Now for the question – should I call my Apple II rev 0 reproductions Mimeo IIs, even though they came first?
Archive for the ‘8008’ Category
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.
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 My original explanation was incorrect and has been replaced by Mark’s story.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.
I still need to add a few 8008 applications I have working to the applications page.
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.
SCELBI TTY CARD UPDATE
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.
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.
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.
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.
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.
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.
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.
The result shows exactly why I was having trouble.
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.
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.
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.
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.
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.
I’m now taking orders for custom made reproduction SCELBI 8H front panels.
Price is $65 plus $10 shipping continental USA or $20 international. I am pricing these more as service to SCELBI PCB kit builders than for profit. When I eventually sell most of the 20 that I have made, there may be a small profit in it for me, but it will not be much. Based on my experience, unless you were already set up for making custom front panels, making a single one for yourself would cost at least two to three hundred dollars.
A couple of things to be aware of.
Send an email to:firstname.lastname@example.org if you need more details on ordering.