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The Memotech MTX Series

MTX PC Keyboard Interface



Board Design

Martin does his prototype board layouts using Excel, this helps him place the components on a matrix representing his prototype board, but they are not schematic diagrams in the usual sense.

Note: the layout is drawn as reference to be used for soldering, so the view is from the underside of the board, i.e., the components are a mirror image of the normal footprints.

This is the layout for Martin's prototype board, the major components being the Propeller, associated serial EPROM and two 74x7 non-inverting buffers. 
The components placed and soldered onto a small prototype board that Martin happened to have available. 

The board sits conveniently on top of the MTX keyboard connector and would allow the MTX keyboard shell to be closed if required but does not have the facility to connect the MTX keyboard at the same time as the PS/2 interface board.
Martin's board fitted to the MTX and attached to a PC PS/2 keyboard I/O slot adapter panel.

You can also see the flying lead connected to the MTX User Port socket from where Martin has picked up 5VDC for the board. 
I took Martin's component layout and converted it to a KiCad schematic as shown. 

As noted above, plugging into the MTX keyboard connector prevents the MTX keyboard from being connected at the same time.
When the Propeller keyboard interface is incorporated into MTXPlus+ it will connect in parallel with the existing MTX matrix keyboard interface, allowing a PS/2 keyboard and an original MTX keyboard to be connected at the same time. I decided that it would be nice to have a similar setup on an original MTX so that a PS/2 keyboard could be connected as and when required, without needing to open up the computer.  

The easiest method of connecting a PS/2 keyboard would be to have a PS/2 connector on a trailing lead exiting the rear of the case, but I also though that it would be nice to be able to include a PS/2 connector on the MTX itself.

There are a couple of options to achieve this, either by drilling an additional hole in the rear plastic I/O port panel, or by drilling one of the end plates. I was not keen on the first method, but was happier drilling one of the endplates as they would be much easier to replace if the keyboard connector was removed in the future. The left hand side end plate is not a good choice as it would be obstructed by any MTX expansion hardware; the right hand side end plate is a better option.

I also considered allowing the PS/2 keyboard PCB to to mounted to the right of the computer board(s) such that the PS/2 connector could be mounted on the PCB, rather than needing a flying lead. If the PCB was mounted at the right hand side of the case, then IDC cables would be needed between the existing keyboard, computer keyboard connector and the new PCB.
To cater for these options, I added additional headers to my design to allow the MTX keyboard to be connected simultaneously with the PS/2 keyboard and/or to run an IDC cable from the right hand side of the case to the MTX keyboard connector.  
These options meant that my prototype design was quite a bit larger than Martin's to allow space for the additional pin headers to be located in the deepest part of the case (at the rear). Whilst this design gives the most flexibility, it does mean that the board becomes quite large and exceeds the target price point for the PCB manufacturer that I use (<100mm).   
My prototype board - with a couple of changes from the 3D model for ease of construction.

I found that the Propeller and EPROM needed to be a little further towards the top of the board than Martin's prototype to allow adequate clearance when the MTX keyboard was fitted. 
My prototype board fitted to the MTX keyboard connector.

You can see the my PC PS/2 keyboard I/O slot adapter panel attached and below the PS/2 cable, the 20 pin DIP header that I have attached to the User Port socket. (Bad Design ! see below)

Although the MTX keyboard connector pin 20 provides 0v,  in addition to the 5v connection on pin 20 of the User port, I have also picked up 0v from pin 16, this provides a little more strain relief on the jumper wire, rather than just having it connected to the 5v pin.
After a little debugging and editing of the code, Bill's Propeller code worked very well, with the same limitations on the mapping between the PC keyboard and the MTX ROM that exist in MEMU on which the Propeller code is based.

These issues are described in the keyboard section on the MEMU emulation page of Andy's site and are associated with issues related to shifted and unshifted keys. Quoting from Andy's page, "... to type certain MTX characters, you must type different things on the host PC keyboard. This remapping is done to ensure an unshifted host PC keypress corresponds to an unshifted MTX keypress and a shifted host PC keypress corresponds to a shifted MTX keypress." and "the whole host PC keypad maps to the whole MTX keypad, based on key position, not what is written on the keys"

"The effect of the shift-state problem :- "

Use host keypress To produce MTX keystroke
^ =
= ^
' @
@ '
# :
<shift> ` `

This behavior is not ideal, but is considered to be the most practical way of using a PC keyboard on the MTX without making changes to the MTX ROM. The difference between the effect of a keypress and what is written on the keys is most noticeable with the PC numeric keypad - see the diagram at the top of the page for further detail.

However, Martin has developed patches for the MTX ROM to map the PC keyboard characters to the desired MTX keystroke when the keyboard "Scroll Lock" mode is active. An updated ROM image is available below.

In standard mode the system works as now with key mismatches. In "Scroll Lock" mode the main keyboard keycaps all match except "back quote". (The keypad in shift mode is still offset because the cursor keys are aligned.)

For MTX users with CP/M or SDX disk systems, Bill has written loadable drivers that enable the "Scroll Lock" mapping mode to be used without requiring changes to the MTX ROM.

For CP/M this works by patching the CP/M BIOS in high memory. For SDX BASIC, a small routine is loaded between the sound buffers and system variables, which hooks into an SDX call vector to provide the additional keyboard decoding.

