Memotech Multi-Function Expansion System

MFX Engineering

MTX CFX Boot Screen

CF Card Directory

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Engineering

This page describes the process of turning Martin's prototype design into a product that we can make available for others to purchase - assuming that there is interest of course.

I should mention at the outset, that one my goals was to design something that, as well as being technically sound, also worked well aesthetically. The Memotech MTX was one of the best, if not the best, looking of the many home computers of the early 1980s and I think that hanging devices off the end of it on the external connector really detracts from its sleek lines.

At the outset, I decided that I wanted something that would be mounted internally and any I/O connections should be as unobtrusive as possible. This inevitably means that the size of the board will be considerably larger than is strictly necessary if only the electronic components were to be mounted on it.

During prototyping, Martin had confirmed that the FPGA development board could be mounted onto a custom PCB to interface to the MTX and fit below the MTX keyboard, but only if the PCB was level with the MTX computer board. The edge connectors that I have used for our previous MTX expansion boards are right angled, which meant that the PCBs are at a different level to the MTX computer board.
It also worth mentioning the way that Memotech designed the internal edge connector does not lend itself to connection! The PCB at both ends of the connector is flush with the connector's edge. As you can see from the edge connector photos, this means that the ends of the connector must be modified to allow the connector to fit over the edge of the board.
It's not as if Memotech used a different type of connector back in the day - all of Memotech's expansion cards had their connectors modified to fit. This close up of the base of an SDX shows how Memotech sliced off the ends of longer connectors to expose the opening between the slots to allow the connector to slide over the PCB.
Straight connectors similar to those used by Memotech on their expansion boards are very rare these days. We did identify a suitable connector made by EDAC, but these are "order on request" items with an 18 week lead time from Digikey, along with a minimum order quantity of 25 with a total price of £150. Once MFX looked like it would likely fly, I decided to take a punt and order connectors, rather than have to wait months once the design was done. (I placed the order in April 2022, by July, the price had more than doubled!)
This "recommended daughter board" drawing from the EDAC sheet highlights where MTX made things difficult for themselves - and for anyone developing modern day add-ons for the MTX. If they had left space to attach the connector, it would have meant that butchering the connector plug wasn't necessary.
On to the board deign . . . . All of the add-ons that I have built for the Memotech MTX computer have been created using KiCad, in its own words, "A Cross Platform and Open Source Electronics Design Automation Suite". MFX will continue that journey . . . In addition to the FPGA and I/O connections, the board has:   512kB SRAM for MTX RAM expansion   128kB Flash for ROM images   32kB SRAM for VDP shadow RAM for the VGA output   GAL22V10 for memory address decoding   74HCT245 for level shifting between FPGA and MTX   3 x 74LVC244 for level shifting between MTX and FPGA   WIZ810 network module
The first step is to take Martin's "blocking" diagram and create a "proper" circuit diagram using KiCad's schematic editor. The first rev of the schematic
Once the schematic has been "finalised" and checked for any errors, footprints are assigned to the components and board is designed using KiCad's PCB designer. The tool has an automatic component placement feature, but I find that it takes ages to run and get better results when I manually place the components, taking into account the main connections between them.
I use an older version of the FreeRouting tool created by Alfons Wirtz. Currently, I run it as a standalone Java application, but will upgrade to the latest version soon. The Autorouter tries to route signals it finds in the KiCad netlist and, if successful, goes on to optimise the solution, which reduces the number of vias in the board.
I find that it is an iterative process - I probably never get to an optimal solution, but I can usually quickly see where improvements can be made and move things around accordingly until I get to something that I am happy with. Spot the difference ? (Here, I have moved a couple of the level shifters around and removed the board mounted VGA connector)
This layout is Eurocard sized (160mm x 100mm) and has headers for the WIZ810, SD card module and VGA connector. It pretty much meets our initial aspirations regarding board size. Mated to an MTX computer board, it would sit about 70mm away from the right hand end plate, meaning that wiring to end plate mounted I/O connectors would require internal cables. It would be possible to run cables for the network and VGA monitor out through the rear of the MTX case, but this would be rather messy and short adapter/extension cables would add some cost. Martin and I had some discussion on the best way of facilitating connection to the PCB and decided that a slight change of approach was needed. Rather than minimising the PCB footprint (and cost), since any extra space inside the MTX would be pretty much wasted, it made more sense to dispense with the need for cables and make the PCB wide enough to reach the end plate. This would end up with a PCB of similar size to Memotech's internal 80 column board, slightly increasing the cost, but this would be more than offset by the savings made by not requiring additional cables.
