Each SCELBI board is protected from over voltage and over current with a fuse in series with a zener diode. For 5 volt supplies I use 1N5341 6.2 volt zener connected between the 5 volt supply and ground. For -9 volt supplies I use a 1N5349 12 volt zener connected between the -9 volt rail and ground. When the voltages are at normal values, the zeners do not conduct and remain inactive, not affecting the circuit. If the voltage exceeds 6.2 volts (on the 5 volt supply) or -12 volts (on the -9 volt supply), the zeners will start conducting, acting something like a short. This should prevent the voltage from rising any further above the switch on value of the zener, preventing damage to the circuitry. Also, if there is enough current, the fuse in series with the zener should blow, removing power altogether and saving the circuitry.

The main concern with this sort of circuit is that the zener cannot conduct infinite current for an infinite amount of time, before the zener itself acts like a fuse and blows. Once the zener blows, there is nothing to prevent the voltage from rising and destroying other components on the card. This is one reason that the SCELBI has different fuse ratings on different cards. The sooner that the fuse blows, the less likely that the zener will fail, first.

The 1N53XX diodes that I use on my reproduction SCELBI are not the same devices that were originally used. Though the 1N53XX devices have pretty good current handling capabilities, I was never quite positive that they would outlast the fuse in the case of an overvoltage event.

Last week, I decided to do a simple test in order to convince myself that these zener diodes would do the job in the case of a problem. The test I put together was quite simple. Simply connect a lab power supply to a fuse and a 1N5341 zener in series. Then crank up the voltage and amperage on the power supply and see if the fuse or the zener failed first. If the fuse went first, I knew the solution would be fine.

I started with a 1 AMP fuse. Adjusting the voltage to something greater than 6.2 volts and turning the amperage up to 1.5 AMPs resulted in a fuse that blew right away. Then I connected a 3 AMP fuse to see how the zener would fare under extended load. I didn`t leave the power supply on for too long, but the zener seemed to survive short tests without a problem. Finally, I turned the power up to 5 AMPs to see what would happen. After a few 10 second tests neither the zener nor the 3 AMP fuse blew. Since I didn`t really want to destroy the zener, I called the test a success.

The 1 AMP or smaller fuses used in the SCELBI would certainly blow before the zeners that I am using. As long as you use the correct fast blow fuses, I am quite comfortable recommending the 1N53XX series zener diodes for over voltage protection on the SCELBI cards.

When checking out a new SCELBI 8B, make sure that there is a 8008 installed on the CPU board. Things will go a lot smoother. 🙂

The SCELBI uses an 11 pin Amphenol connector for output ports. The hardware manuals specify:

That leaves pin 10 as “open”. When I first built the SCELBI-8H reproduction, there was some speculation about possibly using 10 pin for +5 volts, instead of leaving it open. At the time, I thought this probably wasn’t a great idea, because of possible losses over the cables and connectors. However, the same concern really also applies to the ground wire, which is connected.

During research into the SCELBI peripherals, I discovered that the CPU SYNC signal is used on the Oscilliscope interface and the Cassette interface. We have some evidence that at least one SCELBI chassis had sync connected to some (or possibly all) output ports. Since only pin 10 is available, it makes sense to use it for SYNC for these I/O cards.

Sync is on Red Wire running from backplane to I/O ports

This is a crop of an image taken by Jack Rubin at the CHM a few years back. The red wire seems to be connected to the SYNC pin on the CPU card and runs back towards the I/O connectors.

I found a nice selection of old film capacitors for my SCELBI cassette interface build at surplussales.com. Here is an image of what I ordered.

film capacitors

I tested all of these for capacitance, all have the correct value. I did not measure ESR. The most interesting of these are the TRW caps that came individually packaged in brown paper.

TRW capacitor

The West-caps are oval shaped, not perfect for the SCELBI cassette interface, but they will do, at least for now. I bought two versions of the .0068uF size, and will probably use the Mepco versions, since they are closer to the original yellow caps, than the TRW versions. My selection criteria, was to try to find something rated around 100V, with axial leads, and less then 1″ long.

I also found a great deal on Amphenol 78S11 type sockets at the same place. Since starting on this project, this deal, is the best price I’ve seen on this style socket. I bought enough to complete all the peripheral chassis that I plan to build. Here is my surplussales invoice, with parts numbers.

Capacitor Invoice

I have two more parts orders due to arrive this week, at which point I should be able to complete my 8B and start on the cassette interface.

I’ve been working on getting set to print some SCELBI 8B front panels – this decal was part of a trial.

