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Raspberry Pi

 

 
 
 

The Memotech MTX Series

MTX Pi

 

Andy Key's Memotech emulator, MEMU, is a fully featured emulator for the complete MTX range of computers, including the disk drives and video hardware required to run CP/M. MEMU runs on Windows and Linux, including, thanks to Bill Brendling, the Rasbpian Linux distro for the Raspberry Pi.

Using MEMU on a PC or Raspberry Pi is fun, but the experience is spoiled a little by having to use a modern day PC keyboard. I wanted to try using a Raspberry Pi to run MEMU, but use a real MTX keyboard to recreate the look and feel of a real MTX computer. 

Although I have a few MTX machines, all of them are working and I did not want to break up a perfectly good system to "borrow" the keyboard, even on a temporary basis. I had been on the lookout for a broken, and preferably, irreparable, MTX for some time that I could use for this project. I was not having any luck until a MTX keyboard came up on ebay UK. Although I ended up paying more than I wanted to, I bought the item with a view to building MTX Pi.

The ebay keyboard consisted of the top half of an MTX 512 computer, it was missing the bottom half of the case, the end plates and the plastic insert where the I/O ports are usually mounted. However, I did have the other half of a MTX case that Trevor had kindly given me in May 2014, although it too was missing the port mounting insert and I did not have any spare end plates that hold the two halves of the MTX computer together. Jim Wills has donated the port mounting panel from one of his old machines, so I have enough to get started - the only missing items now are the end plates for the case. 

 

Proposed Feature List
Interface Description Status
Keyboard MTX512 keyboard for user input Done
  USB PC keyboard for programming/configuring Raspberry Pi (USB)
Mouse USB PC mouse for programming/configuring Raspberry Pi (USB)
Joysticks 2 x Atari 9-Pin ports (optional) Done
  1 x USB PC joystick (optional) (USB)
Audio 3.5mm Stereo output jack (replacing the MTX Phono Hi-Fi output) Done
  Via HDMI port (optional) Done
Video Composite Video output photo connector  (replacing the MTX BNC video output) Done
  Via HDMI port (optional) Done
Serial (Future) (future)
Network MTX "Node" (future) (future)
  Ethernet RJ45 (optional) Done
Printer MTX Centronics Printer (future - requires additional interface board) (future) 
  Raspberry Pi Network printing (future) (future)
USB 3 x Used - Keyboard, Mouse and Joystick Info
  1 x Spare Info

The Original
The Memotech MTX system board is installed in a brushed aluminium case, with a full travel, 79 key, keyboard mounted in the upper half of the shell. The keyboard includes a separate numeric keypad, 8 function keypad and two blank keys, either side of the spacebar, which, when pressed simultaneously, reset the computer.
The view from the left hand side of the MTX, showing the cartridge port and the profile of the case. The case is held together by 3x3mm Allen bolts through the plates at each end.
The two halves of the disassembled MTX case.

The MTX computer board is secured with a single screw through the base and the heat-sink (hidden by the large capacitor next to the silver UHF modulator).

The rear of the MTX is a plastic molding that slides into a groove in the base and is used to mount the I/O and power connectors.

The keyboard base plate is bolted to the underside of the upper half of the shell, the grey cable at the left normally connects to the pin header in the upper left corner of the MTX computer board.
Photos showing rear panel's I/O ports and connectors, from left to right . . . . .

Cut-outs for two serial ports, RS232-1 and RS232-0 , these were optional extras, the ribbon cable for the FDX exited the case above these ports from the same expansion board. MTXs without serial ports fitted were fitted with covers over the port cut-outs (this MTX has had an RS232 board removed).

Adjacent to them is the BNC connector for the Composite Video monitor output.

Next are the Hi-Fi and power connectors. This photo is of a US export model - the small slot was for the TV channel selector switch fitted to US models

On the right hand side are TV, parallel printer, audio "mic" and "ear" connectors (used for loading/saving software from tape) and two Atari type joystick ports.

