Archive for the ‘8008’ Category

Problems with 8008 serial routines that posted

Saturday, March 22nd, 2014

Just a heads up – the serial transmit routines that I have posted with the 8008 applications have serious issues. Though it was working for me, other were using serial ports that weren’t so forgiving and had problems with transmit. Finally Corey Cohen tracked down problems with start and stops bits used with the transmited data. As soon as I get a chance to check out the fixed routines, I’ll be updating all the applications on my 8008 application and BASIC pages. For those of you that can’t wait here is the code for a version that I think will probably work. This version is timed for 2400 baud.

My OS/X emulator was also broken and needs an update, as well.


NDI 177 ; mask MSbit
LDA ; timing delay
LAI 200
OUT OUTPORT ; stop bit
LDI 100 ; and extra for inter character spacing

RRC ; shift to get next bit
LAA ; nop to delay 5 states to make timing work
LDI 0 ; delay 8 states

Kit and PCB status update and a question

Saturday, February 1st, 2014
  • Coming for VCF east – SWYFT card reproduction kits – special introductory price if you join my SWYFT Card soldering Workshop – $40, drop me an email to register. Part of the proceeds goes to help support MARCH. After VCF east, price goes to $50.
  • Mimeo’s – out of stock – expect to have new batch around end of February
  • Datanetics PCBs – 1 left – rerun timing TBD
  • Brain Boards – 9 left – Note that I’m going to try move the remaining Brain Board kits over the next few months. I might even list them on ebay, to clear them out. If you want one, time to get it, is now. A rerun is not likely to be in the “cards”. I have a vague idea for an enhanced firmware board that may or may not get traction someday in the future.
  • SUPERPROTO kits – 8 left – rerun TBD – these are slow sellers and I haven’t actually made any money on these. However, I find them useful for my own purposes, so I might do another run (someday).
  • ACI kits – plentiful (currently I’m running low on PROMs, but expect I’ll be able to find another batch without too much trouble)
  • PS/2 keyboard adapter kits – plentiful
  • A2 rev 0 – ample supply
  • SCELBI 8H – I have less than 10 8H board sets remaining. I’ll make extra SRAM PCBs, when needed. Front Panels are in limited supply (less than 15, I think). TTY boards are plentiful
  • Now for the question – should I call my Apple II rev 0 reproductions Mimeo IIs, even though they came first?

    SCELBI/8008 Page Update Complete, What’s Next

    Sunday, December 29th, 2013

    Finished the update of my SCELBI/8008 pages with a bunch of work on my new 8008/SCELBI applications page.

    I’m glad I have completed this SCELBI web site update, as it is something I wanted to do for a long time. Now, if I only had time to write a complete guide to building and operating the SCELBI.

    Next up… I have new Apple II rev 0 PCB‘s on hand. The PCBs look good, but I need to build one up to test, before offering the PCBs for sale. I’ve also been cleaning up and repairing a craig’s list Apple II plus purchase. Tomorrow, I hope to have the IIplus done, and perhaps start on the rev 0 soldering.

    SCELBAL Page Update

    Friday, December 27th, 2013

    Mark Arnold sent me a paragraph with the story behind SCELBAL. My original explanation was incorrect and has been replaced by Mark’s story. Check out the update.

    SCELBI/8008 Web Pages Get Update

    Wednesday, December 25th, 2013

    I still need to add a few 8008 applications I have working to the applications page.

    Rev 0 Repro Progress and Retro Workshop Update

    Saturday, November 30th, 2013

    Apple II rev 0 Progress
    My revision of my original reproduction, the Apple rev 0, is coming along. This morning, I did an extensive design review of the 35th revision, and only found 3 things that needed “fixing”. I’ll probably repeat the effort tomorrow on revision 36. Hopefully that review passes cleanly. If so, I’ll get quotes, pick a vendor and kick off board fabrication next week.

    Projects in the Lab
    Like usual I have a lot of other projects in progress. Some have been in progress for while, others not. Here is a snapshot of my workshop. It keeps getting more crowded as time goes on.

    My Retro Workshop

    My Retro Workshop

    My “LAB” is currently setup for checking out the SCELBI TTY card, which is interfaced to an Apple II serial card, which also supports 110 baud current loop. The hardware is working, but I have to do some software work on the Apple II side of the serial current loop connection in order to make it usable. The standard Apple drivers seem just a bit quirky,as they are tied into Apple II monitor functionality. I was hoping to get by with Apple’s standard PROM drivers, but it looks like I’m going to have write a custom driver for interfacing to the SCELBI. I’m hoping to get the IIe running such that it works just like a real TTY, perhaps eventually including emulating paper tape using floppy discs. I decided to use the IIe instead of a II or IIplus because it supports 80 column output without extra plug in cards.

