The Altair 8800: More Retro Computing

I’ve been working on a Z80 computer kit off and on for a while. I have to admit that soldering small parts onto tightly packed boards is less fun than when I was younger.

A few weeks ago I came across an Altair “clone” that was available:

Altair Clone

I was tempted, but I decided that I really needed to finish up the Z80 kit before investing that much in my next retro computer. Then I came across an kit that was bit lower priced that really caught my eye:

Arduino Altair Clone Kit

At the time the kit was sold out, but I ordered one as soon as they were available again. The kit arrived about a week later and I spent some time over two days of vacation building it.

Overall, I’m very impressed. If this looks like fun to you, then I’ll guess you would enjoy it and would not hesitate to recommend it.

You can load an SD card with up to 15 different disk images or choose from several preloaded ROMs that are on the system.

I’m still getting my feet wet on running it, but can offer a few thoughts that might help you if you build one.

The Arduino is attached to the main board using header strips. When I install header strips, I normally will just pull pins instead of breaking the strips into smaller ones. I’ve attached a picture of where I was able to do this on one side.

header

Pulling a pin just involves grasping the long end of the pin with needle nose pliers, touching a soldering iron to the short end for a few seconds, and the snagging the plastic part of the header under your fingernail and pulling. This approach makes it easier to keep the pins lined up.

There are two other sets of headers. One is a dual row header that is a single length of stock and the other uses an offset spacing for part of it to act as a “key” to ensure the board is correctly oriented.

There are two ribbon cables (one pre-installed on the Arduino) and the colors may not match the pictures in the build instructions, but the places they connect are well labeled and it is pretty obvious how they go. (You would have to work at it to mess this up.)

One interesting feature of this kit is that it offers a “serial port” connection from the USB port on the Arduino and a second serial port that can be built two different ways. (You choose your preference when ordering.)

The second serial port can either be an RS-232 port or it can be a Bluetooth device that will act as a serial device when connected. You can swap this second port to be the primary interface if you want. With the Bluetooth connection that means you could open a terminal emulator on your Android phone and play with the Altair.

I choose the Bluetooth option. I had some of the plastic from a project that had required removing it from a row of header pins once installed. I used two of these “blank” single pin plastic pieces as “spacers” at each end of the BlueTooth card after straightening the pins on the card.

bt1.jpg

This gave me a neat installation with some spacing without having to juggle things with my hands in the attempt.

bt2.jpg

The power supply connector is a fairly standard make with three lugs. That allows you to connect one lead from a battery to the third lug and have it disconnected when power is supplied by an external supply. (You don’t want to run a supply and batteries in parallel in most cases.) That doesn’t apply here, but the lug is still there and can just be ignored. The proper wiring is shown here:

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The supplied USB cable is stout and a bit short. I was worried it would pull over the Altair clone, so I used a lighter cable I had around. I also used a wire tie on this cable, tying it to one of the standoffs. This will keep any “tugging” on the cable from putting mechanical stress on the Arduino’s USB port.

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The computer comes with a wooden (some bamboo but also some pressed board with an imitation bamboo lamination) enclosure. I used the front panel as a template to drill the holes in the back, but still had trouble getting things lined up. I tried to install from the back first and then the front panel last, but that didn’t work well, So I used a trick I’ve found helpful before and took a pocket knife, put the blade through the holes from the inside, and twisted the knife a few time. One hole seemed to be a bit off so I gave it a few extra twists. This will create a bevel (or cone) on the inside. That, in turn, helps catch and guide the threads to and through the holes. I messed around with it for probably 15 or 20 minutes before trying this. After doing this, it went together on the first try and with no “wiggling” to try to get things lined up.

A few extra parts were included. This is a really nice touch that you won’t appreciate until you have trouble. For a first time builder who gets an LED or a transistor installed wrong, it is much easier to clip out the old one, pull the leads, clear the holes (something like a toothpick works great if you don’t have desoldering tools), and then install a new component. For someone without a lot of practice and without the right tools, trying to remove a component so that it can be reoriented and reused often leads to things like solder bridges and lifted traces. I’ve kitted a few different projects up over the years–it can be a pain and little things like this tend to show the person doing the kitting is more interested in you being successful building it than in saving a buck or two in parts.