(Photos courtesy of John Halliwell)

The Imposter
I have a number of Raspberry Pi Model "B"s, including the original Revision 1 board (256MB) and Revision 2 (512MB).

These boards have 2 x USB 2.0 ports and a 26-Pin GPIO connector with 17 available I/O pins, a further 4 GPIO pins are available if a header is fitted to the unpopulated pads on J5.

Though it would probably do the job, I decided that having all of the GPIO pins on a single header would be better so I would not use the Revision 1 board.
The Model 1 "B+" board has 4 x USB 2.0 ports and an expanded 40-pin GPIO connector with 26 GPIO pins available.
The Model 2B has the same USB and I/O capabilities with an enhanced processor and more memory, but the physical dimensions and port positions are the same.

The design will be based on using a Model 1 "B+" (which I have), rather than a Model 2B (which I don't).
A simplified mechanical drawing of the Pi Model B+ and Model 2B.

Having dimensions of 85mm x 56mm, the Pi board will obviously fit inside an empty MTX case, but the board is small enough that it could also fit inside an MTX with the system board and up to 1 expansion board already present.

Given the low profile of the MTC case and limited clearance below the keyboard, the more critical dimension is the height of the RPi board, for the Model 1 B+, this is ~21mm from the top of the USB connector to the bottom of the micro SD card.
Bill Brendling's Raspberry Pi connected to an MTX keyboard during the software development phase

Preparing the MTX Case
Since the MTX "Atari" type joystick ports are just connected to the keyboard drive & sense lines, I shall be adding joystick ports to the system.

A Eurocard size prototype board fits the MTX PCB guide slots perfectly and will be used to mount 9-way "D" type connectors for the joysticks.
The "D" connectors soldered to the prototype board.

The pitch of the connectors does not match the 0.1" of the proto board, but with a little persuasion, they could be bent to fit without stressing the pins too much.

The screw sockets have been reversed to allow the connectors to be mounted flush with the molding, the joysticks won't use them, but they are also used to secure the metal "D" shell cut-out.
The MTX keyboard has two unmarked keys, either side of the spacebar, that are used to reset the MTX. They are hard-wired in series to the Z80 Reset circuit but will be repurposed in MTX Pi to provide two, independent, MEMU control keys, by patching one of the keyboard drive lines between the reset keys and taking the return lines to two GPIO inputs.
To provide the flexibility to easily convert back to the standard MTX configuration, jumpers are used to disconnect the additional drive line and reconnect the reset key return to ground.
The board will also be used for the MTX keyboard header, a 40-pin IDC header that will be cabled to the Pi GPIO header and the patch wiring between the headers.

The columns of header pins at the left are for the keyboard connector and the 40-pin header in the centre of the board is for the Pi GPIO cable.
The patch wiring maps the MTX keyboard lines to the Pi GPIO header, the I/O pins were allocated before the start of the build and should not need to be changed.

Although it was not necessary, to provide maximum flexibility, the keyboard headers allow the wiring between the keyboard lines and the GPIO header to be re-patched as required.
To allow one of the keyboard drive lines to be connected between the reset keys while keeping the changes to the keyboard PCB to a minimum, I just soldered a single header pin onto the inboard solder pad for one of the reset keys.
Jumper cable connected to the header with a Dupont connector
The Pi has ports and connectors on all sides, I found that the best place to position it was more or less in the mid position of the base, with the GPIO connector at the rear as shown.

Later models of the Pi, including the Model 1 B+, have mounting holes in the PCB, but, as might be expected, given the size of the board, they are sized for M2.5 screws.
I had hoped to use self-adhesive PCB mounts, but could not find ones small enough for the Pi mounting holes. Instead, I found that the MTX circuit board fixing hole was in the perfect position to align with one corner of the Pi PCB. With 1 screw through the base, the Pi was adequately secured but I put screws in the other three holes to make "feet" for the other corners.

