Sunday, October 30, 2016

RC2016/10 -- Happy, Happy, Joy, Joy!

After several hours of wrapping wires, I completed the wiring of my COSMAC ELF replica. At that point, I flipped it over and stuffed the various parts into the sockets. The ICs installed without problems, while the discrete components required a little bending and trimming to fit into the sockets but were not really a problem. Two 0.1" jumpers (a.k.a. "shunts") were required by the design yet not included in the package, but luckily the local Radio Shack is still in business and had them in stock. With everything in place, I attached the battery, flipped the switch, and...nothing happened! Now what?

Follow The Process

The exact procedure for loading code in to the COSMAC ELF is not entirely intuitive, so I reviewed all of the available information to ensure that I was following the correct process. Convinced that I was trying to do the right things and yet still not getting any results, I broke out the schematics and started my analysis...

After basic circuit "first aid" like checking power and ground lines and ensuring that the clock was ticking, I started trying to isolate specific bits of functionality on the board to make sure that flipping switches was sending the right signals to the correct pins on the CPU. While poking around, I found that the output of one of the inverters did not change when it was supposed to change. A bad chip? Well, not so fast...

Chip Test

Long ago I picked-up an EPROM programmer that includes the ability to test logic chips, including a variety of 74xx and 40xx chips. I took advantage of this piece of kit by testing the 40106 chip that seemed to be producing the wrong output. Oddly enough, the chip worked fine in the tester! I took the precaution of testing most of the other logic chips on board, all of which proved to be working fine. Hmmm...

The culprit...
Circuit Inspection

Reassured that the chips were working fine, I refocused my attention on the possibilities of bad wiring. Not surprisingly, I found one wire out of place. Despite having checked along the way during construction and further checked all the connections afterwards, one wiring mistake had escaped detection. Oh well, at least it was just the one!

With that one wire removed and replaced correctly, the machine is working! Too bad I don't have any COSMAC ELF software lined-up for Halloween. But for your amusement, I have included my recreation of the classic COSMAC ELF construction photo above... :-)

Anyway, this project is mostly done. I'll probably still wire-up the expansion port and maybe secure a few bits here and there with some hot glue or something in the near future. Hopefully there will also be some bigger and better ELF or RCA1802 projects in the future as well. But no matter what, I hope that you will continue to stay tuned!

Tuesday, October 25, 2016

RC2016/10 -- Wiring Complete

October is winding-down, and with it the Retrochallenge RC2016/10 event. As we round the last turn and start the sprint to the finish, how are things going? Well, not too bad -- the birth of my new COSMAC ELF is coming along well... ;-)

Wiring Wrap-Up

As predicted, learning the skill of wire wrapping was not without its hardships. I think I have achieved basic competency at this lost art, but I am still far from comfortable with the technique. Just as I settle into the groove of things, usually I will hit a run of a dozen premature wire breaks in a row or something equally frustrating to the builder. Nevertheless, the directions provided in the VCF ELF kit and a little patience and diligence have allowed me to complete the basic wiring for the new machine.

Just before wiring the toggle switches...

Pre-Flight Check-Up

With the wiring completed, it is tempting to charge ahead to power-up! However, prudence demands some basic testing be done, lest one enjoys the smell of "magic smoke". Using the audible continuity checker on a multimeter, I ensured that the pins at each end of every node were connected. Testing between the outside pins of a node is a shortcut to prevent walking pin-to-pin testing for continuity at ever point along the way. This shortcut should be fine...hopefully... In my hubris, I have neglected to do any sort of extensive checking for short-circuits -- wish me luck?


At this point, there is little left to do but to stuff the parts into the sockets on the board and apply power. Considering the risk of shedding tears at that point, I may delay a day or two just to ensure that I have had a chance to remember anything I might have forgotten! I need to line-up some simple ELF programs to use for testing as well. But really, at some point we must proceed...

There is only so much time left in this competition and little excuse not to proceed at least until power-up. Whether that turns out well or not, I suspect that I will see this project through one way or another. If you are reading this, then I imagine that you want to see this ELF in action...well, stay tuned!

Friday, October 14, 2016

RC2016/10 -- Halfway @ Half-time...

So it turns-out that walking through a wire-wrapping project isn't as thrilling as one might imagine. Hopefully the end of the project will enable new COSMAC adventures, but the steps along the way are mostly a 'lather, rinse, repeat' sort of process -- one wire after another, for dozens or hundreds of wires. But it has enabled me to build the paleotechnical skill of wire wrapping, allowed me to exercise my engineering sense to discover a mistake in the docs I was provided, and introduced me to some new friends on the Internet.