For comparison, here is a photo of the Memotech internal 80 column card, used for the SDX CP/M video output. You can see the connectors for RGB and composite video, as well as a hole for an external power supply (which was not used unless the 80 column card had the DART option installed). We had toyed with the idea of including a second bus connector to allow MFX to be connected externally, but despite the larger sized PCB having ample space, having a board this size hanging off the end of the MTX is silly, so the idea was dropped.
For development, Martin's FPGA board was powered from an external DC PSU. Since MTXs and/or their PSUs can have some variability in their power characteristics, we checked how Martin's system performed when the FPGA was powered from the MTX. This resulted in a small amount of "wobble" on the VDP AV output. This may be unique to Martin's MTX, but provision for an external 5VDC supply was added to the PCB design.
Draft layout of a square, rather than rectangular, board. Although there appears to be a large amount of wasted spaced, a design such as this would allow direct connection of the I/O through the right hand end plate. An added benefit is that there is no need to place components where the MTX audio and video connectors are on the rear panel. The fixed VGA connector has been reinstated, along with a barrel jack for an (optional) external 5VDC power supply.
Martin suggested that the VGA connector could be made accessible through the RS232 port cut-out and 3D printed a possible insert to fill the cut-out. Unfortunately many VGA cables are too bulky to fit into the RS232 recess. We considered a couple of ways that this might be made to work, but decided that end plate access was definitely the way to go.
I spent quite a bit of time trying to decide what type of SD card module I was going to use, whether full size or TF sized; whether bare board or something more complex? Martin had used a bare SD slot in his prototype, the only addition being a bypass capacitor for the power supply to it. Breakout boards like this are quite common and have a footprint not much bigger than the SD card slot itself. However, many SD modules have quite a few passives fitted and I wasn't sure why.
Andy had used a board like this for REMEMORizer. The SMT components are a voltage regulator, a few resistors and a few capacitors.
The voltage regulator is there to allow the 3V SD card to be interfaced with 5v devices (this version has no level shifting on the data lines though.) In this image of the circuit diagram, you can see that resistors are used to pull the SD card data lines up to 3.3V. I was not able to find a definitive answer on the web as to whether these are required or not. Martin's system obviously worked without them and from what I have been able to find, it is not an SPI requirement. It seems that some SD card manufacturers specified pull-ups for their SD cards. It may be that some older SD cards need them, but newer ones do not and many SD cards seem to have weak internal pull-ups in the card. In any event, I think it safer to include them than not, so I will be making sure that the PCB can cater for this, larger sized, type of module.
A very small number of the Chinese suppliers of these modules ship them without the header pins attached, but the majority fit the headers to the top of the PCB. So as not to restrict the number of sources of these module, I will use the more common ones with the headers preinstalled. This means that the SD card module will need to be mounted on the MFX PCB upside down - as Andy did with REMEMORizer.
The SD card modules with components on them are relatively long, at around 35mm, which means that if the module is not to be permanently soldered to the PCB, a header socket on the PCB needs to be some distance from the right hand edge of the PCB. The restricted clearance below the MTX keyboard PCB means that there is not enough headroom to mount an SD card module as it is shown in this first drafts for the larger PCB.
To get clearance for the SD card module, it was necessary to swap the positions of the VGA connector and SD card module. The VGA connector can easily fit in the shallow part of the case as the keyboard PCB stops some 30mm short of the right hand end of the case. Moving the VGA connector closer to the front edge of the MTX end plate was not ideal, but appeared to be the best option. With the components and I/O connectors placed, I felt that I was pretty close to getting a trial run of boards made. However, whilst this was not going to be a first for me, I am still a novice at this stuff and was sure there was lots of scope for improvement.
You can see in this photo of a MTX RAM card and an RS232 board, that although Memotech did not have ground planes, the expansion boards have large ground strips at the top and bottom of the board. There are "holes" in the solder mask on these strips to allow the ground to contact the case metal work. Memotech were not consistent in their use - neither the MTX computer board or the 80 column board had these strips.
Many of the 2-layer hobbyist PCBs that I have seen include ground and/or power planes on the top/bottom of the board. I think that this helps with noise reduction, particularly for boards that operate at much higher frequencies than the MTX. It appears that good design practice is to have at least a ground plane on one or both sides of the board - this has been added here. There is not 100% coverage, but additional VIAs have been added to fill the major void spaces left by the auto-fill algorithm in KiCad. I also removed areas of the solder mask at the board edges to allow the ground to contact the case - though I'm sure they'll be of no benefit here.