    SCELBI Galaxy
    I was hoping to make a video of the SCELBI running tiny SCELBAL basic and the SCELBI Galaxy program, which I recently got running. The Galaxy program is SCELBI’s version of the Star Trek game. It was published as documented source code in the SCELBI book “Galaxy”. I had to OCR a scan of the book and then covert it to the AS8 assembler format that I prefer. This took a considerable effort, but I was able to exactly reproduce the original program. When I first went to download it into a physical reproduction 4K SCELBI 8H, I discovered that there wasn’t enough room in memory for a boot loader and the game. I had to spend a lot of time creating a second version the source code to make it fit into 4K with room for a boot loader, without altering the play of the game. This was particularly difficult as the originally published source, didn’t have labels attached to any of the messages, just hard coded addresses pointing into a huge block of characters. In order to move anything in memory, I had to convert the hard coded addresses to labels and add the label to original block of bytes. Oh yeh, there were some page boundary assumptions that I also had to deal with. It wasn’t easy. Once I get the Apple IIe TTY emulation going, I’ll definitely make the video of both the Galaxy game and tiny SCELBAL running on the 8H.

    SCELBI Webpage Update Coming
    I have an update to my 8008/SCELBI web pages coming. Right now, there is too much on my 8008/SCELBI page and I’ll break it down to a few smaller pages. One page that needs a lot of work is the SCELBI/8008 software page. I have managed to get a number of 8008 programs running, some new and others, like Galaxy, old programs that I believe haven’t been run in decades. I need to set up a dedicated software page in order to share them all, plus the cross development tools that I have used to develop them. Most 8008 software from the era will need to have I/O drivers tweaked in order to run in a particular environment, so I will need to publish source and tools. Some of the software, I didn’t develop, so I need to get permission from creators. I would also like to find time to covert my SCELBI blog posts, along with original documention into some kind of book form, as information about building a SCELBI is rather scattered around. I don’t know when or if I will get around to this, but I am motivated, as I think better documentation might help me to sell more SCELBI board sets.

    Apple IIplus

    Apple IIplus

    Apple IIplus

    At top of the first picture you can see the keyboard for an Apple II plus that I recently picked up as part of a Craig’s list transaction. My intention is to clean up and repair this Apple IIplus for resale. I think it will make a good first II plus system for someone, since it has a pretty late serial number along with an RFI board. My experience with those later systems with RFI boards is that they tend to be more reliable than earlier systems. In fact, except for some keyboard issues, this system came in working condition. As you can see by comparing the inside of the top with the rest of the case, the plastic hasn’t yellowed much, if at all, so it should clean up real nicely.




    Along with the IIplus, I picked up a TRS-80 system, with expansion chassis and floppy drive, along with documentation. It doesn’t work, but it should be a fun project to fix up. I can’t decide whether I will keep or flip it. Initially I was going to add it to my collection, but the engineering isn’t what I’m used to, so I’ve already somewhat soured on it. A friend of mine had a TRS-80 briefly, back in the 70′s, but took it back and got an Apple II. Having played with his TRS-80 briefly in the 70s, is what spurred my interest in having one, now. If I keep it, I’ll need to get a Commodore Pet in order to have one of each of the first machines of the “big three” computer manufacturers of the 70′s.

    I keep telling people that I’m not a serious collector, as I like to work on, learn about and operate these old machines. I don’t usually acquire vintage computers, just to own, which is a sign of a serious collector. However, as you can see from this blog post, I seem to continually find ways to increase my “backlog” of projects.

    SCELBI TTY interface revisited.

    Thursday, November 7th, 2013

    I finally found time to revisit the SCELBI TTY card. The last time I tried, I had issues getting it to work with my Apple II serial card. To start with, I have been focussed on getting the SCELBI TTY receive interface, running.

    Teletypes and the SCELBI TTY card use a current loop interface, which is a technology that I’ve long been aware of, but have had no practical experience with. Instead of using a signal level, current loop uses a current to indicate 1s or 0s. This current can be sourced from either side, but the transmitter will pass on a current of around 20 milliamps to indicate a 0 and a zero to a few milliamps to indicate a 1. The reciever’s circuit in digital equipment will typically convert the current to a voltage level for later processing by a circuit that will deserialize the incoming bit stream.

    Here is a crop of the SCELBI TTY schematic showing the RX circuit.

    SCELBI TTY Receiver Circuit

    SCELBI TTY Receiver Circuit

    The circuit is pretty simple. At the top is a 5 volt supply, that is current limited by resistor R7 going to the edge connector, pin M. This can be sent to the transmitter as the current source, or can be ignored, if the transmitter has it’s own current source. One of the challenges of using current loop technology is determining which end point sources the current. The SCELBI TTY reciever can support either sourcing the current or not. If the transmitter supports it’s own current source, there must be a ground return path between the SCELBI TTY card and the transmitter.

