My OS/X Macintosh emulator now supports reading and writing SCELBI compatible AIFF audio files, but I have to do some major work on the associated TTY terminal emulator window, so that I can successfully run the MEA editor in emulation.

A while back, I had introduced 8008 cycle counting and timing on the OS/X version of the emulator, so that I could support SCELBI front panel operations and run it at prototypical speed. This made implementation of the tape read fairly straight forward in that environment. I recently added a configurable memory menu option to switch between an 8H and an 8B configuration and have a built in MEA PROM image that can be enabled, as well.

SCELBI OS/X Emulator Reading a “tape”

Once I clean up the emulated tape interface a little, I’ll make a new version available for download. The editor part of MEA will still have problems until I fix the TTY terminal behavior, but there are a lot of other improvements in this version that may interest people.

Once I get the TTY emulation fixed, the OS/X emulator will have practically all the functionality of a real SCELBI 8B or 8H with TTY and audio tape interfaces.

I’ll look into what it would take to add it to Apple’s app store. I also might work on converting it into a IOS app for iPhone or iPad at some point. I did some experiments with IOS a while back and had basic 8008 instruction execution working, so I know a little bit about what it would take to do that port.

TTY Chassis Front

SCELBI TTY Chassis Back

TTY Chassis

Inside TTY Chassis

Here are a couple of pictures of a rare original SCELBI factory TTY chassis. Can anyone find what’s wrong? Hint – the schematics can be found at scelbi.com.

This program turns SCELBI 8008 code and data that is in Intel hex format into AIFF format sound files that is readable by the SCELBI cassette interface when played out of a computer or digital music player earphone or headphone out port. Note that the SCELBI only will read continuous data using the cassette read command. At this time, the program does not fill in gaps, so the Intel hex format file must also be continuous.

The program is a work in progress, and there is some ugliness in the commenting, coding style, etc, but it works well enough, that I figured I’d share it, rather than wait to find the time to perfect it. I put the source in the retro-restore newsgroup files section under 8008 Systems/SCELBI. Be aware that you need to download an AIFF “C” language library from source forge to build it. I had to make a change to the library source to make it work for this application. In function AIFF_ReadOpen(const char *file, int flags) add this code to the end of the function to initialize buffer2 pointer and length.
r->buffer2 = NULL;
r->buflen2 = 0;

Since it doesn’t divide perfectly into normal audio file sample rates, like the Apple II sound format does, the SCELBI’s FSK encoding took considerably more thought to reproduce, which made this an interesting little project.

It should be possible to use the program as a starting point for other audio format conversions that use FSK or similar encoding methods. See my previous program that converts from Audio to Intel hex format.

Height is obscured by the ruler, but it is 3 1/2″. I can’t recall where I read about it, but I think the lettering on SCELBI enclosures is rub-on Letteraset or equivalent. It was clear coated after being applied.

Here are some pictures of the binding posts.

Binding Post Front

This picture also shows how the opposite pairs of pins on the edge connector are bent and soldered together. This can be seen toward the right of picture where a couple of wires connect to the edge connector.

I have images of the cassette interface unit at the CHM and the wiring was done with two color purple and red wire, so the tech must have used whatever he had available. The edge connectors on that CHM unit are blue and marked 67040 over 6748. It’s hard to tell from the images that I have, but I think those CHM connectors had a single row readout. The overall form of the connector design in the CHM unit seems to be a fairly close to match these AMP units. However, the color is wrong and the wire wrap lead length of .75″ is likely to be too long.

Many SCELBI ethusiasts have found the image of the Audio Cassette Interface in the Computer History Museum’s catalog page. I recently had a chance to check out an extremely rare original example and took the opportunity to take some pictures and measurements. Here is how the front of the chassis is set up. Wiring follows the schematic. Note that this picture is actually taken upside down, as the cards are mounted to the top half of the enclosure.

Inside Cassette Interface

Though the mechanical design is simple, building this chassis clearly involved a lot of manual labor. I wonder what would have happened if the SCELBI had started to take off like the Altair did. I think that Nat and Bob were very lucky to be able to move the company so successfully into publishing. The SCELBI Computer clearly would have required a complete redesign in order to compete with the coming wave of inexpensive, mass market, micro-computers.

From this upside down perspective, the read card is mounted facing up, in the upper slot, and the write card, in the lower slot. There are holes in the bottom of the enclosure for viewing the LED on the read board and a hole in the rear for adjusting the POT, also on the read board.

The binding posts for power and ground are very nice examples with heavy gold plating. Unfortunately, I couldn’t find any manufacturers markings on them.

What is not so good, is that the horizontally mounted cards are not supported by anything other than the edge connectors. This means that rough handling could easily cause damage or connectivity issues, particularly with the rather long cassette cards. Many systems, such as the wildly successful Apple II, used non-supported peripheral cards in edge connectors, but the cards are usually mounted vertically so that gravity will help hold the cards in the slots.

It’s not visible in this view, but the pins from the top and bottom rows of each edge connector are soldered together in each position where there is a wire connected to the edge connector. I’m not sure why this was done, as no signal lines were connected to the top side of the edge connectors on the cards. I also got to look at a TTY enclosure. That enclosure had a different type of connector with single row of pins installed.