This mock up shows how a 3D printed end plate might look with apertures for the I/O connectors placed to suit the PCB above.
"Final" version of the PCB design, reflecting the changes shown on the rendered image above
OK, first draft done - including needless complications such as ground planes and solder mask voids as described above. Now I just need to bite the bullet and submit an order for PCBs, then a few weeks later - find out how many mistakes I've made along the way! I will likely wait until I have a definite shipment date for the EDAC connectors and have gathered together more of the components that I'll need before I "push the button" on the board's manufacture, so there is still a little time for "tinkering".
Here we go ! I tried pricing up the board with the PCB manufacturer that I usually use, but found them to be quite expensive for this size of PCB. There are a number of different Chinese PCB manufactures that cater for hobbyist projects, and by shopping around a did find that most were cheaper. Time will tell if the one that I chose has the quality of my previous supplier, but I wanted to minimise costs until I had a proven design.
This supplier also has a neat on-line visualisation tool - here is their representation of the PCB. Now I just need to wait for delivery ! Damn ! Whilst I way playing with the online viewer, I realised that I had made an obvious mistake and the boards would have at least 1 issue
Can you see the problem ? . . . . . . . Looking at the reverse side of the board, you can see that the solder mask layer (grey) is missing on the edge connector pads :-( When I cloned the pads on the footprint's component side for the solder side, I didn't change the mask layer from front to back! It will be difficult to remove the mask from the pads, but we'll see if it can be done when I get the boards. But I am not optimistic . . . .
This image, from pcbgogo.com shows the structure of a typical 1.6mm PCB like the one ordered. The copper layers are "1 oz" thick, equivalent to 35um, 1.4 mils or 0.035mm! There won't be much copper to play with when I try to remove the solder mask.
By design, the solder mask layers are difficult to remove, it can be done using some quite harsh chemicals, but I have read that good results can be achieved by mechanical means, i.e., scraping, using, for example, a fibreglass brush such as this one from RS. I have one of these in my toolbox; they need to be used with care as they shed tiny shards of fibreglass which can cause irritation and pierce the skin, but this is what I am planning to try.
In the meantime, the EDAC connectors from Digi-key arrived - some 4 weeks ahead of the projected delivery date. The size of the package took me somewhat by surprise! To show scale, here's a photo of the box on my computer chair - the only contents were the 25 edge connectors that I had ordered!
The EDAC shipping carton looked like it was designed to hold 300 connectors in two layers of 150 each. It appears that they don't expect smaller order quantities such as mine.
Front and rear view of the connectors. The datasheet advises that the contact material is copper alloy with "Gold Plating on mating area and Tin Plating on tails with Nickel underplating all over".
The PCBs have arrived Component side of the board. It looks OK and some brief electrical checks were fine, but I need to check positions of the components - particularly the alignment of the headers for the FPGA module.
Solder side of the board. With the solder mask coating the edge connector fingers as described earlier shown.
I managed to removed the solder mask from the edge connector, but found that the fibreglass brush struggled to get through the coating. I used a new Stanley knife blade, removed from the knife and scraped off the coating, using the flat of the blade, trying to avoid digging the point into the copper trace. The KiCad ground plane fill had placed some narrow ground zones between a couple of the fingers and I was a little worried that removal of the solder mask might make it hard to solder the connector without creating bridges to these areas.
I did consider scraping off these thin areas of ground plane between the fingers, but decided not to. The close up photo shows some small remnants of mask that shouldn't be an issue, I didn't want to go too far and damage the thin copper, but it was actually much more robust than I had expected.
Edge connector soldered to the board. Even with the exposed areas of ground plane between the fingers, I managed to fit the connector without creating any solder bridges. The board would look better without the EDAC label being so prominent - next time I will mount the connector with the EDAC label on the bottom.
The headers for the FPGA seemed to be in the right place, there was nothing else obviously wrong, so I made a start on assembling the board. I was still waiting for some items to be delivered, mainly the SD card modules, so there was not much further I could go until they arrived.
However, I could at least test that the ROM was booting and the RAM expansion was working. ROM, RAM and PAL installed with the MFX PCB pinned for a MTX500 where the PCB was installed. At this point, you can see probably the largest (by area) memory only upgrade board ever installed in an MTX !
Screen shot of the MFX boot screen displayed on the VDP output The system is reporting 512kB of RAM - the maximum useable. Although 544kB is installed (32k + 512k), this is constrained by the number of available terms in the GAL22V10. The serial number is reporting "noise" as the ROM cannot read valid data from the FPGA.