    In the middle, coming in from the edge connector, pin L, is in the input current source. Capacitor C3 acts as a low pass filter, to prevent switching due to incoming noise. I don’t have TTY experience, but I’m assuming that TTYs were very noisy and adding this filter improved reliability. Transistor Q2 acts as a current to level converter. Low levels of incoming current will leave the transistor turned off, allowing +5 voltage source to pull the inputs of NAND gate Z1 high through resistor R1. Higher levels of incoming current will switch on Q2, causing the inputs to Z1 to go low.

    During my previous attempt at transmitting from an Apple II serial card to the SCELBI TTY card, I was unable to receive any data. This is despite trying several connection methods.

    This time around, I first built a LTspice model of the circuit, including both the Apple II Serial card transmitter and the SCELBI TTY reciever in order to determine theoretical behavior of the circuit. Here is the final schematic that I ended up with.

    Apple Serial TX to SCELBI TTY RX

    Apple Serial TX to SCELBI TTY RX

    Since I couldn’t find a model of a 7400 NAND gate used as the output of the TTY card, I modelled it with discrete components. The connections between the boards are simple.

  • pin23 of the Serial card’s DB25, which is it’s current loop output is connected to edge connector pin L of the TTY card, which is the current loop input
  • pin 7 of the Serial card’s DB7, ground, is connected to ground of the TTY card
  • Next I setup a 1000 KHz square wave input into the serial card in order simulate the 2400 baud serial signal that I was trying to send from the Apple II to the SCELBI.

    Spice Simulation of Apple Serial to SCELBI TTY at 2000 baud

    Spice Simulation of Apple Serial to SCELBI TTY at 2000 baud (current loop)

    The result shows exactly why I was having trouble.

  • The green square wave shows the serial signal being input into the serial card
  • The blue signal shows the current loop signal being sent from the serial card to the TTY card
  • The red signal shows the output of the TTY card.
  • Once I ran this simulation, it became imediately obvious why I was having trouble. The TTY’s input low pass filter is filtering out the 1000 KHz signal. I confirmed this simulated result with a scope on the actual hardware.

    Changing the input signal to a simulation 100 baud (50 Hz) square wave results in the following simulation.

    Apple Serial to SCELBI TTY RX at 100 baud

    Spice Simulation of Apple Serial to SCELBI TTY RX at 100 baud (current loop)

    Clearly, if I had run my original test at 110 baud, I would have had better results. I confirmed this, by changing the baud rate on the serial card to 110 and monitoring the output of the TTY card with scope.

    Simulation shows that replacing the .1uF capacitor C3 with a .001uF capacitor should enable the circuit to work at 2400 baud, at the cost of much lower noise immunity. I have yet to test this simulated result on actual hardware, but I am pretty confident that it will work.

    Next up: I need to check out having SCELBI TTY transmit data to the Apple II serial card. Hopefully that will go a lot smoother than the TTY receive tests.

    More SCELBI Power Supply Information

    Monday, November 4th, 2013

    I just made contact with a SCELBI employee, who added some details about the power supplies that SCELI made. The power supply design that I’m talking about, is the one seen in the CHM’s online catalog.

    This is one of no more than 20 that were made. The same design was used for both the 8H and the 8B. The design incorporated 2 standard Power-One linear supplies. A 5 volt version and a 12 volt version. The 12 volt version was modified by SCELBI to produce 9 volts.

    The enclosure was lettered with rub on lettering, which was then clear coated with lacquer.

    SCELBI 8H is now capable of running BASIC!

    Friday, October 18th, 2013

    Thanks to Mark Arnold, who stripped down SCELBAL to the barest fundamental elements, I now have BASIC running on a 4K SCELBI 8H for the first time, ever. Mark, who created SCELBAL back in the mid 70′s calls his new stripped down version of BASIC, “tiny SCELBAL”. He removed all non-essential features, including support for floating point math in order to make it fit into 4K. There are only about 350 bytes for program space left in a 4K SCELBI 8H, but that is enough for some very simple games. I have ported a version of HILO from Ahl’s “BASIC COMPUTER GAMES” book, and made it fit. I’ll be making a video, posting source and object files very soon. I’ll also post the source of HILO and describe some space saving tricks.

    Mark says SCELBI considered doing this back in the 70′s, but determined the result would be too limited to be useful for most practical purposes.

    Exciting SCELBI 8H News Coming Soon…

    Wednesday, October 16th, 2013

    I hope to announce within a few days that a SCELBI 8H has done something that a SCELBI 8H has never done before…

    This is something that was possible back in the heyday of the SCELBI mini-computer, so no, I’m not connecting a SCELBI 8H to the internet.