When I get a chance, I’ll be posting more images and make up drawings with measurements.

Bud Industries CU-3009 A enclosures seem to be an exact match for the enclosures used to build the originals. I hope to find time in the coming weeks to post details on external and internal modifications needed to turn the enclosure into an accurate reproduction SCELBI peripheral chassis.

I’ve also been working on adding cassette support to my OS/X SCELBI emulator application. The other day, I was able to read an SCELBI formated AIFF file for the first time using the cassette driver normally included with MEA. While working on this, I think I may have discovered a bug in the cassette read driver. More about that in a future blog post, once I do a bit more analysis.

Also, I discovered the diodes on the cassette read and write boards may actually be 1n4148 types, not 1n914s.

This program converts SCELBI Audio files that have been digitized into AIFF format into Intel hex files. My 8008 emulators use Intel hex files, so I know that this will be an efficient way to move files back and forth between the two worlds. It works well enough that I was able to recover a 40 year old original SCELBI tape that my reproduction SCELBI couldn’t completely handle.

The program is a work in progress, and there is some ugliness in the commenting, coding style, etc, but it works well enough, that I figured I’d share it, rather than wait to find the time to perfect it. I put the source in the retro-restore newsgroup files section under SCELBI. Be aware that you need to download an AIFF “C” language library from source forge to build it.

It should be possible to use the program as a starting point for other audio format conversions that use FSK or similar encoding methods. At some point I’ll write a program that goes the other way from Intel hex to audio, like I have done for the Apple 1 and II.

regards,
Mike Willegal

In a previous post, I described how I discovered that the SCELBI cassette write card didn’t work with a standard 7493 in position Z4 without changing the value of the capacitor located between Z13 and Z14. I went ahead and ordered some 7493A parts, which arrived yesterday. Last night, I installed one in position Z4 of the cassette write card. As expected, with the “A” version part, the SCELBI cassette write card works with the standard .02uF capacitor.

Also, I have posted a new version of the Cassette BOM V2.1.

My last update was in December of 2014 and I’ve made a bit of progress on a couple of fronts.

Documentation

I’ve done a substantial amount of work on completely OCRing the original 8B hardware manual. This is the first step of making a new manual for people interested in reproductions. I plan on going through the manual and adding all the information that I’ve accumulated that is relevant to people building reproductions. Much of this information is scattered about over several years of blog posts, emails and in some cases, web pages.

I just started work on putting together a build guide for the cassette interface boards. It will have the look and feel of the other SCELBI construction guides, but as we haven’t found original build instructions, it is new work. Though build instructions are missing, some original documentation on the cassette interface has turned up and that will be OCR’d, as well.

Mechanical

I have built reproductions of the 8H using a BUD AC-413 chassis and the 8B using a custom chassis that Corey had fabricated. I still need to put together reproduction chassis for the peripherals and the power supplies. For those, I’m using off the shelf enclosures of approximate the same dimensions of each type of original enclosure. At one point I was looking into making my own reproduction enclosures, but I haven’t found a good deal on a box and pan brake that is sufficiently capable to handle these jobs, so that idea is on indefinite hold. Screen printing of the 8B front panels should happen in the next week or so.

PCBs

My estimate of getting the cassette boards done in the first half of 2015 was off by a few months, but it did happen in 2015. I do feel a bit bad that the people working on reproduction 8B projects had to wait so long for those cards.

My estimate of getting keyboard and oscilloscope interfaces done in 2015 is not likely to happen, though I expect the fairly simple keyboard card should be done in early 2016 and I hope the oscilloscope cards are done by end of 2016. Both of these cards will take some software work, as no one has come up with original drivers for either.

In all, there were 16 different PCBs made by SCELBI for the 8H and 8B computers and associated peripherals. For quite a while, I’ve dreamed of having reproduced the entire set. It’s hard to believe, but at this point, I’m only three PCBs away from having completed that task. For those that are into numbers, that means that I’m over 81% of the way there!

The current status of my reproduction efforts of each of those cards follows.

Main System Cards – working reproductions of all 10 cards types have been completed.

Reproduced
1100 CPU – 8H/8B
1101 Data bus buffer – 8H/8B
1102 Input – 8H/8B
1103 Backplane – 8H
1104 Front Panel – 8H/8B
1105 1K SRAM – 8H
1106 Memory Expansion – 8B
1107 4K SRAM – 8B
1108 Backplane – 8B
1109 PROM – 8B

Peripheral Cards – 3 reproduced, 1 in progress, 2 haven’t been started.

Reproduced
2102 Audio Tape output
2103 Audio Tape read
2104 Teletype interface*

In progress
2105 Keyboard

Not started
2100 Oscilloscope digital**
2101 Oscilloscope analog

* not as high as normal vector boards
** double width vector board

I just put a video demo of the SCELBI 8B with MEA and SCELBAL. Like usual, with my videos, the quality is substandard, but at least you’ll get to see a glimpse what I have working. The truth is creating quality videos is extremely time consuming and I’d rather spend my spare time (and money) on the vintage computers, themselves, rather than movies about them.

podcast feed is on iTunes store under “Mike’s Hobby Pages”
or direct feed at http://www.willegal.net/feed.xml
or direct link to movie at http://www.willegal.net/podcast09-18-15.mov