Archive for the ‘Vintage Computing’ Category

SCELBI Oscilloscope Analog Board Components

Sunday, January 17th, 2016
SCELBI Oscilloscope Analog Board

SCELBI Oscilloscope Analog Board

Here is an image of a SCELBI analog board. Overlaid on top is my current front copper layout and a silk screen layer showing component mix. As you can see, the layout is very far along, considering that I’ve only worked on it for parts of two weekends. There is still plenty of tweaking and checking to be done, but there is good chance that the layout, as it stands, would work.

Note that this board has several component locations without components stuffed and a couple of 56K resistors tacked on. There are no pads for these resistors, but they show up in the schematics and on the SCELBI placement diagram. I wonder if there was a second batch of cards made with corrections?

The 130 ohm resistors are listed as 100 ohms in the schematic. It would be hard to know why there was a difference without doing some experiments on actual hardware or a circuit analysis. This change could be functional in nature or do to using components that were close enough and on hand. The 3K resistors are listed as 3.3K in the schematics, but I think they used what was on hand, and there probably isn’t a functional difference in that case.

The pads in the lower left are for decoupling capacitors. I don’t know why they weren’t stuffed in this example. The component mix is pretty basic. There appear to be some film and mica capacitors. Based on what I’ve seen on other SCELBI boards, the transistors are probably 2n2222 and 2n2907 types, but I don’t have confirmation on that. Probably the most expensive parts to source are those pesky 72741 op-amps in 14 pin packages. They seem to be available on eBay, but the asking price is very high. I’m going to be on the lookout for a better source.

One final thing that threw me for a loop on this board. When compared to all other SCELBI boards, the DIP packages are mounted upside down. I’m going to have to pay attention when assembling this board.

The Great American Prob. Machine

Thursday, January 14th, 2016

A few days ago, I was searching through my small personal archive of Apple II floppy disk images, looking for an Apple II clone of Colosal Cave. As fate would have it, among those images, I ran across a program called “THEGREATAMERICANPROB.MACHINE”. Vaguely recalling it being a fun program from the “old” days, I started up an Apple II emulator, loaded the DSK image and ran the program. Here is a screen capture



and a movie of the program in action.


Just as I sort of remembered, it was a pretty cute Apple II low res color graphic animation. However, when the credits rolled, I discovered something totally unexpected. It was written by none other than Bruce Tognazzini. Tog, as he is now known, is now a respected authority on user interface design.

Having had a brief email interaction with Tog a few years ago, and knowing he was quite approachable, I decided to send him a message. I let him know that this old work wasn’t forgotten. I figured that he would appreciate that. The response I recieved, had some quite unexpected news. Here is Tog’s reply.

Thanks. It was when Steve Jobs saw that program that he decided to hire me. It was the first ever full-screen animation done on the Apple II.

I sent him another message thanking him for his reply and asking him if he minded if I shared it on my blog. He responded with more details about how “THEGREATAMERICANPROBMACHINE” helped get him hired at Apple.

Not a problem. Specifically, I took a piece of code I’d written that added a new command to Integer BASIC down to Apple to show Steve. After selling him on that, he asked me what else I had done. I showed him the Probability Machine, throwing it up on Apple’s large-screen Advent projector. He got really excited, left the room, and gathered up everyone he could find in the building. (Apple was still all in a single building in those days.) I was quite surprised it caused so much excitement; I had no idea I’d pulled off something that, at the time, was a breakthrough. A week or so later, Jef Raskin called me up and said that Steve thought we should talk. I assumed he was going to want to buy some more of my software. It turned out, he and Steve wanted me,


Initial Look, SCELBI O’Scope Board and Keyboard PCB Update

Monday, January 11th, 2016

Over the weekend, I started work on the SCELBI O’Scope PCB layout. There are two boards that make up the package, a standard size analog board, and a double width digital board. The standard size analog board looks like it will be pretty easy to layout. However the double width digital board looks pretty complicated. It has 35 chips, and around 220 VIAs. By comparison, the Apple II, rev 0, which has to be the most complex PCB that I have recreated has about 380 VIAs. The chip count is the second most for any SCELBI board. It trails behind the 4K SRAM board, which has 37, 32 of which are SRAM chips. All the SRAM chips on a SRAM board are connected the same way, which simplified things quite a bit on that board.