The only other thing that was a bit of research was getting the Arduino drivers installed. You have to download a zip file, unzip it, and then point to the driver. (At least on Windows 7.) It wasn’t difficult, but it did require downloading a 120 meg file. Anyone who regularly plays with Arduinos is probably not going to have that issue.

Things worked fine on the initial test, but as I was finishing up I had an issue where when powered up the lights would all come on, but then they would go off and the HLDA light would flicker dimly. It happened with both the power supply and powering over the USB. I carefully looked at things, trying to figure out what might have changed. When I tested it after finding nothing with a close visual inspection, it worked again. I suspect I had a piece of lead that had ended up gumming up things someplace and that I knocked loose through later handling.

A couple of other hints…

Do a full format (not the “quick format”) of the SD card and place all the disk image files in the root directory.

Also, when clipping the short leads of the LEDs (or, for that matter, any leads), I would strongly recommend wearing safety glasses. They are tough to cut, short, and will go flying if you aren’t careful.

So now it is time to make it do something!

altair.jpg

Here is the assembled computer being tested. The main background on the screen is a web page, but the window in the center of the upper part of the picture is the terminal session I’m running. The device is being power from the USB port in this picture.

I’ll keep you posted as I play around with this and some of my other retro computing toys.

Thanks,
Jim W4JBM

“With a soldering iron in one hand, a schematic in the other, and a puzzled look on his face…”

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Retro Computing

Computers, tablets, smart phones, etc. are nearly ubiquitous today and it is hard for many to imagine the early days with computers offering nothing more than a few front panel switches and some LEDs. Adding a few numbers together and being able to see the results by looking at the contents of a few memory locations was rewarding “back in the old days”.

There was a period where it seemed like many vintage systems had emulators that would run on the PC so you could relive those memories. And, of course, some of the older hardware is still around and has been restored to working order. But one doesn’t really have the same excitement and the other can be expensive and time consuming.

So in the last few years, retro computing has emerged…

Retro computing uses modern hardware and software to recreate vintage systems.

My first experience was with the 1802 microprocessor based Membership Card which is similar to the COSMAC Elf (one of the early “affordable” computers) but fits in an Altoids tin. It makes use of an actual 1802 chip and has switches and LEDs along with a serial port. Mine is running with 32 kilobytes of RAM and 32 kilobytes of ROM. (The ROM has the serial drivers, a machine language monitor, and integer basic.)

That was purchased on an impulse, but then I came across the P112 Z80 system which can run a derivative of CP/M.

The Dodo is a 6502 based hand-held “game system” that includes an online development environment.

And then there was the kickstarter campaign for the ZX Spectrum Next (due out in January 2018) and the efforts of the team working on the Mega65 (the realization of Commodore’s successor to the C64, the C65 which never made it beyond prototypes).

These later two make use of field programmable gate array logic (FPGAs) to implement many of the features including the video and the sound. A gentleman named Grant Searle has a website for those interested in home brewing a simple and cheap FPGA based computer that can run a Z80, 6809, or 6502. (The USB programming interface for the FPGA he uses runs between $15 and $25, but the FPGA card itself is around $20 on Amazon.)

Tinkering with these projects has been fun, but the tone of some of what I’ve read about the ZX Spectrum Next resonated.

A lot of people from the UK who went on to careers in engineering, computer science, and related fields credit the original ZX Spectrum and the BBC Micro with their start down that path. Three decades later, they are seeing the number of people in those fields taper off.

We need more young people interested and pursuing careers in the area of science, technology, engineering, and math. (Referred to as STEM for short.) The Raspberry Pi team has realized that and formed “coding clubs”.

This is great, but I would offer a “call to action…”

Except for a few highly motivated members, these clubs can offer only a limited glimpse into the world of STEM if those of us involved in the field do not make the effort to become involved.

There is another aspect for me personally–I worry that the chances for exposure to STEM related activities is lower for those who parents struggle paycheck to paycheck or who are in foster homes. If each of us were able to help one young person move past the limitations of their circumstances and towards a career in STEM, the results would be rewarding from so many different aspects.

