Final Lap

The end of the month is here, and with it the end of the Retrochallenge Winter Warmup for 2014.  Despite a few setbacks and unresolved issues, this has been a good one for me!  But before I declare my contest entry complete, I should at least get some code running on this thing... :-)

The Micro Chroma 68 Expressing Enthusiasm

Some Assembly Required

The TVBug monitor on the Micro Chroma 68 board only provides for loading machine language programs.  So, the first step is to find an assembler for the Motorola 6800 family of processors that runs under my Linux development environment.  There are a number of options for this, although it seems that not all assemblers are created equal!  We must be cautious...

In the days of yore, Motorola operated a "freeware" BBS.  This BBS was originally available by modem, and later over the Internet as an FTP site.  Some of the files there included source code for assemblers targeting the full range of Motorola's CPUs, including the 6800.  Alas, the site is no more and it is now difficult to find even an old archive of it.  If anyone knows of such an archive, please leave that info as a comment on this blog -- that site needs to be preserved!

Fortunately, others found those tools useful as well.  While the unadulterated sources are difficult to find, a variety of slightly modified versions are available from a number of places.  So for now, I will be using the as0 assembler from the "masm" package.  It is a simple tool, but it more then fills the need at hand.

Manual Code Entry In TVBug

Hands-On Programming

The Micro Chroma 68 is already limited in its accessibility for loading programs, providing only an audio cassette interface and the video/keyboard console.  Since I still have not received those magic capacitors for the audio interface, I am limited to just the console for loading programs.  The keyboard adapter I am using has a provision for input through a serial port instead of the PS/2 keyboard.  But with the continuing "repeated characters" problem, manual intervention is still required.

Undaunted, I wrote a small program to put a message on the screen and assembled it with the assembler mentioned above.  The assembler outputs an S-record file by default, but I also had it generate a listing file.  That file had the hexadecimal values for all of the opcodes and data for my program, allowing me to enter them into the TVBug monitor manually.  The process was a bit tedious, but it was manageable for the small program in question.

Flash In The Pan

The original program merely cleared the display and wrote a new message to the screen.  At the end of the program, I simply inserted a busy loop to keep my display on the screen rather than returning to TVBug.  This worked fine, but didn't have quite enough sizzle for the end of the project.  I checked the schematics and found that the video timing signals from the VDG are available for the CPU to read on the Micro Chroma 68.  So, I changed the end of the program to spin in a loop that counted those timing signals and periodically changed the display's color set between the green and orange options -- exciting! ;-)

While changing the end of the program, I was faced with a problem.  I had structured the code such that adding new code at the end would force me to reenter a significant amount of data in memory.  My inherent laziness kicked-in and found another option -- I just jumped over the data and added new code at the end.  Maybe this sheds some light on why "spaghetti code" was so prevalent and accepted back in the early days of computing?  Or, maybe I am just a horrible coder...

Thus ends my entry for the Retrochallenge Winter Warmup 2014 event.  I hope that all of you have enjoyed reading about it as much as I have enjoyed doing it!  A few problems remain, and ultimately I would like to do something a bit more useful (or at least something more interesting) with the Micro Chroma 68 in the future.  I can't help but think that it would look right at home mounted inside of an arcade cabinet...hmmm...

Well, anyway...if you want to find-out what happens in the future then you'll just have to stay tuned!

Two Steps Forward

My Micro Chroma 68 project is still in need of a solution for keyboard input.  The original (and long-term) plan is to use the Radio Shack ASCII-encoded keyboard, but that isn't working at the moment.  Until that gets fixed, I'll have to find some other option.  Fortunately, others have already faced this sort of problem and at least one of their solutions is available...

PS/2 -> ASCII Keyboard Adapter

Problem Solved?

The Micro Chroma 68 was roughly contemporary with the Apple I, and both used the same sort of ASCII-encoded keyboard.  While original Apple I kits are as rare as hen's teeth, replica kits are available and many people have built them.  These Apple I builders have faced the same sort of keyboard problem that I am facing.  So what do they do for keyboards?