CHM SCELBI O'Scope Interface

Image of the Inside of the O’Scope Interface at the Computer History Museum (courtesy Jack Rubin)

The good news is that, so far, I haven’t run across any components that will be excessively difficult to find.

Another complication on the digital board, is that I only have three good images of two different O’Scope boards. The front and back of one board, and the front view of a second. The board that I have front and back views of, has quite a bit of rework on it. I don’t know if these are design issues or customer modifications. It will be interesting to sort through these changes and see if there are any differences between these two boards.

The final challenge will be figuring out the details of the power supply that is integrated into the O’Scope chassis. At this point, all that I have to go on, is a picture. When you put everything together, the SCELBI O’Scope interface turns out to be far more complicated than many single board “trainer” type computers.

The keyboard interface layout is complete. I have done design checks and have a quote in hand. I probably will not order boards until the O’Scope layouts come together, or at least until they get further along. I think of the keyboard and O’Scope boards as kind of a matched set, the keyboard for input, and the O’Scope for display output. One, without the other is kind of a half solution, though I will be able to test them out independantly.

New Version of my OS/X 8008/Scelbi Application Available

Thursday, January 7th, 2016

I think this version has some significant improvements that bring operation, look and feel to a new level. Besides incorporating some support for teletype, front panel and cassette “sounds”, a help system has been added that should provide enough support for even a novice to be able to boot a SCELBI to the MEA monitor prompt.

Check it out at

If you like what you see, let me know. If you have ideas for more improvements or discover issues, input is always welcome.

Lunar Lander for SCELBI and is there a “Solution”?

Sunday, January 3rd, 2016

While putting the finishing touches on a new version of my OS/X SCELBI app, I spent some time porting over the classic Lunar Lander game program to SCELBAL. I started with the version from a 1978 edition of “BASIC Computer Games” by David H. Ahl. I bought my edition of that book back in 1978, along with my Apple II.

BASIC Computer Games

BASIC Computer Games

I remember trying to type in a game from this book, the first night that I had the Apple II, only to almost immediately run out of memory. I initially purchased a 4K Apple II system, and only about 2K was usable by BASIC, so it didn’t take much to exceed it’s memory capacity.

Anyway, back to the present. I found porting Lander to SCELBAL was pretty easy. I had two issues.

  • First was an issue with the calculations exceeding the 6 digits of precision supported by SCELBI floating point, which caused a math error. This was fixed by changing the compare instruction at line 340 to IF S <5E-1 THEN 260 instead of S<5E-3 then 260, which reduced the amount of precision required.
  • The second issue is that SCELBI doesn’t support multiple statements on one line, which was easily rectified.
  • While working on this, I wondered if there was a single number solution for Lunar Lander. Using a simple binary search mechanism and multiple copies of the SCELBI app, I was able to determine that with 6 digits of precision allowed by SCELBAL that there isn’t a single number that can be entered every time and result in a safe landing. The closest I could get is a burn rate of 76.4386, which results in a crater only 33.83179 feet deep. Entering a number of 76.4387 results in an out of fuel situation, as the lander just misses landing before proceeding to accelerate upward after which it runs out of fuel and then comes crashing back down.

    As far as releasing an updated SCELBI app, it is in pretty good shape and I should be releasing it within the next week. I’ve taken advantage of a holiday break from work in order to make significant improvements and revisions and I think this version will be a vast improvement over the last release.

    Oh and here is a link to the ported version of Lunar Lander.

    New Version 2.2 of SCELBI OS/X Emulator Posted

    Saturday, December 5th, 2015

    I’m really excited to be able to release version 2.2 because of it’s capabilities. After several years of on and off effort, this version is good enough to completely support all the features of MEA, SCELBI’s 8008 software development environment.

    You should be able to develop and test 8008 applications without ever leaving this application. In addition, the audio files it reads and writes are completely compatible with the actual SCELBI cassette tape interface hardware. This means that you can transfer object and source files back and forth to real SCELBI hardware with ease.

    Read more about it on this webpage

    The MEA manual can be downloaded from

    Instructions for operating the front panel, which is necessary to boot MEA can also be found at

    To start MEA, you must toggle a JUMP to location 060-000 with the front panel.