I’ll explore more ideas along these lines in the future, but for now consider taking a look at the retro computer field with an eye towards both having fun and figuring out how to involve others so they can also share in that fun.

Thanks,
Jim W4JBM

Streaming a Scanner

A while back, I purchased a Uniden Bearcat BCD356HP. As I understand it, the original concept for this scanner was that it would be able to stream to a remote client over the Internet letting you listen even when you weren’t home. Unfortunately that capability has never been realized.

Some of you may be familiar with RadioReference.com and the relationship they have with broadcastify.com. There are hundreds of scanners across the country that are being streamed. I use an Android app called Scanner Radio Pro on my phone and tablet. One of the really cool features is that it can alert you when a lot of people begin listening to a particular scanner feed. More than once I have been listening to breaking news live from a police scanner feed before the story hit any of the news sites.

I stream the local police and sheriff channels off the regional P25 system and it has been an interesting experience.

I started with Ubuntu Linux and have now migrated to VectorLinux 7.1. Even though the box set an ran for over a year, something cause the Ubuntu installation to begin dropping the stream.

Streaming has two components. I use darkice to stream from my local computer to broadcastify.com. I’m not sure what they use, but there is a different piece of software on their servers to handle connecting and streaming to the end listener.

Trust me, I enjoy being able to listen to the traffic from back home when I travel. When I was having problems getting a stable feed, it seemed like some people felt like I was going out of my way to interrupt their listening. It is a hobby and I only have so much free time to work on issues and resolving this last batch of problems required trying to install several different distributions of Linux before I found one that would both run on the computer I have free to use and allow me to run the darkice software.

I also have certain features that are important to me that are part of the configuration. I do local recording (in 30 minute chunks) so I have my own archives. (By streaming, you get a premium membership to broadcastify.com which also allows access to archives. I would rather just have my own that I capture locally.)

A few people have sent me harsh emails saying I am putting officers lives in danger by streaming the traffic. I once talked to a sheriff in another county about his feelings about streaming feeds and he made a good point: the suspect setting in the back of a cruiser can hear the radio just fine. His point was that most critical traffic is carried on “tactical” channels which are separate from those normally used or sensitive information is typically handled by cell phone these days.

But overall it has been educational as I worked on the Linux scripts that allow me to stream and capture the audio. In addition to the public safety feed, I am considering using the second channel (it is a stereo connection) for either the small regional airport near me or one of the local ham radio repeaters.

If there’s a feed for you local public safety channels, give it a listen. If there isn’t consider taking an older PC and putting it into service with your own feed. If you need some help with getting a Linux box streaming, drop me a message and I’ll see if I can help you.

73 de
Jim W4JBM

“With a soldering iron in one hand, a schematic in the other, and a puzzled look on his face…”

General Radio Frequency Standard

I recently picked up a rack of old gear from the garage of a local silent key. They were in rough shape with leaky caps, rust, and corrosion, but deep inside a crystal oven I found this.

IMG_20160717_110659

This was part of an FAA frequency standard. It was mounted in an aluminum enclosure with about 1/4″ walls. This was surrounded by about a half inch of styrofoam and then a slightly thinner aluminum enclosure. That outer enclosure had three resistors on each side used to heat it and that was enclosed (with some air space) in a wooden box.

To align this, there was a WWV receiver and various other gear. I’d only be guessing on the accuracy, but I recently ordered a GPSDO (GPS disciplined oscillator) that is better than 1 part-per-billion in accuracy. I don’t doubt that is significantly better than this could offer even when it was freshly calibrated.

There is a slight chip in the upper right hand corner of the quartz bar. It is held in place by what looks like fishing line and a series of springs.the leads aren’t visible–they are whisker thin and soldered to the gold plating on the bottom of the quartz bar.

I was going to build a 100 kHz marker generator with this and still might on day, but for now I’ll probably stick with the 10 MHz GPSDO around the home lab.

73 de
Jim W4JBM

“With a soldering iron in one hand, a schematic in the other, and a puzzled look on his face.”

Book Sale! Mysteries of the Ether

Are you a radio enthusiast who likes true stories of the strange and unusual? Here is a special offer for you to enjoy over the holiday weekend. On sale for 99 cents (from the list price of $4.99), this book will convince you that shortwave listening is anything but boring!

