunday, February 26th, 2017

I finally have it all working together as a system.

8B Scope Setup

8B Scope Setup

To go along with the hardware setup, I’ll be putting together some graphic displays explaining what the SCELBI was, the hardware architecture and why it was significant. I’ll also have some additional HW showing internal construction of the peripherals and the main chassis.