SCELBI 8B I/O Port Wiring Progress

August 6th, 2015

There are 80 wires to the eight output ports on a SCELBI, and 54 wires to the six input ports. All wires must be cut to length, stripped and soldered by hand. This wiring is, by far, the most tedious and time consuming single task involved in building a reproduction SCELBI. When I wired the I/O ports on my SCELBI 8H, I did it in one marathon session that took almost an entire day. For my 8B, I’m not in so much of a hurry and so far, have had 3 shorter sessions and have 56 of the 134 wires connected. The connected wires are all the ground wires for both input and output. I also have 6 bits out of the 8 bits of the output port bussed together, but not connected to the backplane. For connecting the output data bus, I found I could cut and strip a batch of equal length wires, then solder them between connectors.

SCELBI 8B I/O Port Wiring in Progress

SCELBI 8B I/O Port Wiring in Progress

The blue painters tape is there to protect the edges of the enclosure while the system is being constructed. I think another three or four sessions and I’ll finally have the wiring task done.

SCELBI Over Current/Over Voltage Protection

August 4th, 2015

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.

Reminder to Self

August 2nd, 2015

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. :-)

SCELBI Output Port Wiring

July 31st, 2015

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

  • pin 11 is connected to ground
  • 1-8 are connected to the output data bus
  • pin 9 is connected to the output strobe
  • 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

    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.

    Film Capacitors for the SCELBI Cassette Card

    July 28th, 2015

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

    film capacitors

    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

    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

    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.

    ME1702 programmer artwork

    July 25th, 2015

    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

    ME 1702/A programmer by Martin Eberhard

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

    Apple Service Sucks

    July 15th, 2015

    I have had two negative experiences.

    A while back, a Powerbook trackpad failed while still under service contract and Apple didn’t want to fix it because the service person claimed something was spilled on it. Though the machine was used by other family members, I know of no actual spill. They wanted something like $900 to repair it. I ended up getting an aftermarket trackpad off of ebay and fixing it myself for $40. So much for the extended warentee. I must admit that a service manager called me after I sent negative response to their after service questionnaire. He offered to have another look at it, but by then, I decided I could handle it, myself.

    My iMac hard drive was recently recalled due to a high failure rate. I made an appointment and took it in to their store. Result – after waiting around for a while, someone checked out my machine and made sure that it operatated correctly, found that they had the replacement drive in stock, then said that they would have it ready in 3 to 5 days. I asked if they were going to migrate the data. The answer was no, it was going to take so long because changing hard-drives was a “delicate” operation. 5 days to change a hard drive, without migrating data – Apple, give me a break – you can do better than that.

    While waiting for help on the iMac hard drive, a woman came in complaining about intermittent WI-FI connectivity with her iPhone. I can’t imagine the Apple “genius” solving that problem.

    “C” language 8008 emulator, debugger, disassembler source posted

    July 4th, 2015

    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.

  • b – set break point
  • d – dump memory
  • r – run – note that unless you have a break point set, or the program encounters a halt instruction, there is no way to halt the program
  • s – step
  • n – next – steps over calls
  • h – display numeric values in hexidecimal (except for disassembly)
  • o – display values in octal
  • u – disassemble range of memory – use more readable format
  • a – disassemble range of memory – use format compatible with AS8 assembler
  • x – exit to shell
  • 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.

    SCELBI Cassette Interface Software

    July 2nd, 2015

    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 The other source is SCELBI Newsletter #3, which also can be found on 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.

    First Pass at Cassette Interface BOM

    June 28th, 2015

    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

    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.