Mysteries Of The Ether: The puzzling and unusual side of shortwave radio!

73 de,

Jim W4JBM

“With a soldering iron in one hand, a schematic in the other, and a puzzled look on his face.”

The Tekpower TP-3005D-3 Power Supply

Most of the power supplies around my shack and on my bench are homebrew affairs built with junk box parts. I have a few commercial supplies that I’ve picked up over the years, but did not have one that really met all my needs. I wanted one that could handle at least 3 amps up to at least 28 volts. I also wanted dual tracking because of the analog circuit work I do. An I wanted meters–preferably individual meters for each voltage and each current.

I seriously considered building another, but after reading lots of reviews, reviewing lots of designs, and coming to the realization that “metering” was going to be a challenge if I built one from scratch, I finally decide to buy a Tekpower TP-3005D-3 power supply off of Amazon.

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There has been a recent chance and the Tekpower supply that arrived actually had LEDs to display the voltage and current instead of the earlier LCD displays. The LEDs are bright, large, and easy to read. Voltage is displayed in green and current in red. The earlier LCD displays had received positive reviews, but I can’t imagine anyone not being just as happy with the new LED display. (I actually prefer LEDs to LCDs in this type of application.)

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I carefully read the reviews for this and several other supplies before making my choice and there is some good information out there. But let me run through what I did and what I found as I put the supply through its paces.

Estimated shipping time was two weeks and it actually arrived several days early. (In time to have a weekend to play with it!) It was double boxed. The interior box was the original factory box and the supply was wrapped in a large, heavy plastic bag and held in place with a pair of foam forms. The outer box had taken a beating during shipping and showed it, but the interior box looked fine and the supply itself was in perfect shape when unboxed.

In the box was 1) the power supply itself, 2) a power cord, 3) one pair of polarized (one black, one red) banana connector to alligator clip cables, 4) a second pair of polarized banana connector to alligator clip cables that used two wires each (for higher currents), 5) a short connecting cable with banana plugs on both ends for series operation of the supply and 6) a short printed manual.

As I discuss the supply, I will give some readings or measurements in places. When I do, I will give the right / master side reading first and the left / slave side reading second.

I powered it up and found 32.5 volts and 32.4 volts on the outputs. I confirmed these with my bench meter to verify the accuracy of the supply’s meter. If I tweaked, I could get them one digit out of synch, but the meters were definitely within the plus or minus 2.5% specified for both the voltage and current reading I took at various times.

Initially I connected the supply to a 50 ohm dummy antenna because that is the thing I have setting around that can dissipate significant power without straining. I was kind of surprised that the output voltages stayed the same when a load was connected—basically I was drawing 0.65 amps per side and the voltage stayed the same as the no-load voltage with the regulator control fully clockwise.

There are two buttons in the middle and they are marked with the following configurations:

Up / Up is “Independent” and you can vary the voltage and current limiting individuals for the two variable supplies.

Up / Down is “Series”. This did not exactly work like I expected. The manual indicates this is the “tracking” mode, but it isn’t. More on that in a bit.

Down / Up is “Parallel” and the two supplies are connected in parallel with the voltage of the slave supply tracking the voltage you set for the master supply and the current being evenly split between the sections.

There is a cable that lets you jumper between the two supplies so they can operate in what I would describe as an “independent” series mode where the voltage of each supply can be adjusted independently.

Now to the mode that isn’t listed on the panel:

Down / Up seems to be the true “series tracking” mode. With the buttons in this configuration, the supplies track each other and the included jumper is not needed—the positive of the slave supply and the negative of the master supply are connected automatically and internally.

So basically, I think that the mode most people would want when operating in series (tracking) is actually Down / Up, not Up / Down, and that the jumper is unnecessary.

Let me come back to how the protection and current limiting operate in modes other than “Independent” in a bit.

With a 1 ohm power resistor, I was able to see where the current maxed out and it was 5.08 amps on the master side and 5.24 amps on the slave side. I just don’t have enough power resistors around to see if I could get 5 amps at 30 volts, but from everything I saw I don’t have any reason to doubt I could.