At least one person solved the issue by using a microcontroller to drive a (somewhat) modern PS/2-style keyboard on one side and the ASCII-encoded interface on the other.  Even better, such a kit is available for sale!  I bought one and quickly built it.  Initial testing even suggested that it was working...

Close-Up Look At The Dead Bug Hack

Dead Bug

Despite initial impressions of success, I soon discovered a problem -- when pressing a single key multiple times, only the first keystroke was recognized by the Micro Chroma 68!  This is at least inconvenient and it could be crippling for entering data into the monitor program.  What might be causing this?

The ASCII-encoded interface was at best a 'de facto' standard and at worst it was no standard at all.  Such interfaces typically had 7 data signals and one strobe signal that signified when the data was valid.  But, the polarity of those signals and the timing of the strobe could vary between implementations.  The fact that any keys were being read by at all suggested that the data line polarity of the keyboard adapter matched the expectations of the Micro Chroma 68.  But what about the strobe signal?

The documentation for the PS/2 -> ASCII adapter indicated that the strobe signal goes high to signify that the keyboard data is valid.  Checking the source code for the Micro Chroma 68's TVBug monitor revealed that it was expecting the strobe signal to go low to indicate valid data.  I could not visualize how this might account for the observed loss of repeated keystrokes, but it seemed worthwhile to at least make the interface match the expected behavior.

I found a 74-series inverter chip in my parts stash and added it to the PS/2 -> ASCII adapter circuit using a 'dead bug' style of construction.  The hardware functions just as expected, and I verified that all of the connections were correct.  Unfortunately, instead of the Micro Chroma 68 only accepting single keystrokes it then didn't see any keystrokes at all...

Yet Another Hack...

Make It Work

Obviously, that was unacceptable.  But, simply putting things back like before was only marginally better.  So, what to do?

When first observing the "no repeated keys" problem, I did some poking around and found that I could get the Micro Chroma 68 to accept a repeated key if I used a probe to drive the strobe line myself.  As I indicated above, I have verified the connection between the microcontroller on the keyboard adapter and the strobe line on the Micro Chroma 68 parallel keyboard input.  So, I'm at a bit of a loss for why driving the signal manually works...

I can only figure that the Micro Chroma 68 is somehow missing the strobe signal.  I may be able to correct that with a one-shot timer, or there may be some other problem that I'm not yet seeing.  In the meantime, I have inserted a momentary push-button switch in series with the strobe line.  This allows me to manually drive the strobe line when I need to repeat a keystroke, so at least I can get around the problem in the short-run.

Once again I am left with an imperfect solution.  In the long-term I hope to have a more polished result, but for now I'm just trying to get each thing working well enough to make the next step possible.  I guess that is a bit like life, or parenting, or something... :-)

Anyhow, now I've got a mostly working Micro Chroma 68 board (sans audio cassette storage interface), I've got a way to view its video output (via a hack on a satellite board), and I've got a way to put information into the machine (with an extra step for repeating keystrokes).  I may be limping a bit, but I think I can make it across the finish line!

Now I suppose I have to brush-up on 6800 assembly language and figure-out how to write a little program to run for the end of the contest.  I've got some big distractions coming this week, so don't expect too much!  But until then, please do stay tuned...

Keep It Clean

When last we left our daring hero, the Micro Chroma 68 board was just barely showing some signs of life.  The video output was stable, but the garbled mess on the screen was a telltale sign that something was still wrong with the circuit...

Micro Chroma 68 Display

By The Book

The manual for the Micro Chroma 68 board has some simple procedures defined for debugging a non-functioning board.  The first step, of course, is simply to check your work:  check power and ground connections; ensure that parts are installed correctly; and, check for shorted or open connections on the board itself.  I completed those checks, yet the problem persisted.  Fortunately, the next round of testing was more fruitful.

The manual described a few simple and temporary modifications to make to the circuit.  These modifications hold the CPU in reset while forcing the address decode circuitry to select the ROM chip.  This puts predictable values on the address and data lines, allowing for some simple testing with a voltmeter.  These tests showed that the CPU was behaving correctly, but that the data bus held the wrong value.  Further testing showed that while the CPU was driving the address bus correctly, the ROM was seeing the wrong values on its address lines.