End on view, the photo is not too clear, but hopefully, shows that there is more than adequate clearance between the top of the Pi Network and USB connectors and the underside of the MTX keyboard.
As noted above, the original plan was to read the status of the MTX reset keys in the same was as the other keys, using drive and sense lines. Since the reset keys are not used by the native MTX software, this is not necessary and the keys can be read directly by the Pi MEMU code to initiate MEMU control functions.

For maximum flexibility, jumpers are provided to configure the reset keys as required.
Circuit diagram showing the reset jumper options
The two switches can be either separate or commoned
    JP1 directs the return from Key 2 to either a GPIO input or ground
        (JP2 should be open in this case)
    JP2 sets the common point for Key 1 and Key 2 to either DR7 or ground 
        (When using the two reset key switches for separate functions)
 
Wherever possible, I wanted to have the I/O connectors use the same positions as the MTX connectors and have the port identification markings correct. The power connector was a little awkward as the original MTX power plug is just inserted through a hole in the plastic back panel and plugged directly onto the PCB.

I used a relatively large (12mm diameter) 5.5mm x 2.1mm DC power socket like this one and made a couple of supports put of a piece of strip-board to allow the power plug to mount in the power input cut-out.
To save board space on the Model B+, the PCB has a 3.5mm jack socket which is used to connect the stereo audio and composite video outputs that were on two  separate connectors in earlier models. 

Although I could just have used a suitable cable and terminated the ends directly onto the connectors on the rear of the MTX, I did it slightly differently.
I mounted a cheap (1 off ebay) breakout board on the keyboard PCB, cabled the pin header to the AV sockets on the MTX rear panel and connected the breakout board to the Pi with a standard 3.5mm 4-pin cable.
The patching for the power and AV connectors.

The new 5.5mm barrel jack socket on the rear panel is connected to a pin header wired to a standard USB socket and a USB to Micro USB cable takes power to the Pi.

The audio and video signals from the PI 3.5mm connector are split out for new 3.5mm stereo and phono video connectors on the rear panel.
Installing software on the Raspberry Pi is made much easier if the device is connected to a network and the internet. In the short term, I had planned to just have a short length of network cable hanging out of the back of the MTX case, but another trip to ebay revealed short (0.5m) RJ45 extension cables like this one.

The flats on the female end make it suitable for attaching to the base of the MTX case with a double sided "sticky" pad.
The Pi connected up and booting to the Raspbian GUI, using my cheap 7" LCD monitor for initial testing.

The  keyboard interconnecting ribbon cable has been removed for clarity and the photo is also missing the network connection cable, the easiest way of getting the MEMU software on the Pi was to copy it over from a network share.
The HDMI and USB ports have short patch cables fitted that exit the right side of the case. The cables are fitted with bulkhead connectors - I had intended to mount the connectors on the end plate, but it would be better to move the Pi to the extreme right and have the USB and network ports flush with the end of the case.

An SD card extender cable moves the SD card slot to the left edge of the case
With the Raspbian Jessie operating system loaded and Bill's MTX-Pi version of his MEMU installation extracted to the Pi home directory, the system can be started from the command line by entering :

            ./mtx/memu/memu-pi

(In these screenshots, I am using the composite video output to a Dell 2007FP monitor which results in less than optimal video output)
Success !

The familiar MTX "Ready" prompt

(At this point, the MTX keyboard has not been connected to the Pi, only due to the fact that the interconnecting ribbon cable is faulty.)
Final fit, showing the keyboard interconnect cable - a standard 40-pin IDE cable
   
To be continued . . . . .      
   

Preparing the Raspberry Pi
At the time of writing, the latest version of the Raspbian operating system for the Raspberry Pi is based on the "Jessie" version of Debian Linux. The Raspberry Pi website has an overview of the differences between Jessie and the previous Wheezy releases, but the most important difference for MTX-Pi is that sudo is no longer needed to access the GPIO functions.
The Raspberry Pi website recommends that a clean install of Jessie is to be preferred over an upgrade to Wheezy. Download the Zip file (1.24GB) from the Raspberry Pi downloads page to another PC - in my case, running Windows.  
The Zip file contains an SD card image which decompresses to 4GB, the built-in unzipping tool in some versions of Windows, including the 64-bit version of Windows 7 Professional that I am using, is not able to handle archives of this size and generated the error shown.