I did pull out a 12 volt motor that drew around 0.6 amps. I ran the voltage up and down a bit and things worked fine.

I do a lot of RF work and for me the “gold standard” for those applications is being able to power a direct conversion receiver. Until now, virtually every commercial supply I had used had to be “RF proofed” with bypass capacitors on the rectifiers and a number of other modifications. With this extra effort, most power supplies will induce home (by modulating the carrier from the local oscillator in the direct conversion receiver that “leaks” out). A lot of times this forces QRP operators with simple rigs to operate from battery instead of a power supply. The Tekpower supply worked fine as it came from the factory. That should be enough to impress any amateur radio operator who has struggled with these issues before.

I usually separate a direct conversion receiver from the power supply by at least three feet anyway, but found that I could set my FOXX-3 transceiver (which is in an Altoids tin and uses a direct conversion receiver) on top of the supply and there was no induced hum.

Part of that may be because of the use of a toroidal transformer. This type of transformer construction is going to offer more self-shielding than a conventional transformer with open windings.

And the transformer in the supply is impressive. It is about 5 inches across and 2.5 inches high and bolted into the middle of the chassis on the bottom. It looks fully capable of handling the 300+ watts the supply can provide.

Okay, by saying that I have to admit I “popped the hood” and took a look inside.

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I was impressed. Construction is neat and professional. They made liberal use of heat shrink to cover connections. The boards use a mix of surface mount and through-hole components. (Most of the power components such as diodes and the filter capacitors are through-hole.) There were small touches like a dab of glue on the filter capacitors (which are on a board that is mounted vertically) that most manufacturers probably would not have done.

For each of the two independent supplies, there are four pass transistor and eight rectifier diodes. For a five amp supply, that should offer some healthy headroom and I doubt anything is being pushed particularly hard.

The physical construction is also impressive. The top of the supply have a conventional “U-shaped” cover. It is held in place by twelve screws. Two of these also hold the handle in place. The handle is secured to a cross brace that runs front to back. The handle is a soft rubber / plastic and has a metal strap in its center. There are also two other cross braces—one on each side of the supply.

Bottom line on physical construction is that it is solid, uses about twice as many screws as they could probably have gotten away with, and the handle is solidly mounted. I move the supply from my workbench to my ham shack (both in the same room, but about ten feet apart) on occasion, so it is nice not to have to worry when I grab the handle.

Also, there are no sharp edges. The heat sink is inside the enclosure and there are two fans that indicate they kick on at around 50 degrees (I assume that is Celsius).  I hooked motors, resistors, and everything else I reasonably could to load the supply down. Even after letting things set and run for an hour the heat sink was barely warm and the fans never kicked on. It is rated for operations up to 40 degrees (Celsius) ambient temperature which is pretty hot—with those kinds of external temperatures you might see the fans operating.

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A lot of other supplies have the heat sink as part of the physical enclosure on the back and the heat sink fins often have sharp edges that can give you a nasty cut if you reach behind the supply without thinking. Having them internal is a nice touch.

There appeared to be surge suppression on the input line and there was a line safety filter cap which may be another part of what helped it perform so well with a direct conversion receiver.

One thing that jumped out at me was that the filter capacitors are not huge monstrosities.  At 6800 uF capacitors rated at 63 volts they are decent sized. I have seen a lot of “cheap” supplies try to make up for a lack of “iron” (i.e., a transformer that is being push a bit harder than it should) by adding larger capacitors.

Why does this matter? Well what made me first notice this was that when I shorted one of the outputs there was not even a tiny spark or spit. With a short, you essentially have to drain all the energy off the filter capacitor. With a large filter capacitor this can give you the small spark you often seen when a supply is shorted. The bad thing with that is that if the filter capacitors are after the regulator, that energy is not throttled down by the overcurrent protection. If you design things properly and don’t try to push every component to the limit while shaving a few dollars in parts, you don’t have that issue. And this supply definitely does not have that issue.

Okay, let’s talk about over current protection. I skipped earlier it because this takes a bit.