However, with the FPGA installed, the boot screen displayed on my VGA monitor was pretty much unreadable. Martin did not see the same level of corruption on his screen, but he was using faster VRAM. When when using the same speed as me (70ns), he got the same results. (Martin also noticed that I had made a minor error on the VGA pin-out - another correction to the PCB was needed.) I ordered up some faster RAM (Alliance AS6C62256-55) but my testing was pretty much stalled until it arrived.
With the faster VRAM, the VGA output from Martin's board was much better, but did exhibit some noise that was not present on the prototype board. On the boot screen, this noise appeared as "sparkles", typically looking like a single pixel line in random character positions as shown here. These "sparkles" were noticeably worse after an attempt was made to read the SD card from SDX BASIC.
The biggest concern though was that reading the SD card was very unreliable, more often than not, the system failed to boot to CP/M and operation of the SD card in SDX BASIC mode failed too. We lost quite a bit of time here, trying to get to the bottom of the SD card issue. During the development of the prototype board, Martin had some issues with noisy signals interfering with the operation of the SD card, but that was resolved by tidying up the wiring and removing an unused header. Although there was a little noise on the PCB SD card signals, it was trivial compared to when the issue was seen on the prototype.  The most obvious difference between the prototype and PCB boards was that the PCB was designed to use one of the common SD card modules with pull-ups on the data lines.
To eliminate this as a possible cause of the problems, Martin removed them from his SD card module, but this did not make a significant difference.
Examples of the quality of the SD card data signals, demonstrating that there is no appreciable noise on the data lines. (I had made a minor change to the FPGA I/O allocation to help with routing of the signals for the PCB, but none of these were associated with the SD card and in any event, the properties of the relevant FPGA I/O pins was identical. It turned out that, while we did consider this as possible cause of the issue, it proved to be a red herring.
The 'scope traces show the MTX CPU clock signal on the input and output sides of the level shifter. The raw clock signal has a relatively large overshoot on the falling edge but this is likely not an issue and is much better on the level shifter output to the FPGA. Although there was no indication that there was a problem with the clock, we discussed the differences between the SD card implementations in Andy's REMEMOTCH and REMEMOrizer. In REMEMOTECH, the whole MTX is synthesized in the FPGA and the SD card is clocked by the FPGA at 25MHz. In REMEMOrizer, the SD card is clocked by the MTX CPU at 4MHz.
The SD interface in MFX was changed to mimic that it REMEMOrizer, i.e., it was clocked at 4MHz and the data transfer synched to the MTX clock. This resolved the SD card issues. This was particularly puzzling as the SD card worked without issue on the prototype, the interface between the FPGA was simple and the PCB checked out fine. However, since the switch to the REMEMOrizer like implementation resolved the issue, there was no real need to investigate further. The video noise ("sparkles") was equally puzzling - the video on Martin's prototype board was perfect but building the hardware on the PCB had made things worse! We spent a lot of time checking that the PCB was consistent with the schematics and Martin's blocking diagram. Martin also spent a lot of time checking for noise and other issues but was unable to spot anything untoward. The eventual "fix" turned out to be quite interesting too. The FPGA allows the user to configure a range of drive strengths for the I/O pins. The range for LVTTL (3.3v) is from 4mA to 24mA with the default at the higher level. Tucked away in the device data sheet is the text "Using minimum settings provides signal slew rate control to reduce system noise and signal overshoot ". When Martin changed the output pin configuration from 24mA to 4mA, the "noise" issue magically went away. Again, the fact the prototype board did not have the "sparkles" problem was interesting, but as the issue now appeared to be resolved, no further time was spent on it - following the guidance in the data sheet seemed to do the trick!
The fully assembled PCB, with all its sub-modules fitted     The large blue board is the FPGA module     The smaller board is the SD card module     The white module is the WIZnet network adapter (The FPGA module arrived with a few areas of excess solder flux, it looks a bit ugly, but as it does not affect its operation, didn't bother to try and clean it off.)
Installed in the MTX
End on view, showing the fit of the FPGA, WIZnet and SD card modules below the MTX keyboard PCB. As anticipated, it's neat, but it fits with no problems.
