MSX COMPATIBLE BOARDS FOR RC2014

Next module to be downsized, is the YM2149 Game Module

Here is the new module in the Stegosaur series. The Stegosaur YM2149 Game Module.

Like the other modules, it is an alternative to the Yellow Module. With some difference - and the big visual difference is the height reduction.

Let me highlight some of the key changes:

Pseudo Stereo Output

By popular demand, I have updated the design to incorporate the ‘golden ratio’ stereo mix. I am not 100% sure where it first appeared - I see references to https://hw.speccy.cz/ayinterface.html - so not 100% sure who to credit. Many kits based on the YM2149/AY-3-8910 chip have implemented this output circuit.

The chip has three separate channel outputs (A, B and C) - and by using some resistors dividers to spread the signals across the left and right channels, we get a very full wall of stereo sounds for our games and music playing programs.

Updating the GAME module to produce stereo output, also meant I needed to update the design of the MSX Music module to pass-through this stereo mix. So its design also had to be updated. More on that in the next post.

Controller Connectors

In comparison to the Yellow MSX module, I remove the game controller DB9 connectors to an optional expansion board.

This new revised packaging should help anyone who wants to place their kit in a custom designed case. As most games and applications will work just fine using the keyboard, the lack of a controller interface is not too big a deal.

Optional Support for AY-3-8910

An extra jumper and an optional on-board clock divider circuit, and we can support the very similar and compatible chip - the AY-3-8910.

Dropped the on-board clock

In the interest of saving space and simplifying as much as possible, I removed the on-board clock generator. This module is expected to get a clock signal from the main bus (CLK1 or CLK2). Typically that will be generated by the V99x8 RGB Module - which will be used by this module and some other modules of the series.

I have more details of the module and schematic up on my site now: https://www.dinoboards.com.au/stegosaur/game/

And its listed on my shop for sale as a kit https://shop.dinoboards.com.au/product/stegosaur-ym2149-game/

Next module to be downsized, is the Turbo CPU module.

I manage to reduce the PCB size, by removing the on-board slow clock, and relying on an external slow clock source, such as a RC2014 Dual Clock module, or the RGB Video Module.

I am particularly proud of this module – as it was perhaps the first design I did that wasn’t so much based on stock MSX schematics.

For those that don’t know - the Turbo CPU module runs a standard Z80 40 pin chip (Z84C0020PEG), at up to 20Mhz. Its not overclocking it - nor does it require the backplane clock to run at 20Mhz. It dynamically switches the clock of the CPU between the bus-clock and its 20Mhz turbo boost speed.

The clock speed of the CPU is switched to the bus-clock, when the IOREQ line goes low. It holds the processor at the ‘slow’ bus clock for 31 slow clock ticks.

This means when the processor is communicating with any I/O device, its running at the modest bus clock speed - to ensure compatibility. But when its just ‘processing’ or reading/writing to memory - it runs at its boost speed.

This turbo boost process gives it a lot of compatibility with existing devices - but can really improve those very slow IX/IY+n operations a lot.

It was an interesting challenge, to do the clock switch - can’t just do an immediate switch between the two clocks. You have to ensure when it transitions from one clock to the other, that its not done too quickly, that we end up momentarily having a super high frequency sent to the CPU’s clock input; the 2 clock sources (20Mhz fast and the standard bus clock) are not in phase - so when switching it would glitch unless it waits. When transitioning, the clock is ‘stretched’ a little until we sync up with the other clock.

This module will work just find as a replacement for the stock RC2014/RCBus Z80 processor modules as well as a perfect speedy CPU solution for the MSX2 configuration.

I have more details of the module and schematic up on my site now: https://www.dinoboards.com.au/stegosaur/turbo-cpu/

And its listed on my shop for sale as a kit https://shop.dinoboards.com.au/product/stegosaur-turbo-cpu/

It’s a new year, and I have some new modules under development.

I have started to look at updating my Yellow MSX series with new PCB designs that fit the more conventional RC2014/RCBus height of around 5 to 5.5 centimeters.

The original Yellow MSX PCB have a height of around 8cm - near double the typical height. The greater height was generally required, as I needed the extra PCB space to fit all the components for each of the various modules.

In particular, the V99x8 module is quite a challenge - the 64 PIN DIL chip takes up almost half of the PCB area of a conventional height of ~5cms. Then when you add the RAM chips (4 of them) - well its just not physically possible.

That is, unless I reduce the RAM to just 64K (down from the 128K). With just 64K, it is possible to fit all components onto the smaller PCB. The lower RAM still allows the VDP to displays all the game graphic modes, just with less RAM available for paging - so some games might not work - but I think many still will.

But I still want to support the 128K of VRAM - and in fact, it would be very nice if I could extend support to the VDP’s maximum of 192K VRAM.

