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The Memotech MTX Series


PAL14L4 Reader for the Memotech MTX



Given that there are 14 inputs, there are "only" 16384 (2^14) possible input combinations to drive the four possible outputs. Tony Brewer made a PAL14L4 Reader and wrote the Z80 code to control it. It takes just over 10 seconds @ 4MHz to step through all of the possible input combinations and read the 16K nibbles of output, search for active low outputs, remove redundancy and give the logic expressions as a text string, e.g. XX X010 000X 1111

With Tony's help, I have drawn up a schematic for the PAL reader interfaced to the MTX User I/O Port using KiCAD.

Despite a few limitations, KiCAD allows you to take a design all the way from a schematic through to the finished PCB.

Using the KiCAD design suite, including the on-line automatic "router", can produce finished designs such as this board for the PAL Reader.

However, it is uneconomic to have a board professionally made just for a single unit, so an alternative solution for the production of a working prototype was called for. Stripboard is a widely used type of prototyping board, and ideally, I would have liked to have used a stripboard add-in for KiCAD. Unfortunately, I could not find a free add-in that would have done auto-routing of a KiCAD schematic, so I had to use a stand-alone stripboard design program. I looked at a number of free stripboard design tools including Stripboard Magic (would not run on Windows 7), Stripboard Designer 1.0 (extremely basic), VeeCAD Free (black and white only) and VeroDes (written in VB5). Whilst it did look good, I was originally put off by VeroDes by its requirement to install a host of dependencies before the program would install on Windows 7 and was intending to use VeeCAD Free, being a bit reluctant to have VeroDes screw up my Windows 7 machine. However, Tony had successfully used VerDes and recommended it, so I installed it after all.

While I was having a go at designing the layout and routing myself, Tony quickly came up with a VeroDes design, which I have to say, is far batter than the attempt that I would have made. Simple things (being easy when you know how) like tying unused inputs and outputs together on chips having these signals on opposite sides by not cutting the stripboard between them saves cluttering the board with unnecessary links.

Having seen a photo of Tony's prototype board, I had bought some stripboard (95 x127 mm) that I thought would have been more than adequately sized, as it turned out, if it had not been for Tony's compact design, I would have struggled to fit the parts on my board without it becoming an untidy mess.


Tony's VeroDes design for the PAL Reader.

The green lines indicate the wire links on the component side of the board and the red "X"s denote breaks in the tracks on the solder side. Normally, these are at holes, but for the IDC connector, the break must be between the connector pins and respective holes. VeroDes is not able to show this accurately and breaks must be shown aligned with the holes.

(This prototype uses the 5V supply from the MTX I/O port - see below for why this is not feasible and for the finished design.)

A really useful feature of VeroDes is its ability to print overlays for the copper and component sides of the strip-board, these can be used to place the components and identify the track breaks and wire links required.

On the solder side overlay, the position of the track breaks are indicated by the red dots and the link ends or component legs, by the grey circles.

Preparing for the "operation", scalpel at the ready!

 - The surgical mask and gown are just out of shot :-)


[If I do something like this again, I will invest in one of those hands free solder stations with a light and magnifying glass - it would have been a great help - particularly when cutting the tracks and checking the solder joints at the end.]

The first task was to make the required cuts in the tracks. You can buy track cutters for stripboard, most of these are basically just a drill bit in a handle that is turned on a hole to cut the narrow strips of copper on either side of the hole.

(These tools are not suitable for cutting the strips between holes.)

When making the track breaks it's wise to "measure twice, cut once" !

I had intended to use the trimming knife just to make the cuts that were positioned between the holes and to use a drill bit to cut the rest. Using the knife, two parallel cuts were made through the copper layer and the copper picked off with the tip of the knife.

Since the knife was working well, I used it to make all of the cuts. Once all the cuts had been made, I smoothed off the cut edges of the tracks and used a multi-meter to make sure that there was no continuity between the two sides of the cuts.

When cutting the tracks, I obviously handled the board a fair amount which, more than likely, had transferred some "grime" onto the copper tracks.

Prior to soldering, I used a PCB eraser (a small rubber block with a fine abrasive embedded) to clean up the strips and gave the board a final wipe over with isopropyl alcohol to ensure a clean surface for the solder to adhere to.

The first "components" to be mounted were the wire links. As well as  identifying the positions of the links by their x,y coordinates, the VeroDes  component overlay was very useful for making up the links themselves.

Starting at the right hand side of the board, I first made a 90o bend on one end of a piece of link wire and laid the link on a copy of the overlay to measure the required length, prior to making the second 90o bend. The links then had bends at the correct distances to match the holes in the stripboard and could just be dropped into place prior to soldering and cutting off any excess length with a pair of small side cutters.

The resistors were installed next, followed by the sockets for the counters.