The file can be successfully unzipped with the free Windows version of the 7-Zip archive utility.
The image file should be written to a [micro] SD card with a disk imaging tool - it cannot just be copied to the media.

Win32 Disk Imager is available from SourceForge on its Project page
When written to an 8MB SD card, the image produces a 54MB Windows readable FAT formatted partition, an approximately 4GB Linux partition with the remaining space unallocated.
With the SD card inserted, powering on the Pi should boot to the Raspbian X-Windows desktop as shown.

For convenience, at this point, I was using a Dell 2007FP monitor in composite mode. Eventually, I will be using the Pi HDMI output, but this was fine for setting up the system, although the quality was not great.
Since this Pi is intended to run MEMU full time, some changes to the system configuration should be made from the Raspberry Pi Configuration option, available from the Menu bar.
In normal circumstances, you will not need to use the GUI and want the system to startup and log on automatically.

From the System tab, set the Boot option to CLI (Command Line Interpreter) and enable Auto Logon as the pi user.

(If you need to use the GUI at some point, entering "startx" at the command line will start the X-Windows interface)
The default CPU speed for the Pi is 700MHz, but it can be safely over-clocked using the Overclock parameter from the Performance tab.

Although it is not really necessary to run MEMU, I opted to use the Medium setting and run the CPU at 900Mhz. The system will almost certainly run faster, but as I wanted a stable system, rather than necessarily the fastest, 900MHz will be more than adequate.
Although it's highly unlikely that you would use anywhere close to the free space on the default 4GB partition (~580MB free) for MEMU, the Expand Filesystem option under System preferences is used to make the rest of the free space on the SD card available to the Pi. (A reboot is required for the change to take effect.)

In my case, using an 8GB SD card, the free space increased from 580MB to 3.8GB.
When Bill was looking to add GPU acceleration to MEMU he passed on some information on how video scaling worked for MEMU on the Raspberry PI.

"The resolution is (in effect) the same as the original MTX resolution. This is then scaled up to fit the available screen pixels, each MTX pixel becoming a number of pixels on the RPi display. Exactly what the scaling is will depend upon the pixel size of the display and on the MTX screen size (256 x 192 pixels). The up-scaling is always an integer number of pixels in each direction."

Bill added an additional configuration option to the memu-pi.cfg file : -gpu-mode.

This option defines whether the up-scaling is done in the ARM processor or in the GPU.

The screen shots below are taken with my iPhone camera, the images suggest that the colour of the text has a bluish tinge, but in all cases, it is a pretty good "white". The images are intended to show the effect of the -gpu-mode setting on the "crispness" of the text display.
-gpu-mode 1

Up-scaling is performed by the ARM processor

 
-gpu-mode 2

The GPU performs up-scaling with interpolation.

As Bill says, this mode "emulates a fuzzy 1980's TV" - you can use this mode for the real 1980's retro experience !
-gpu-mode 3

The GPU performs up-scaling, but with no interpolation (temporarily changes the scaling kernel).

Modes 2 & 3 are making more use of the GPU and less of the ARM, so they should be better than mode 1, particularly on high-res HDMI screens. Based on my limited testing, Mode 3 seems to be the best one for me with my Dell FP2007 monitor using an HDMI-to-DVI-D cable.
Now with an HDMI to DVI-D cable connected between the PI and my Dell FP2007, I can use HDMI mode and take advantage of the much bigger X-Windows desktop.

The DVI-D connector does not support audio, so I am still using the PI's 3.5mm audio output, connected to the Dell Soundbar audio input.
   

Preparing MEMU 
A "normal" installation of MEMU on a Raspberry Pi is described on my MEMU-Pi pages, there you can find a description of how to compile and install MEMU from its source files.

For MTX-Pi, Bill kindly created a pre-built installation for me, all that required was to copy the .tar file into the Pi home directory and decompress the archive.
   
To be continued . . . . .    
   


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