What I ended up designing is a main board - at the 5.5cm height, with support for 64K - and an ‘expansion board’ that allows the module to support the full 192K. The height is still the same, but the width is increased a bit.

Here is a picture of the new design, with the 64K RAM on a single 5.5cm PCB:

And here is it with the expansion installed:

With the expansion board, its a tight fit - and the width of the 2 combined PCBs does extend beyond the RC2014/RCBus’s typical spacing. (If the RAM was soldered directly and not socketed it would probably fit in the BUSs’ standard spacing).

I am calling this series of boards - the Green Stegosaur Series. The intention is that the series will be compatible and mostly the same as the Yellow MSX series, but with the smaller height. It will probably necessitate that some modules are split into 2 - eg the MSX Memory module will definitely need to be split into 2 separate PCBs. So this series will probably require more slots on the backplane.

If you are not interested in MSX, though, this new Video Module’s form factory, might be a better fit for your RC2014/RCBus rigs.

I have more details of the module and schematic up on my site now: https://www.dinoboards.com.au/stegosaur/v9958/

And its listed on my shop for sale as a kit https://shop.dinoboards.com.au/product/stegosaur-msx-v9958-rgb/

Art is never finished only abandoned

Don't worry - I haven't abandon the project.

I've been working on the MSX Music Module (and listed a new revision in my Tindie store).

This revision further reduces some of that annoying digital background noise that could be heard when no music or sound playing. Its still emanate some background noise -- I think it be very hard to eliminate as so much of the various RC2014 modules will generate a lot of EMF noise.

The latest revision (1.9), increases the gain of the op-amp stage and adjusted some of the capacitors on the feedback. This seems to improve the signal to noise ratio, and it also improved its higher frequency responses.  Its sound much brighter.

Updated schematic on the github

Can you tell the difference between these modules - they are both sold as CH376 Modules?

Both were purchased from ebay/aliexpress traders. Both are sold as CH376 modules.  Both have the same CH376 chip.  

The one of the left though, has a different pin out.  Notice the 2x3 header pin.  Whereas, the one on the right only has a 1x3 pin header.  And just to add to the change,  the main headers have been swapped.  For the left/original, the outermost pins are the data lines, and for the right, its the inner most pins.

This alternative form-factor is not compatible with my PCB design.

On my Tindie store, I had packaged up some kits - and only at the last minute did I notice this very different pinout/form factor change.  I very nearly sent out kits with an incompatible module!  

I have not been able to source any more of the module as shown on the left.  Only seem to get the one on the right.  

It now seems that the ebay/ali-express traders are only selling the one on the right -- even if their product page shows the original 2x3 pin header.  You just can not trust what you will end up getting. 

So I had to redesign the PCB for my Cassette+USB kit to accommodate the change.  As this is just a form factor/pin out change - the software and electronics all still work the same.  Only need to re-route and reposition the cut outs on my PCB.

So now I have 2 PCB versions for this kit, to accommodate the 2 form factors:

I have just assembled a kit using this new module form factor - and so far all seems to work just fine.  I do need to do a bit of testing to double check everything.  So if u are waiting to get one of these kits - I can hopefully have new stock listed next weekend.

I have been updating the USB software over the last week or so and making some progress with support for additional hardware types.

USB Keyboard

I managed to get a USB keyboard to work.  At this stage, it has very limited functionality.  (arrow keys for instance don't work).  But I was able to boot and control the kit without the matrix keyboard attached - using a USB keyboard only.

Almost certainly, few games would work with the USB keyboard, but MSXDOS/NEXTOR and CP/M apps seem to work just fine.

The kit would need the PPI card (part of the keyboard kit) as without the PPI, the system will not boot. 

I still need to add extra support - arrow keys, caps lock -- and I do wonder if I am able to control the keyboard's capslock LED.

FTDI Serial Adapter

This one is interesting.  Have a couple of USB/Serial adapters I've used to connect my PC to the original RC2014's SIO/2.  But I thought, can I now go the other way?  

But this USB adapter is a vendor specific class.  That is, the USB protocol to control the adapter is not part of the USB specification - its the manufacturer's proprietary protocol.  I could not find any published specification - but there is the public Linux driver for it. The code is quite simple, and so by reviewing that code, it was quite easy to figure out how to configure the adapter and send/receive data over the serial chip.

This is still very early in the development - managed to get some experimental code to do a loop back test - (connect the TX/RX lines of the adapter).  The code confirms what it sends is also what is receives.  Have this operational at baud rates up to about 38400 - I think I can go higher!

I've only coded for the specific FTDI 232R chip - as I don't have any other chips to test and verify.  Including from other manufacturers. 

Also I am starting to run out of ROM space.  All my USB code, mostly written in C using Z88DK/SDCC compiler needs to fit in about 16K.  I am at about 15K!  All part of the challenge.

Okay,

This one has been quite an effort to get here.  But I finally have a version of the Cassette/USB module completed.  About to list the product on Tindie.