In the series, parallel, and the series tracking mode, the current limiting when the supplies kicked into constant current mode was a bit unpredictable. In parallel mode you should be able to get 10 amps in constant current mode. I don’t have anything that can sink that amount of current at any reasonable voltage on the bench so I didn’t explore it much. For the series and series tracking mode, things seem to work best if you crank current limiting on the slave all the way up and just used the current limiting adjustment on the master side.

If you think about it, this isn’t unreasonable. Protection does not drop into some type of voltage foldback on overcurrent with this supply, instead the supply switches to a constant current mode. And if one side of the supply is in constant current mode while the other is trying to stay in constant voltage mode they are not necessarily going to play well together.

I rarely use constant current mode supplies in any of the work I do. Honestly, I would rather have voltage foldback instead of constant current. But at the same time, if I was the designer, I can see where the constant current mode makes the supply much more flexible in the types of applications it can handle.

The bottom line is that the over current protection scheme is fine and was a good design decision, you just need to understand how it is working. The fact that there are indicators that light green for a supply in constant voltage mode and another that lights red if it switches to constant current mode gives you a quick indication of what is going on. For most of my work, either of the red lights coming on is an indication that something is wrong. And I can still set the current limit to a position that protects the circuit I’m working on, but the circuit will still be receiving current even if it has shorted out.

I am a big believer in spending the time to understand how your test gear really works both when you are using it like you should as well as when you encounter something unexpected. This supply is intuitive enough that ten or so minutes of playing around with it and a power resistor is more than enough to get a solid sense of what how it behaves.

There is a 5 volt, 3 amp supply that is independent of the others. It has a green LED that is labeled “Over Load”. Really, if the light is on things are fine and if you overload the supply the light goes out.

I haven’t tried it yet, but with the ability to get a bit over 60 volts DC out and an independent 5 volts DC, this supply might even be able to power one of my small homebrew vacuum tube transmitters or receivers. The 5 volts is a bit lower than normal for operating the heaters, but I suspect you could make things work. Anyway, that’s just a somewhat random thought.

So to wrap up…

There are little things I would change if I was designing one from scratch like the use of voltage foldback instead of dropping into constant current mode and maybe a fixed 12 volt output instead of (or, even better, in addition to) the fixed 5 volt output. But those are nitpicks and design decisions the engineers made and not really shortcomings of this particular supply or design.

The only two things I could legitimately ding the supply on are two labels. Really, I think the “Series” mode should show using the two switches as Down / Up instead of Up / Down and the LED for the 5 volt supply says “Over Load” when actually the opposite is true (the light is on when there is no overload and goes off when it is overloaded). You are instantly reminded of the first one because the voltages don’t track when you use the labeled series setting for the switches. The second one is also fairly intuitive.

Overall I am very impressed with the supply’s performance and construction. It seems like the components are not being pushed to anywhere near their limits. The supply is solidly built and not going to buckle or bend. You can grab it by the handle and not have to worry about the handle snapping off. There is no external heatsink that is going to snag things or that could give you a burn if they get warm. The LED display is bright and easy to read. There are indictor LEDs for other things that keep you aware of how the supply is performing.

This supply is a great value for the price and I don’t hesitate to recommend it. I suspect this one will still be going strong a few decades down the road.

Interesting Oscilloscope Patterns

Over the Independence Day Weekend, I spent some time with my vintage Tektronix 7614 o’scope looking at various ways to generate “pretty pictures”. My goal is to get some of this documented in a way that allow easy demonstrations and such. This, in turn, might help interest other people in the radio and electronics hobby.

I have posted a video with some of the results on YouTube.

One very useful piece of equipment that made life a lot easier was a Samsung tablet running the audio function generator software app available from Black Cat Systems. The output level on the tablet was a bit low, so I built a dual 5x amplifier using a 1458 dual op-amp chip. After that I was in business and able to generate some interesting patterns. Most were “stable”, but the last example in the video actually shows a primitive demonstration of what the output from an analog computer looking at the orbit of the moon around the earth as the earth orbits around the sun might look like. (In other words, it shows a “dual orbit” on the o’scope screen.)

I hope to be sharing more soon!

73 de
Jim W4JBM

“With a schematic in one hand, a soldering iron in the other, and a puzzled look on his face”