Ancient Anti-Static Foam

Baked-On Gunk

More checking continued to show the proper connections on the board between the CPU socket and the ROM socket.  Digging deeper revealed that the disconnect was between the CPU socket and the pins on the CPU itself!  This is a new (or NOS) socket, and it shows no sign of being worn-out.  So, what was happening?

The CPU is one of the chips that were original parts included in the box of the Micro Chroma 68 kit.  The chips were inserted into a block of anti-static foam which served to contain the chips and to protect their pins from damage, and there they sat for roughly 35 years.  When I removed the chips from this foam, I noticed that some bits of the foam stuck to the pins of the chips.  I scraped the visible bits of foam away and gave it no more thought until I witnessed the results of the testing described above.

I acquired some parts cleaner from a local electronics retailer and used it to clean the pins of the chips that were included with the Micro Chroma 68 kit.  After this cleaning and closer visual inspection, I was satisfied that the chips were clean and serviceable.  I re-installed the chips and repeated the tests described above.  Some of the pins now properly reflected the state of the address bus, but to my chagrin some of them were still incorrect.

Strangest Electronics Tool Ever

File It Away

After some thought, I decided on a more aggressive strategy for cleaning the pins of the chips in question.  Using some small hobby files, I filed the pins until they were slightly shiny.  I then hit them again with the parts cleaner, just to remove any remaining filings.  I re-installed the chips onto the board, and another round of testing showed correct values for the address and data lines at both the ROM and the CPU.  Then I removed the test hacks to the reset line and the ROM address decoding, and I powered-up the board.  The picture at the top of this entry shows the result -- the TVBug sign-on message and an apparently working Micro Chroma 68 board!

So, where does that leave things?  I still need a keyboard solution to allow for input -- more on that in the next post, most likely.  I'm also still missing a couple of capacitor values related to the audio cassette interface for storing programs -- hopefully they will arrive soon!  Finally, it would be nice to see at least one program running on the board by the end of the month.  I'm not sure if I can solve all of these issues between now and then, but I might be able to keep things interesting in the meantime.  I hope you will stay tuned!

A Few Things Amiss

This weekend brought me a bit more time for working on my Micro Chroma 68 build.  A few more parts have arrived, and I found an alternative for an important one that was still missing.  The board is showing some signs of life, but perhaps just barely so...

Garbled Output on the Micro Chroma 68

Substitution

I had collected most of the parts required for this build over time, and I ordered the remaining parts during the first week of January.  Unfortunately, there are still parts that have not arrived.  I received two of the missing capacitor values this week, but I am still missing three more.  Two of the missing capacitors are only used in the audio cassette storage interface, so they aren't required for board bring-up.  Unfortunately, the missing parts also include one of the capacitors in the clock circuit.  So, the board has not been testable until recently.

The missing clock circuit part is a 50 pF capacitor.  While poking around in my parts stash, I found a bag of 47 pF capacitors.  The Micro Chroma 68 kit instructions specifically mention the use of a 47 pF capacitor as a substitute for that part.  Since that capacitor is mated with a variable capacitor on the other end of the crystal, the clock can still be tuned to the proper frequency.  I installed the capacitor, tuned the clock using the frequency counter feature of my oscilloscope, and moved onto getting a video display!

Perfboard MC1372 Circuit

Reinvention

The Micro Chroma 68 board is designed to provide an RF signal for feeding a TV through its antenna connection.  As I mentioned in an earlier post, I attempted to hack the circuit to disable the RF signal generation and to just output a composite video signal.  Alas, I don't seem to have understood the MC1372 datasheet as the device was not generating any video output at all.

I went back to the drawing board and built a new MC1372 circuit based on a schematic in the MC6847 datasheet.  The new circuit was similar to the hack I had attempted, but different in a few details.  I built it on a piece of perfboard, and used some wires to plug it into the MC1372 socket on the Micro Chroma 68 motherboard.  Thankfully, this circuit worked to provide the video output in the picture at the top of this entry.

Deduction

Of course, it doesn't take a lot of expertise to deduce that the screen output pictured above isn't exactly what the board should be producing.  I would have expected to see a sign-on message from the Micro Chroma 68's built-in monitor software, TVBug.  The garbled mess on the screen certainly suggests that something is wrong.