Just as we thought that we were done, a really worrying issue came to light. Although the WIZnet interface seemed to be working fine, we found that intermittently, files transferred to the MTX over using FTP had errors when they were saved to the SD card. One rare occasions, the odd byte was being dropped from the file. For example, when sending over an 8kB RUN file, one byte towards the end of the file had been missed and the subsequent data was shuffled up in the file. Strangely, when copying the same file multiple times, most were OK, but perhaps 1 in 10 was missing the same byte in the saved file. Quite a bit of time was lost trying to locate the source of the issue; the signals to the WIZnet module appeared to be "clean" and "noise" didn't really explain how the same byte could be missed on just the odd occasion. The WIZnet module used the CPU's Read (\RD) and Write (\WR) signals. These signals are driven by the CPU at one end of the MTX computer board and are distributed to various locations on the computer board and around the MFX PCB. Thinking about the positive effect that de-glitching the CPU clock signal had had, Martin wondered whether de-glitching the Read and Write signals for the WIZnet module might improve things. Fortunately, there were two spare I/O pins on the FPGA module that could be used for this purpose. Martin modified his PCB and added logic to de-glitch the signals for the WIZnet module. Ta-Da! After testing multiple transfers, Martin was confident that this had resolved the issue. We don't quite understand why this is the case, but Martin theorized that it may be due to the way the the WIZnet auto increments the pointers after a transfer like the VDP does. However the WIZnet is MUCH newer so a glitch that goes un-noticed by the older devices gets accepted as an valid transaction, when the proper signal arrives it too starts a transaction, but the pointer has moved. In any event, the important thing is that the issue has been resolved.
The modification on my PCB It was a fairly simple task to break the traces for the CPU Read and Write signals below the WIZnet module and patch the pins to the spare I/O pins on the underside of the FPGA module. In true Memotech fashion, I used yellow for the patch wires :-) As with Martin's PCB, extended testing demonstrated that the issue had be resolved.
By now, Martin had also produced a design for the 3D printed end plate. Here you can see his test print using white filament.
As well as being cheaper than the black filament that will be used for the final print - the white allows the detail to be seen more clearly. You can see how Martin has added small shrouds around the connectors to increase the rigidity of the thin end plate.
Whilst the positions of the connectors were acceptable, they were not exactly flush with the printed end plate. Moving them a couple of mm here and there will allow them all to be flush and look better.
So, it's done - Almost ! The two PCBs assembled by Martin and myself, with the minor modifications described on this page, are working as designed. Having proven the design, I had planned on using the additional PCBs that I have left over from the first production run to supply "early adopters", but the need for the two patch wires has made me reconsider that. Whilst they are on the underside of the PCB and will never be seen once the PCB has been installed, I don't like them - though they do look like a lot of Memotech's boards :-) My intention now is to get another run of PCBs done to incorporate the changes to the WIZnet configuration, in addition, I will take the opportunity to correct the other minor issues identified with the V1 board :-     Correction to the solder mask layer     Correction to the VGA connector pin-out     Reduce board width by 1mm to increase clearance to the end plate     Minor I/O connector placement changes to better align with the end plate
Final version of the schematic - incorporating the WIZnet modifications Version 1.1 of the PCB has been ordered to this design.
OK, updated PCBs have arrived and they appear to be OK. I have just ordered some trial pieces of the 3D printed end plate (in different materials) and have just started assembling the first few "production" MFXs. They will be ready to ship once I have checked the fit of the end plate - probably another 10 days.
Martin sent me an end plate that he printed on his 3D printer, shown here fitted to my MTX500 with VGA and network cables attached and the SD card inserted. It has a slightly textured finish, but other than that, it completes MFX perfectly!
And here with the cables disconnected.
And the SD card removed.
The 3D end plates that I ordered from a commercial 3D printer have now arrived, apologies for the poor quality photos, but hopefully, you can see the differences between them - particularly if you open the full size photos. From top to bottom . . .     Martin's example     FDM(ABS)     SLA(Resin)     SLS Nylon The three bought in items all have pretty good print quality, the difference is obviously in the materials used. My observations are :
FDM(ABS)     The most expensive     The "blackest" of the three Slightly grainy finish     Perhaps the most rigid SLA(Resin)     The cheapest, 40% of the ABS price     Mid range colour between the ABS and SLS     Very smooth finish, pretty close to the texture of the Memotech         metal end plate     Not quite as rigid as the ABS, but there’s not much in it SLS Nylon     The mid range material, half way between the cost of the others     Quite coarse texture     Probably the least black one, slightly grey looking     A little more flexible than the others, but still acceptable
Although the colour is slightly greyer than I'd have liked, if I do go to this vendor, then I think that the cheapest material is actually the best. The smooth texture looks the best of all three, but based on the samples above, other than convenience, there is probably little to choose between a "home" print and a professional one. In terms of colour, Martins filament gives a much "blacker" finish than the commercial version, though the surface finish is not as nice; this is a function of the print bed that Martin uses, other 3D printers have a glass print bed which will likely produce a much smoother finish - closer to that of the commercial resin print.