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The Memotech MTX Series

MTX PC Keyboard Interface Installation

 

This page provides guidance on fitting one of my PC keyboard interface PCBs to your MTX.

Please read this page and be sure that you understand the process and are happy with your ability to follow it before purchasing one of the interface PCBs. Fitting the PCB requires opening up the MTX - which is straightforward, but also requires attaching flying leads to a couple of components on the MTX computer board. Again, this is straightforward, but does require some care. The additional steps are necessary in order to provide power to the new PCB.

The most convenient place to pick up 5VDC power on the MTX computer board is from the 20 pin User Port socket, we used this connection for initial testing of the board. Unfortunately, the user port is only capable of supplying 20mA of current - whilst this is enough to power the components on the keyboard PCB, it is not adequate to power the connected keyboard. The keyboard will require at least 100mA and with some keyboards, considerably more.

The preferred method of picking up 5VDC from the MTX computer board is to solder a wire from a 5V connection to the new PCB power connector, but this would mean that installation of the PCB would need skills that not all prospective users might have. Instead, I have chosen to use flying leads to connect the keyboard PCB to the 5VDC supply. (An external 5V supply could also be used if preferred.)

There are many places where the 5V line can be tapped, but connecting to the TTL logic chips is very convenient. The usual pin-out for TTL chips has the 5V supply on the highest numbered pin (the one on the top right, when looking from above) and 0V on the bottom pin on the left hand side. For example, on a 20 pin chip, 5V is on pin 20 and 0V on pin 10, on a 16 pin chip,  5V is on pin 16 and 0V on pin 8. Refer to the component data sheets for confirmation if you are unsure.

Note: the MTX RAM and VRAM chips do not follow TTL chip pin-outs and are probably best avoided for this job.

See below for suggested power connections for the new PCB.

POWER OFF THE MTX BEFORE PROCEEDING

 

Opening the case

This page provides basic instructions for opening up your MTX, full disassembly instructions can be found in the MTX Service Manual on my Manuals page.

 

The two halves of the case are secured by six, 3mm socket head machine screws, three through each end plate.

The front edges of the two halves of the case have interlocking profiles that allow the keyboard to be swung upwards like a hinge.

Step 1

Using a 2mm Allen key, remove the three screws from the right and left hand sides of the MTX.

Step 2

Lift the MTX keyboard at the rear, just above the plastic panel, taking care not to put strain on the keyboard interconnecting cable.

This photo shows a ribbon cable attached to the MTX computer board and to the left hand side of the keyboard. This cable is not the original MTX one, the Memotech cable is shorter and you will not be able to raise the keyboard to the same extent as in this photo without disconnecting the cable first.

(Photo courtesy of Martin Allcorn)

Another view of Martin's non-standard MTX keyboard interconnecting cable.

(Photo courtesy of Martin Allcorn)

This photo shows the underside of the keyboard with an original Memotech ribbon cable attached. The Memotech cable is somewhat unusual - unlike a more typical IDC cable, the ribbon is not supported by the connectors, instead, each core is stripped out from the ribbon 5-10mm from the connector.

(Photo courtesy of John Hancock)

Step 3

Gently ease the  keyboard interconnecting cable from the mother board connector, always using the connector - do not pull on the ribbon cable.

The photo shows the type of cable damage that can result if care is not taken in opening the case or disconnecting the keyboard cable.

(Photo courtesy of Andy Garton)

Step 4

After the cable has been disconnected from the computer board, the keyboard is released from the base by sliding it completely to the left or right, leaving the MTX is two halves as shown here.

(Photo courtesy of John Hancock)

Installing the new keyboard PCB

This photo shows an unexpanded MTX512 with a 4000-05 version computer board.

Optional internal expansion boards, such as memory expansion or the RS232 board, are attached to the edge connector at the right hand side of the board.

The new PCB is attached the the MTX computer board keyboard header in place of the MTX keyboard cable.

A female pin header on the underside of the PCB plugs nto the header pins.
The PCB fitted with all connections in place

Although the new PCB can be fitted and the power connections can be made with the keyboard just swung open on its hinge, it is easier if the keyboard is separated from the base for the tasks.
Fit the new PCB to the MTX keyboard header on the computer board.
Connect one of the test clips to the 5V pin on one of the ICs and the other to the 0V pin of one of the ICs.

An example of the test clip connection that I use is shown opposite. The test clips that I am using are quite bulky, but the connections shown allow the connections to be made without affecting the keyboard shell's closure.

Here I am using the 74LS374 in board position 2E for 0V (brown wire) and the 74LS193 in board position 9E for 5VDC (red wire).

Take careful note which colour wire is connected to which! Make sure that the test clips lie as flat as possible on the computer board, they will not lie flush with the PCB but even lying at about a 30 degree angel will allow the case to close without problem.

Connect the 5V wire to the 5V pin on the new PCB (it is marked and is the left hand pin on the power header, J4). Connect the 0V wire to the other pin on the power header.

DOUBLE CHECK THE CONNECTIONS.

If you get the polarity reversed you will more than likely destroy the buffers and/or the 3.3VDC regulator !
Ensure that the appropriate keyboard type has been selected on the keyboard ID header, LK1. Connect the USB header cable to J2 on the new PCB, pin 1 is marked and is the left hand pin on the header.

The 8 pin header, J3, is a programming/debug header and is not normally used.
Slide the MTX keyboard back into the slot in the lower half of the case.

Check that the power connections have not been disturbed.

Attach the MTX keyboard cable to the header on the new PCB. The header pins need to be angled towards the back of the case at about 30 degrees to provide enough clearance between the cable and buffer U3.
The USB lead should pass thorough the rear of the case, above the printer and joystick ports.

Connect the PC keyboard to the USB connector using the PS/2 to USB converter if using a PS/2 keyboard.
Before closing up the MTX, test the new keyboard interface is working.

To check that your keyboard is recognised by the Propeller, power up the MTX and watch the Scroll Lock, Caps Lock and Num Lock LEDs on the keyboard.

If a USB keyboard is connected and has been recognised correctly, the three LEDs will flash once in sequence 2-3 seconds after power has been applied. If a PS/2 keyboard is connected, the LEDs will flash once concurrently.

If your keyboard is not recognised, the diagnostic LED on the new PCB will flash continually at about 1Hz.

If the keyboard is recognised, the diagnostic LED will flash in response to key presses.

It is a good idea to check the remapped keys (described on the hardware page) to make sure that your keyboard performs as expected on your MTX.

Once you are happy, power off the MTX to refit the end plates.

In the event of any problems, contact me for help.
   

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