Letting the board sit there and run sometimes results in some screen characters getting changed, and fiddling with the reset button or the break key has some effects that offer tantalizing clues that at least something is working on the board.  Removing the video memory results in a more uniform pattern on the screen, suggesting that the 6847 is able to read the video memory.  So, there are some signs of life on the board even if the current state is far from perfect.

It isn't clear to me what the problems are with my Micro Chroma 68, but these clues suggest that the fix could be as simple as replacing a bad chip or fixing a short-circuit or a cold solder joint.  Finding the answers will require a combination of deductive reasoning based on observations and methodical inspection and testing of the board itself.  There is no telling how long that might take, but please do stay tuned! :-)

Rapid Advance

The Micro Chroma 68 build is making good progress!  My schedule aligned on Saturday such that I had several hours which I could fill with the smell of melted metal.  So far the only casualties are a spool of solder and the worn-out tip of my soldering iron... :-)

Progress on Micro Chroma 68 Construction

Progress Report

With most of the required parts in hand, it was easy to make progress on construction.  In particular, I had been waiting on the various IC sockets to arrive.  It is standard practice when assembling a PCB to install the sockets first.  Sockets have lots of pins, and it is easiest to get them installed properly when there is nothing else on the board to prevent them from laying flat while being soldered.  Other parts that lay flat (like resistors and diodes) are typically next in line, followed by capacitors and special items like crystals, connectors, and switches.

I now have the IC sockets, resistors, diodes, and capacitors installed in the Micro Chroma 68 board, with exceptions for the RF tank circuit (discussed below) and the handful of parts which I still don't have.  This includes a handful of oddly valued capacitors, and one socket that I overlooked when ordering the rest.  It should be simple to install those parts as they get here.  As long as they arrive on time, those parts should pose no barrier to completing construction.

Imperfect Fit for C5 and R2

Square Pegs

I have been pleased at the availability of some of the less common parts required for the build.  Between surplus electronics providers and various auction sellers, I have been able to locate usable options for every required part so far.  Unfortunately, it can be difficult to find the exactly correct footprint for every part.  In particular, I had to do some creative bending of the leads on both the potentiometers and the adjustable capacitor in order to fit the holes on the PCB.  This isn't uncommon or unexpected -- I'm just glad that the pins were long enough to reach!  Of course, doing this probably gives the vapors to all of those "period correct" Apple I replica kit builders... ;-)

Engineering Opportunity

One part that I made no attempt to find is the variable inductor that forms part of the tank circuit for the RF modulator.  This part is unnecessary because I don't intend to have RF output for the video.  Such a circuit was useful back in the days when the only way to use an unmodified TV as a monitor was by building a tiny TV transmitter into your device.  But nowadays simple composite video is a widely available option, and it will generally provide a much higher quality display as well.

Fortunately, the Motorola MC1372 part used in the RF modulator circuit can be configured to produce just the composite video without the RF modulation.  As detailed in the datasheet for that part, this mostly requires replacement of the RF tank circuit with a single diode.  The devil is in the details, of course.  But I intend to work-out those details and find a way to hack the PCB in order to produce composite video instead of the RF output.

Other than the video output hack, the board is almost ready for some limited testing.  Unfortunately, one of the missing capacitors is part of the clock circuit.  So while I can do some limited testing for shorts/opens and the like, I can't do much to test the functioning of the CPU or VDG or anything else in the system that needs the clock to function.  The waiting game continues, so like me you will just have to stay tuned...

All Keyed Up

Unfortunately, my Retrochallenge project continues to progress at a turtle-like pace.  I have still been waiting for parts, and tending to some of life's other demands.  In the meantime, I have managed to poke around my project at its edges...

Radio Shack ASCII-Encoded Keyboard

Keyboard Crisis

The Micro Chroma 68 board is designed to take input through a parallel interface to an ASCII-encoded keyboard.  This sort of arrangement appears to have been common in the days of TV typewriters and the Apple I.  Someone even interfaced one to the Bally Astrocade!  Nevertheless, today it seems to be quite difficult to find such hardware...

