SCELBI Demonstrated at VCFse

I figured I would report on my SCELBI exhibit operations at VCF SE 1.0. I will also cross post this to the new yahoo group, retro-restore.

Saturday was dedicated to running the SCELBI calculator program. Toggling in the bootloader before exhibit opening, went without major issues, though I found that focus on toggling was a little hard to keep with numerous other people around getting the facility ready and getting their exhibits ready. No significant problems with downloading and pretty soon I had the program running.

I had left the divide by zero error handler to just halt the CPU. This turned out to be a good way to demonstrate front panel operations, as I just needed to divide by zero in order to halt the cpu. I would then need to toggle in a jump instruction in order to get the program running again. I did have a few issues during the day when the program appeared to stop accepting input. I think the main reason for this, is that the parser is rather particular, expecting only certain characters during input at any given time. Other characters are ignored, making it seem like the program or machine is stuck, when, in fact, the operator is typing the wrong thing. Anyway, the program ran all day, without any need to reload it, until I took it down to demonstrate during my talk.

The second day, I was planning on demonstrating Mark Arnold’s 1K Mandelbrot program. I arrived just 20 minutes before exhibit opening and because I was also demonstrating a Mimeo and had to set that up, and didn’t have time to toggle in the bootloader before opening. I had to do it, while showing off my exhibit. The Mandlebrot program needs to use location zero, so I had to relocate the bootloader. Last time, I did this, I had no issues, but this time my math skills failed me and I spent a considerable amount of time trying to make the Mandlebrot program load right on top of the bootloader. Finally after realizing the error of my ways, I retoggled the bootloader into a better spot and got the Mandlebrot program loaded and running.
If I had been thinking straight, I could have loaded the bootloader into the new location, with the bootloader that was loaded in the wrong place. The SCELBIgenerated Mandlebrot images for the rest of the day, without error.

The bottom line is that the SCELBI ran all weekend, the clock remained stable enough to support RS232 bit-banged I/O without error. The power supply worked great.

A lot of people that had never heard of it, were exposed to the SCELBI for the first time. And finally, I overheard one person say that the reproduction SCELBI was the coolest thing at the show.

Things I would change. I would probably run the Mandlebrot on Saturday, when things were busiest. I also need to find a teletype or current loop terminal,because some people were confused by the Powerbook running a terminal program acting as the terminal.

It was wonderful meeting everyone who came to the show and if you didn’t come I recommend you make an effort to attend a VCF, if you get a chance.

Tragedy in Boston

Too close to home. Yesterdays event occurred in a place I know fairly well and at an event that is a unique part of the culture around here.

I hope that the survivors physically heal quickly and find a way to emotionally come to terms with their horrible experience, sooner, rather than later.

SCELBI Clock – Component Changes for Better Stability

As I noted in a previous post, clock stability on my reproduction SCELBI was pretty bad. The system clock slowed down 10 or 15 percent as the system warmed up. This would have played havoc with my bit banged serial communications, but by the time I was able to toggle in the loader, it would be warmed up and fairly stable.

SCELBI cpu clocks

SCELBI cpu clocks

The timing circuit is made up of 4 interconnected 74121s one shots that control rising and falling edges of 2 interleaved clocks. The timing of each 74121 is controlled by a 5K trimmer resistor and a 300 pF cap. Figuring that the cap was part of the problem, I looked for some NP0 300 pf caps to replace the ones that i was using.

What I found was a modern 330 pF cap, Kemet part # C322C331J2G5TA. While I was at it, I decided to replace the 5K trimmers that I had been using since first bringing up the SCELBI. The original trimmers I bought had the wrong foot print and turned out to be a little jumpy while adjusting. I replaced them with Bournes # 3006P-1-502LF.

Though I don’t know which change made the difference, several trials have shown the new trimmers and NP0 caps seem to have stabilized the clock to the point, where I think I can forget about the “problem”.

The only thing I might do at some point, is try to find some old style NP0 caps to replace the obviously modern Kemet ones.

Updated Apple 1 Reproduction Flyer with Original Image from Ad

I have permission from Wendell Sander to publish a high res scan of an original print of an Apple 1. We believe that this is either the exact image used in the original ad or at least from the same photo shoot. We are so lucky that Wendell saved this very detailed print all these years, and is so willing to share it with all of us. I have updated the flyer and included this scan, which pretty much makes this reproduction about as good as I can make it, at least in available time. I left the image uncompressed, in case you want to blow it up to poster size.

Ad with high res image

enjoy…

CFFA 1 back in stock

It appears that Rich Dreher have made another batch of CFFA 1’s available.

http://dreher.net/print.php?s=projects/CFforApple1

I have tested these in the Mimeo and they work great. If you are tired of dealing with the cassette interface or a serial port (on PS/2 adapter), these are great way to load programs into your Mimeo.

The CFFA 1 also adds in extra memory, so you can run the some of the larger Apple 1 programs that are available from Wendell Sanders web site.

Do note, that in order to preload the compact flash with Apple 1 software on a PC, I think you need to use a Windows program called Ciderpress. I don’t know of a Macintosh equivalent. You can always load the program on the Apple 1, using my PS/2 adapter’s serial port, and then save to the CFFA 1. CFFA 1’s use the Apple 1’s slot, so unless you have a slot expander, you can’t use both an ACI and CFFA 1.