Some Progress
Building Skills

This project is my first time using wire wrap as a construction technique. While the process seems simple, it is not without its pitfalls. Cutting the wires to the proper length requires some expertise, as does proper placement of the wire in the wrapping tool, and proper use of the tool to minimize wire breakage. Good judgment about wire routing and proper use of wire colors also contributes to project success. I am reasonably pleased with the project so far, but I can only imagine that more practice will yield some improvements. As it stands, I am not completely convinced that this technique is in any way better than point-to-point construction with solder...

From PIN6 of IC1?
Finding Mistakes

The documentation included with my VCF ELF kit contained a wiring list for use while building the kit. This list includes each individual wire, grouped by circuit node. Each node's list reads something like "from A to B; from B to C; from C to D". As I reached the node for the /LOAD signal, I saw something more like "from A to B; from B to C; from D to E" -- notice the disconnect? That made me check the schematic, which made it clear that the wire list was wrong. But what is right?

The wire list showed the next to last wire as ending at "PIN6 of IC11", and the last wire as starting at "PIN6 of IC1" -- notice the similarity? Checking the schematic confirmed that starting the last wire at "PIN6 of IC11" makes sense, so I went with that. Later I found that there was an updated wire list at the VCF ELF website which confirmed my correction.

New Friends

For years I have been a member of the COSMAC ELF and 1802 Microcomputing Yahoo group. Until now, I have had little reason to actually interact with the individual members. I was curious about the RCA1802 and the COSMAC ELF, but I didn't have any hardware to use. Now I'm building this kit, so I made the members of that group aware of the project. So far they have been friendly and responsive, taking a healthy and welcome interest in my project. It's always good to make new friends!

More Progress
As I write this, the month of October is almost halfway over. That means half of the RC2016/10 event is over, and half still remains. Will I finish my build? Will it work at the end? If you want to find out, then I guess you will just have to stay tuned... :-)

Monday, October 10, 2016

RC2016/10 -- A Good Start...

So, I've been a bit slow with my updates -- I could blame the extra duties involved with running the contest, or I could just blame laziness.  Which one is more believable? :-)


Before any wires can be wrapped for the VCF ELF, some preparation needs to be done. The base platform for the wiring is a piece of "perfboard", which serves as a framework for holding the sockets and other parts in a fixed physical configuration. In order to facilitate placing the various parts, the kit creators included a paper template that is designed to be laid on the back of the perfboard. This identifies the location of each part and explicitly labels the numbering of each pin, making this information available from the back (where it is needed when wire wrapping). In addition, the bottom of the template folds back over the front of the perfboard to provide some simple graphics for the "console" area of the board. This was a great idea, but...

Check The Scale

Laying the printed paper template provided with the kit down on the perfboard that came with the kit revealed a problem -- the paper was not printed at exactly 1:1 scale! Since the paper was ostensibly designed to match the physical layout of parts exactly, this rendered the paper template a bit useless. Fortunately, the kit creators have made the template graphics available as a PDF at their site. I was able to print a new copy of the template, this time at the correct scale. Using this template I was able to get started with placing parts on the perfboard.

Fit And Finish

With the template in place, I started poking socket pins through the perfboard and the paper template. I added the side rails, and continued placing most of the parts necessary for the wire wrap process. The only real problem for me was making space for the toggle switches.

Toggle switches are used for a variety of purposes on the VCF ELF. These purposes include turning the machine on and off, enabling the memory protection circuitry, and controling execution of the program on the machine. Most prominently the row of switches at the bottom are used for entering addresses and data while loading programs. Unfortunately, all those toggle switches need holes drilled for mounting them...

Drilling the holes is no big deal per se -- just drill the hole! Even "good enough" alignment with the spaces allocated on the paper template is reasonably easy to do. However, aligning holes for eight switches in a row is a bit too much for my skills with a hand-held drill. I would like to think that I might have done better using a drill press...maybe... Anyway, I got the holes drilled. Alignment is less than perfect, but that won't effect the operation of the device!

Overall, the assembly process was relatively simple and a necessary step. Nevertheless, it did take most of my available time for the first day of work -- I failed to anticipate the prep time! At least at this point I was ready to move onto the real wire wrap work. If you wanna hear more about that, then I guess you'll just have to say tuned...