Fortunately, a couple of years back I managed to find the keyboard pictured above for sale on a popular online auction site.  FWIW, the keyboard and the PCB were sold separately in the 1977 and 1978 editions of the Radio Shack catalog.  The manual for the encoder PCB is available online for the curious among us.

Heep Test Module

Heep Test Module

I thought it would be wise to test out the keyboard, given its age and unknown condition.  Fortunately, the manual provides a diagram for the oddly named "Heep Test Module".  I was able to construct the fixture in an hour or so from a handful of scrounged LEDs and resistors, some wire, and a newly acquired edge connector.

As an aside, I thought the "Heep Test Module" name sounded a bit odd.  Coincidentally, I was reading the newly released CoCo book when I noticed mention of a Tandy engineer by the name of Jerry Heep.  I guess that "Heep Test Module" is like a printed electronics manual version of an Easter Egg!

Anyway, I'm glad I took the time to build this fixture.  Applying power to the keyboard lights-up the fixture in strange ways, revealing that the keyboard is not currently functioning as it should -- one more thing to fix...  Alternatively, I may have to buy or build some sort of replacement.

Bags Of Parts

Parts Is Parts

Aside from the keyboard situation, I am running out of excuses on the construction project.  While I still don't have all the parts I need to complete the box, delivery trucks have been rolling and I've got the bulk of the missing ingredients.  I had been missing some key parts, like IC sockets and some particularly valued resistors and capacitors.  A few of those are still outstanding, but most of the stuff is here already.

Hopefully the next post will at least have some parts stuck to the board!  Until then, of course, you'll have to stay tuned... :-)

Building the Micro Chroma 68 Kit

The Retrochallenge Winter Warmup 2014 event started days ago, and I have yet to blog about my entry!  I guess I need to get going on that... :-)

For some time, I've been in possession of a mostly complete Micro Chroma 68 kit that I acquired from someone in the Tandy Color Computer (CoCo) community.  Over time I've acquired most of the parts required to build the kit, and I've been on a bit of a roll lately with electronics construction projects.  So, I guess this is as good a time as any to work on this build!

What Is It?

The Micro Chroma 68 is a single board computer (SBC) based on a derivative of the Motorola 6800 processor.  It was produced as a platform for demonstrating the Motorola 6847 video display generator (VDG).  Along with the amazing graphics capabilities of the VDG, the Micro Chroma 68 features an interface for loading and storing information stored on audio cassettes, and a keyboard interface for interacting with the device as a video terminal.  The device also features an EXORcisor bus connector for interfacing to other devices.

I have seen it suggested that this device provided some inspiration to the designers at Tandy that ultimately produced my beloved CoCo.  Whether that is true or not, it seems likely that those designers would at least have been aware of this device.  While the Micro Chroma 68 is clearly designed for engineers and experimenters, it does show the first few steps toward producing a standalone computer consumable by normal folk.

The Micro Chroma 68 also seems comparable to the venerated Apple I design.  They certainly have similar features and capabilities.  Given the high cost of building even a replica Apple I kit, this is probably the closest I'll ever come to that experience!

Shop 'Til You Drop

The Micro Chroma 68 kit comes with a printed circuit board (PCB), and a handful of key integrated circuits (ICs).  However, it is far from a complete kit and not all of the other parts are still commonly available.  I had made a point of acquiring most of the remaining ICs already, but some of the other parts are equally hard to find.  Luckily, some strategic eBay shopping and a check of a trusty electronics surplus provider (Unicorn Electronics) seems to have located all of the required parts.

Unfortunately, I didn't do this shopping until New Year's Day.  Some of the items are coming from distant suppliers, and I may not have everything in hand until the third week of January or later!  Hopefully I can make enough progress with what I already have and anything that comes early so as to still have a chance to complete the build....

Other Pursuits

I had some time-off around the turn of the new year, and I used it to work on a variety of retro-ish hardware projects.  I fixed old game consoles (even a Vectrex!), made cables, built circuits, and even made a project enclosure out of an old cigar box!  Anyway, I've got a lot of distractions available to keep me busy even if the Micro Chroma 68 parts get delayed.  If that happens, then I'll try to at least post a link or two here as well.  In any case, be sure to stay tuned... :-)