It was a huge effort developing the software for it.  There was a lot of 2 steps forward, 1 step back, 1 step side, and a few loop-do-loops!

Learning how USB protocols work for floppy, flash thumbs drives and old magnetic external HDD has been quite a journey.

After this massive effort, I can now :

1. Load a BASIC program from cassette (and save), then save it to a USB Flash drive. 
2. Boot MSX-DOS/Nextor from a flash drive (and use the same drive on my PC for quick file transfer)
3. Read and write to 1.44MB and 720K 3.5" Floppy disks
4. Format the floppies

And also - something that I think was kind of cool.  I got a USB to Centronics adapter and can now print to my 9pin dot matrix printer.

I have released on new ROM image on GitHub, that has all the new drivers and software.

More info at: https://github.com/vipoo/yellow-msx-series-for-rc2014/tree/dev-msx-usb-4/cassette-and-usb#msx-cassette--usb-module-designed-for-rc2014

Product will be on Tindie soon:  https://www.tindie.com/products/dinotron/msx-cassette-usb-module-designed-for-rc2014/

Its been a while since I posted here about the project. 

I have been really struggling to find the time (the usual suspects, work, life etc)

First, I want to express my thanks to everyone who has taken an interested in my little project, liked/commented here on Hackaday or purchased some of my kits.   Your encouragement and support is fantastic and much appreciated.

And with that, I was finally able to dust off the soldering iron and get back into developing and updating stuff.

I am working on 3 main tasks at the moment.

1. A new revision for the Video Module to support the V9938 chip.
2. A new revision of the MSX Music module that reduces the idle background noise caused by nearby digital signals.
3. And a new kit under development, the USB Module (based on the CH376 module). 

And I also managed to restock my Tindie store.

V9938 version of the Video Module

When I was restocking my store recently, I needed to source some new V9958 chips.  I found that the price for the chip has gone up a lot - at about $45-$50 USD.  That seems like its doubled in price since I started this project.  This would have been a better investment than an AI company! 

But the previous generation video chip, the V9938, still seems to be generally easy to source and at a lower price.  

One day though, these chips will become virtually impossible to get.

Given this, I decided to update the Video kit to enable support for the V9938. (I guess that makes it more retro!).

I also redesigned the power filtering and re-arrange the layout a bit to help reduce noise leaking into the video signal paths.

The V9938 has some minor differences to the V9958.  

It has features that were removed in the V9958, such as a mouse interface (I don't think was used very much) and direct support for a composite video out.

Graphically the V9938 does not support the 'near true' colour screen mode.  It also lack the support for smooth horizontal scrolling that some games took advantage of.  But it still offers a very similar capability, including all the other screen modes, sprites and memory support.  Its programmed in the same way, so the software and games just work.

Hopefully I can have this new revision listed in my Tindie store soon - with schematic and more details to follow.

New Revision of the MSX Music

One of the things I was less than pleased with this kit, is its susceptibility to the digital noise and nearby EMF interference, causing buzzes and hums to be heard when no music was playing.  This was especially noticeable when running the Z80 at the 20Mhz speed. After experimenting with a few inductors and capacitors, I managed to isolate a good chunk of the noise.  Its now much better.

I am perhaps starting to run the YM2413 audio chip at closer to its minimum power specification.  The low pass filter applied to isolate the digital noise, does drop the voltage a little.  If you combine perhaps a slightly lower than 5V source, some general loss in the backplane power rail - we start to approach the minimum spec voltage of 4.75.  But I think there is enough headroom to ensure its always above spec.  I have done lots of testing, and all seems rock solid reliable. 

The USB Module

This has been my main focus over the last few weekends.  The amount of effort applied to get this working is rather ridiculous when I think about it.  I was beginning to wonder if I was insane trying to get this to work.  Most of the effort is in the software.  I have had to learn a lot about USB protocols as well as the specific USB floppy and thumb drive protocols.   Not to mention figuring out how to enumerating devices plugged into hubs.

The CH376 module seems capable enough, but the documentation did leave me pondering a lot.  

Trying to fit all this USB protocol logic into a page of the ROM (approx: 8K) was also a challenge.  The poor Z80 was never meant for such heights.  But it continues, like me I suppose, to learn new tricks!

So far, I have implemented support for USB Hubs, up to four storage devices (mass storage thumb drives and other USB HDDs), floppy drives.  And just last week, I managed to get my MSX to print to a 9 pin dot matrix Centronics printer, using a USB to Centronics adapter cable.  So now my lab echoes with sounds of impact printing.  Oh Joy!

The current PCB requires a few bodge wires, so I need to get a new PCB manufactured and tested.  Stay tune for more details on its development.

If you have read all the way to hear, thanks - i hope you found it interesting...

I will try and keep updating the journal and provide more details of the project updates.