An observant reader may have noticed that the resistor positions do not match the schematic. The design was based on a plain IDC header, as I had bought one with latches, to allow for the extra length of the connector, the resistors had to be relocated further up the board and an additional cut was needed in the track between the bottom of R2/R3 and the I9 input.

Similarly, the link from the header to the 0V line had to be modified to clear the bottom of the longer IDC connector, resulting in the "L" shaped link.

After the IDC header was installed, I found that it was possible to get the 0V link to clear the bottom of the connector by making a small radius curve at the bottom end. This meant that the unsightly "L" shaped link could be replaced with a vertical link to match the others.

The ZIF (Zero Insertion Force) socket for the PAL has also been mounted at this point, you can see that the locking lever is in the open position. ZIF sockets should be soldered with the contacts in the open position to reduce the risk of soldering the contacts together when installing it.

Self adhesive silicon rubber feet were then attached to the solder side of the board to provide a stable base and ensure the solder joints were clear of the work surface.

Solder side of the completed board, exposing my (lack of) soldering skills!

Prior to installing the counters, the tracks were visually checked with a magnifying glass to ensure that no solder joints had overflowed onto adjacent tracks or across track breaks and that any solder "splashes" had been removed.

The board was also checked with a multi-meter to ensure that there was continuity and no short circuits between the lines shown on the schematic, with particular attention being paid to ensuring that there were no shorts between 0V and 5V.

The board is wired to the MTX I/O port with 20 way ribbon cable with an IDC plug on one end and a 20 pin DIP plug on the other. The cable should be as short as possible, although I needed it to be ~12" long to sit on top of my FDX just behind the MTX.

With the proper tool, crimping of the IDC plug is quite easy, the DIP plug is a little more awkward, but I found that a hexagonal pencil was just the right size to fit between the legs to protect them while crimping the plug.


Installation & Testing

The User Port interface is the blue 20 pin DIP socket located at position 8C on the main board. When installing the DIP header into the I/O socket, pin 1 of the cable, identified with the red-stripe, must be installed in pin 1 of the DIP socket.

NB: The User Port socket is installed the opposite way around to all of the ICs on the board - pin 1 is the connection on the upper right hand side of the socket - on the ICs, it is at the lower left.

The ribbon cable connected to the I/O port and exiting the case above the printer interface connector.

I found that the DIP plug did not make a very secure connection to the I/O socket and taped the ribbon cable to the flat on the cut-out above the printer port.

The board was initially connected to the MTX (with the power OFF) with all of the sockets empty.

When the MTX was powered ON, a check was made to ensure that 0V and 5V were present on pins 8 and 16 of the 74LS163s and pins 10 and 20 of the ZIF socket. The 5V supply from the MTX connected to pin 11 of the IDC header was also checked.

To verify that the counters can be correctly controlled from the MTX, the following code can be used and the voltages on the CLR and CLK pins measured on any of the counters. The expected voltages on the pins are shown in the table.



OUT 7,A ;POT7=0, POT6=0     [From BASIC, OUT 7, 0]

LD A,40

OUT 7,A ;POT7=0, POT6=1     [From BASIC, OUT 7, 64]

LD A,80

OUT 7,A ;POT7=1, POT6=0     [From BASIC, OUT 7, 128]


OUT 7,A ;POT7=1, POT6=1     [From BASIC, OUT 7, 192]




74LS163 Voltages
Pin 1 (CLR) Pin 2 (CLK)
0 0
0 5
5 0
5 5

After verifying the supply and counter control voltages, the MTX was switched OFF and the counters were installed in the sockets.


When the MTX was switched back ON and the voltages rechecked I ran into a problem, the supply voltage had been pulled down to less than 1V. According to the MTX manual, the MTX is only capable of supplying a maximum of 20mA from Port 7 and the counter datasheet states the typical power dissipation is 93mW, i.e., 18.6mA per counter. With 4 counters on the board, I thought that the Port 7 5V supply was not going to be adequate for the counters.

An external 5V supply was required, fortunately, I had enough bits & pieces to hand to make a suitable modification.


A two pin header was added to allow connection of an external 5V supply and the 5V line from the MTX was disconnected by breaking the track at position 3,10. A new link was installed between 12,22 and 12,23 with an additional break at position 15,23.


The modified VeroDes design is as shown.

With the external supply connected, the following code was used to check that the counters were incrementing as expected



OUT 7, A

LD A, 80

OUT 7, A


LD C, 40

LD B, 0

LOOP: LD A, 40

OUT 7, A

LD A, C0

OUT 7, A








Zero "A"

Reset all POT bits (clears counters)


CLR = 1, i.e., NOT clear


Counter incremented

    Repeat . . . .

        until . . . .

All 16384 counts done


If all of the tests are successful, the PAL Reader is now ready for use

Return to the main PAL Reader page for info on using the tool





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