5V Power Connection

Although the User Port was used to provide the 5VDC to the boards for testing, we did not pay enough attention to the restriction included in the MTX Operator's Manual - the User Port is only specified as being able to supply 20mA - the propeller and more significantly, the PC keyboard requires considerably more than that. Inspection of the MTX computer board shows that the 5V supply to the User Port is on a very thin PCB trace - I suspect that it was this that set the maximum current available from the User Port. It would be a relatively easy task to solder an additional wire from a 5V connection to the 20 way socket, but that would prevent the PCB from being "plug & play". Instead, we will likely use a test clip to pick up 5VDC from a convenient point on the MTX computer board.

(Taking power from the MTX PCB may be problematic if the computer is fully loaded with internal and external expansion boards. If the MTX 5V supply is close to capacity, then an external 5V supply would be needed.)
Inclusion of USB Keyboard Compatibility

The prototypes that Martin and I had built operated as intended, however, I had not appreciated that PS/2 keyboards were as obsolete as they actually are and the difficulties that might bring when implementing this project on a wider basis, should it be of interest to others. Whilst legacy PS/2 keyboards are still available, it appears that the PS/2 adapter plates that Marin and I had used are pretty much impossible to find.

During my testing, I used a legacy PS/2 compatible keyboard with 6-pin mini-DIN connector. Bill and Martin had used USB keyboards with "dumb" USB to PS/2 adapters which worked perfectly. When I tried to use one of my USB keyboards in the same way, the system failed to recognise the keyboard and respond to key presses. As it turns out, older USB keyboards can detect when a PS/2 interface is being used and use the PS/2 protocol so a passive adapter works fine. Newer USB keyboards do not all support the PS/2 protocol and an active adapter would be needed to do protocol conversion.

Bill had predicted that this may be an issue and had already found USB interface code for the propeller that was "worth thinking about". It was obvious that my original intention to have a PS/2 compatible interface was short sighted and a USB keyboard interface is pretty much essential. With that in mind, I asked Bill if he could look at developing the code for a USB interface too.

Based on Bill's initial research - a proposed design for a dual function PCB.

It seems that the USB interface needed the USB signal lines connected to low address pins on the Propeller (A0-A7) and the USB and PS/2 interfaces use different resistor values and voltage levels. This design allowed the pull-ups / downs to be selected by jumpers.
We hoped that it may have been possible to find a pair of resistor values that can work for PS/2 or USB keyboards (to be determined by testing), in which case, it would be possible to simplify the design as shown here, where only a single jumper is needed to select the keyboard type.

The status of the single jumper could also be used to identify the type of keyboard (PS/2 or USB) so that the Propeller could choose the appropriate code to execute.
Testing on the prototypes suggested that the PS/2 resistor values would also work for USB keyboards, so a single set of resistors has been configured in the "final" schematic used to generate a prototype PCB.

As of March 2018, the prototype boards had been proven to work in PS/2 or USB mode, the dual mode code had not been tested but as the hardware appeared to work, I committed a trial run of PCBs to manufacturing.
The KiCad 3D model of the PCB. The components are pushed as far up the PCB as possible and the board size reduced to 100mm wide to minimise the PCB production cost.
The first run of the PCB has identified a couple of minor issues . . .
1. The DIP-14 sockets are too close together, which makes installing the buffers in sockets a tight, but workable, fit. (The tolerances in the sockets allows the chips to be installed at the opposite ends of the sockets.)
2. The top DIP-14 socket is too close to the MTX keyboard pass-through connector, but bending the pins at about 30 degrees allows the connector to be plugged in and the keyboards operated in parallel.
Assembled PCB, fitted to MTX.

The MTX keyboard has been disconnected for the photo, but fits without problem with the pins on the pass-through connector bent slightly as shown.

(On this board, the activity LED is orientated so that I could see it more easily for testing, normally, it will be mounted on the surface of the PCB.)
A typical PC Motherboard USB header header is shown opposite. It is designed to support two USB connections; there are a number of varieties of mating cables which support 1 (4/5 pin cables) or 2 (9 pin cable) USB devices. Most of these cables should be suitable for use with the MTX keyboard PCB USB header when connected appropriately.

NB : The keyboard PCB USB pin header is not keyed, Pin 1 is the left hand pin, closest to the MTX keyboard pass through connector.
Single USB adapter cables are available in 4 and 5 pin varieties. Either may be used with pin 1 (5V) connected to pin 1 of the header. If a 5 pin cable is used, pin 5 (usually a slightly thicker core used for the cable screen) will be unconnected and overhang the keyboard pin header.

Future Development

MTX Mouse interface ? (Cancelled)

Martin has been working on an additional hardware interface to support the use of a PC PS/2 mouse with the MTX. To avoid the need for new driver software or modifications to applications software and games, Martin's interface is designed to mimic the cursor keys in the same way as the Memtoech joystick interfaces worked. Martin's design uses an Atmel ATMega328 microcontroller and supporting GAL and also connects in parallel with the existing MTX keyboard (and PC keyboard interface if fitted).

Whilst Martin's mouse interface is a nice technical solution, I am not sure whether there is enough demand to justify making it a "product". However, it **may** be possible to add mouse functionality to the keyboard interface without having to make significant hardware modifications. Adding a PS/2 mouse appears to be relatively straightforward and likely to fit in the available free space of the Propeller. Adding a USB mouse is more complex and further work is required to determine whether it is possible.

 In either case, it will not be possible to upgrade a keyboard only PCB to include a PS/2 and/or USB mouse.


Based on Martin's results when testing his mouse interface with NewWord, it is apparent that using the ROM keyboard scanning functions to update the mouse position is too slow to be practical - certainly with CP/M software. Given that the limitation appears to be the keyboard scanning speed, using the Propeller to provide the interface to the mouse would also have the same issues as the AVR interface.




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