    SCELBI 1702 EPROM Contents Troubleshooting

    Sunday, November 22nd, 2015

    I’ve recently encountered some issues assembling the small program that I use for checking out MEA on my SCELBI 8B. 1702 EPROMs are notorious for “forgetting” data and my first thought was to verify the contents of those EPROMs.

    Since the cassette write function still worked, it turns out that the verification process is pretty easy. Here is what I did.

    1) I captured the contents of the MEA proms (060-000 to 077-377) to an audio AIFF file on my PC by connecting the audio output of the SCELBI to the mic in port of the PC. This assumes that the MEA cassette driver/utility to write the data out still works.
    2) I converted the audio file to an Intel hex file using the utility that I had previously written.
    3) I used an UNIX shell program named “diff” to compare this new INTEL hex file with the one that can be found at

    What I found, was a couple of errors in the EPROM at page 73. I removed this EPROM from the PROM board, reprogrammed and verified it. After reinserting the EPROM and board, assembly of my little test program, once again, worked as expected.

    As far as part 3 goes, I actually did a couple of additional verification steps by loading my OS/X SCELBI emulator with the Intel hex image converted from audio tape format and verifying that assembly of my test program also failed in that environment. I then dumped contents of suspicious memory locations with my emulator loaded with a good version of MEA and compared to dumps executed on the real SCELBI to confirm that those locations really had problems. Those extra steps aren’t strictly necessary, but helped confirm the issue, before I started the process of pulling boards and chips.

    Note that I didn’t erase or use a new EPROM in this repair, but simply overwrote and verified contents of the existing EPROM with bad data. It will be interesting to see if these locations act up again in the future.

    The whole repair process, including the extra verification steps and updating my ME1702 programmer with new firmware, probably took an hour or so. This was one of the easier vintage repairs that I’ve experienced, at least in recent memory.

    SCELBI Keyboard Interface Design Notes

    Saturday, November 21st, 2015

    First of all, much to my surprise, the layout for a reproduction SCELBI keyboard interface is nearly complete, well ahead of schedule. I’m not sure when I’ll pull the trigger on getting boards made, as I think it really needs to be used in conjunction with the oscilloscope interface or something similar.

    Using the interface with many vintage keyboards may present a bit of a problem, as it was designed to interface with the keyboard design in the April 1974 issue of Popular Electronics magazine. The basic interface design is described in the first SCELBI Computer Digest, which can be found at Cameron Cooper’s website. The PE keyboard is unusual, in that it is ASCII decoded, but does not latch data. This means that data is only present while a key is actually pressed, or shortly thereafter (the keyboard circuit has a RC network that will hold data for a little while). This is why the SCELBI keyboard interface has latches incorporated into it.

    Since there is no latching of data on the PE keyboard, the actual latch mechanism on the SCELBI interface is triggered whenever data is actively being sent by the keyboard. This will not work with keyboards that have onboard latches, since they present data to the interface all the time. Some kind of modification with have to be made for interfacing with those kinds of keyboards. This is not a huge problem, but will require some additional thought.

    Of course, a more interesting solution would be to build a reproduction PE style keyboard to go with this interface. I couldn’t entirely rule out the possibility of taking on that project sometime in the future, but first I’m going to finish the few remaining SCELBI boards that are begging to be built.

    Cassette Tape Rub On Lettering

    Friday, November 20th, 2015
    Cassette Rub-on Lettering

    Cassette Rub-on Lettering

    Though I still have to add a protective clear coat, it looks like a successful trial. You can get your sheet by contacting at Since the film/plate is already made, the price should be $32 for a 4×6 1 color (black) sheet. You will have to tell him that this is the same sheet that Mike Willegal had made. I can’t make guarantees, but I think this sheet should have enough transfers to make a couple of complete SCELBI systems.

    After trying a couple of different items for burnishing the transfer, I ended up using a cheap ball point pen.

    New TTY Enclosure

    Sunday, November 15th, 2015
    New TTY Enclosure

    New TTY Enclosure

    After I clean the leaves off of the yard, I should be able to finish the TTY enclosure. I have been wiring the edge connectors first, labelling the wires with port and pin. After the edge connector is wired, I can put it in position, cut wires to final length and solder to the input and output ports. I think this works pretty well.

    With this enclosure, the port for the TTY device exits from the reverse end. Here is a view of bundle of wires that will be routed out that end of the enclosure.

    TTY Enclosure Reverse View

    TTY Enclosure Reverse View