ME 1702/A programmer by Martin Eberhard

I’ll be taking a shot at a SCELBI 8B front panel later today.

Here is a link to the “C” language source for my 8008 debugger/disassembler. Though it has been a while since the code diverged, the core 8008 instruction emulator is essentially the same as used in my Macintosh OS/X app. Some of the output could be cleaned up a bit and I want to add an emulated cassette interface, but I think it has been pretty useful in creating and debugging 8008 software.

It takes 1 or 2 arguments, an Intel HEX file with the program to debug and an optional starting address (in decimal).

Here is a list of commands.

Thanks to Cameron Cooper helped me with the initial conversion of the step display to include instruction nmumonics.

Things to do – add support for emulated SCELBI cassette interface.

Though I have complete description of the Cassette Interface PCBs, hooking those boards up to the main SCELBI chassis requires more research. One key to understanding this hookup, is understanding details of the software. The MEA software was written to work with some specific connections between the SCELBI I/O ports and cassette interface cards. With changes to the software, this could be done in a number off different ways. Since I want to run standard MEA software, I need to hook up the hardware to match the original configuration.

The cassette interface software driver resides in page 77 of a SCELBI set up with the MEA software. I have found two sources of this MEA cassette interface software driver. First is the hex file for page 77 of MEA that Cameron Cooper and Mark Arnold reconstructed. This file can be downloaded from scelbi.com. The other source is SCELBI Newsletter #3, which also can be found on scelbi.com. In order to figure out what the relationship between these two images is, I took the hex file of page 77 and disassembled it and then compared to the listing in the newsletter. It turns out that the two drivers appear to be identical. Since the newsletter listing, includes comments, it greatly helped the understanding of this driver.

While I’m not completely done investigating, I have been able to create a lightly commented source listing that can be assembled with my version of the AS8 assembler. Once I’m done investigating and have a cassette interface hooked up and working, I’ll be sure to post the description of the connections.

SCELBI Cassette Interface Source

SCELBI Cassette Inteface Assembly Listing

Stay tuned for an upcoming blog report, where I’ll describe what I did to create an 8008 disassembler, as well as post the source for it.

Here is my first pass at BOM (bill of materials) for the SCELBI cassette interface.

Initial Cassette Interface BOM

This is taken from the schematics, with some reference to images and my PCB layout. I haven’t check all the resistors and capacitors to see if what is populated on the boards matches the schematics. The sum and count rows in the document compare the count of devices in the document (sum) with the number (count) that are found in images of the original circuit boards. The intention is too make sure that all devices are included in the document.

Other than the schematics, I haven’t seen any original documentation for the SCELBI cassette interface, though we do have the driver for MEA and there is some articles about the cassette interface in the SCELBI newsletters (available at scelbi.com).

The most unusual device is the 72741 opamp. This is a classic 741 opamp in a 14 pin DIP package. I have been able to find a few on eBay. If you want to use Ohmite carbon composition resistors, much to my surprise, it appears that Mouser has everyone of these values in stock.

I’ll have to research the capacitors a bit more, but I’m sure that something that will function, can be found for each of these values.

Since I haven’t built one and haven’t verified all the discrete components by examination of an original card, there could be some problems with this first pass BOM. In particular, I don’t know what specific transistors and diodes were used. Based on usage in the circuit, I expect that pretty generic versions will do the job, so that is what I listed in the BOM.

The last part that may cause you to scratch your head, is the DIP relay. In the past, I found a matching part is still made, but I failed to record the manufacturer and will have to dig around to locate it again.

Projects are starting to back up on my bench.

Assembly Line

I’ve never been that excited about repetitive work, but I have a few things to catch up on, before moving on to the “new” stuff I have on the backlog.

Right now building the 2nd set of “core” boards is in progress. When I first built the 8H, I bought enough parts for 2 sets of “core” boards. However some of those parts got used up in other projects. I’m building with what I have and when I get everything put together as far as I can, I’ll order whatever I have run short of. At the same time, I’ll put together a BOM for those “new” boards in the background and get what I need for those. Postage costs will really kill you, if you are constantly ordering 1 or 2 parts at a time. That is something I try to avoid, though often unsuccessfully. Here are a couple of facts that will give you an idea of how many parts that I have gone through, while building two SCELBIs.

If you like to build complex gadgets, the SCELBI’s are amazing projects. The best part is that they are relatively well engineered, so if you work carefully, once completed, they usually work. I can’t think of any project that has given me more pleasure. Even, the repetitive work of making the second set of “core” boards has been fun, at least once I found the gumption to start on them.