Genial Joysticks      (C) Copyright 1983-2003 Simon N Goodwin

First Published in Digital and Micro Electronics, Autumn 1983
Scanned by Andrew Hague & updated by the author, January 2003
Adapted to HTML by Knut Roll-Lund, September 2003

A switched joystick can easily be added to a Genie computer (or to other popular micros), to duplicate a feature of far more expensive systems.

Have you ever wished you could use arcade-style joysticks to play games or edit data on your computer? This article explains how you can alter a Video Genie to accept input from standard joysticks. The modification can be carried out in about an hour, at a cost of £6-7, and if you build your own joystick mechanism the cost could be less than a pound!

This simple hardware modification gives you the convenience of being able to steer items around the screen in the currently fashionable way. You could even add a home-made 'mouse' to your computer, similar to the one used on Apple's kilopound 'Lisa' system. Another advantage is that a joystick reduces keyboard wear and tear - frantic button-pressing has caused many a computer keyboard to fail!

Joystick In Action

Many programs for the Video Genie accept input from four cursor-keys (on early Genie machines these are confusingly labelled Esc, Ctrl, Backspace and Tab), and this project means that a joystick can be used whenever the cursor keys are normally required. The stick's 'fire' button will duplicate the space-bar, which is generally used to fire shots in computer games.

Once the modification has been carried out it is possible to move the cursor around in programs such as Scripsit (the word-processor upon which this article was written) and Visicalc, so this project is useful not only to games-players!

The joystick does not disable the keyboard, so that all of the keys may still be used while it is connected. The modification has been tried and tested for over a year on the author's machine. It has also been successfully adapted for other computers such as the TRS-80 and ZX Spectrum; this article will describe only the Genie modification in detail, although the principle will be described for those who wish to adapt other computers.

Lateral Thinking

A joystick is really nothing more than a set of switches arranged in a cross pattern, with an additional 'fire' button. Figure 1 illustrates the internal circuit of such a stick: as you move the joystick a plastic arm pushes one or more of the internal switches.

Figure 1. Switched joysticks invariably contain a circuit similar to that, shown. Alternatively, you can build one...

When you play a game using the computer's keyboard you are effectively throwing switches to control your laser base, gunsight or whatever. The Video Genie keyboard consists of switches in a 'matrix', as shown in Figure 2.

Figure 2. The Genie keyboard switch matrix: the details may be different, but most popular home computers use a similar format for the keyboard.

The computer can present a voltage to any of the wires on the left of the diagram (A0-A6), and it is able to sense whether or not the voltage has made its way to the bottom row of terminals (D0-D7). If no key is pressed, then the horizontal wires will be isolated from the vertical ones, and consequently the Computer will not sense a voltage.

However, consider what happens if the top horizontal Wire is supplied with electricity and the '@' key is pressed: current will flow through the switch to the 'D0' line at the bottom of the diagram. Likewise, if the 'C' key is pressed current will flow to the 'D3' line. Every key in the top row connects to a different line at the bottom of the diagram, so that the computer receives a different signal for each combination of keys pressed.

Whenever the computer needs to read the keyboard it merely connects a voltage to each horizontal row of keys in turn. As it tests a row it also reads the voltages at 'D0' to 'D7', and in this way it can test to find out whether or not each key is pressed. If the 'Break' key is pressed, for example, a voltage will arrive at line 'D2' when the botton row ('A6') is energised.

Joysticks From BASIC

Program 1 shows how the BASIC PEEK statement can be used to read keys. The internal design of the Genie means that if you PEEK address 14400 the computer sends a voltage to the bottom row of the keyboard matrix. If no key is pressed the value returned by PEEK(14400) will be zero, since no electricity can make its way from the row to the columns.

Program 1. Joystick Test Routine

90 CLS
100 LET P=PEEK(14400)
110 IF P AND 128 THEN PRINT @156,"BANG!" ELSE PRINT @156,"     "
120 IF P AND 32 THEN PRINT @147,"LEFT" ELSE PRINT @147,"    "
130 IF P AND 64 THEN PRINT @170,"RIGHT" ELSE PRINT @170,"     "
140 IF P AND 8 THEN PRINT @31,"UP" ELSE PRINT @31,"  "
150 IF P AND 16 THEN PRINT @286,"DOWN" ELSE PRINT @286,"    "
160 GOTO 100

If the space-bar is pressed then current flows from 'A6' to the 'D7' wire, producing a value of 128. If the 'Clear' key is pressed a value of 2 is produced - the 'D1' wire is energised. Likewise the Esc key (up arrow) connects line 'D3', giving the value 8 or 2 x 2 x 2.

At first sight it seems odd that this binary pattern should be used - 'D0' produces the value 1, 'D1' gives 2, 'D2' gives 4, and so on. However this approach means that the computer can recognise when more than one key is depressed. If you pressed Clear and the space-bar together you'd get the value 130, or 2 + 128. If you pressed all eight of the keys on the row at once you'd get the value 255, which is 1 + 2 + 4 + 8 + 16 + 32 + 64 + 128.

If you're mathematically minded you will notice that each successive value is one more than the total of those which preceded it. The important thing is to realise that the value of PEEK(14400) tells you precisely which keys in the bottom row of Figure 2 are pressed. The sequence of binary numbers used means that there is no way of producing ambiguous answers. There is only one combination of keys which could produce each value.

Keys Compared

So far it may be hard to see why this explanation is necessary. We've established that the keyboard of a computer is just a collection of switches and, since a joystick is also made up of switches, we ought to be able to connect up a joystick in place of a keyboard. In fact we can do better still. We can connect up a joystick as well as a keyboard, by wiring the computer in such a way that the joystick's switches have the same effects as some keys.

If we connect an extra switch between 'A6' in Figure 2 and the 'D7' line, we effectively give our computer an extra space-bar. If either the new switch or the space-bar is pressed, electricity will be able to pass from the 'A6' row to 'D7'. The computer will have no way of knowing which path the electricity took - it will simply sense a voltage at 'D7 when it energises 'A6'.

In fact you can add more than one extra switch if you want to. Figure 3 shows the circuit diagram of the joystick again. The layout has been altered, but the interconnections are identical - but notice how the figure resembles the circuitry which connects the Genie's arrow keys and space-bar.

Figure 3. A joystick circuit diagram re-drawn to show how it can be connected to the keyboard switches.

So, you can wire up the joystick so that it duplicates the effect of those five keys. Any program which uses the direction keys can now be controlled using the joystick, and Table 1 indicates some of the programs which may be used in this way.

Table 1. Programs compatible with the joystick

Scripsit Word Processor
Visicalc Spreadsheet
Pascal-80 Compiler/editor

Plus most other word-processors and program editors. Dozens of games, including Robot Attack, Gobbleman, Asylum, Meteor Mission, Space-Vaders, Penetrator, Starfighter, Voyage of the Valkyrie, Starfighter, U-Boat, Attack Force, Defend, Firebird...!

Wiring Up A Joystick

We have shown that it is possible to connect a Genie and a standard joystick together, but how are the actual connections made? At this point it should be said that a joystick modification will involve some small-scale rewiring inside your computer. This will invalidate the warranty, so don't attempt this unless you have some experience of electronics. However the mod requires only six soldered joints to be made, and no sensitive components are affected, so it is' not a very demanding project. The procedure is explained in very great detail to minimise the chance of mistakes.

You can buy a standard joystick for about £6 from most shops which sell computers or video games. They are marketed as add-ons for Atari and Vic computers, but they are ideal for the Genie if a few simple modifications are made. Note that the joysticks used on computers such as the BBC Micro and Dragon are not compatible with the Genie - they generate a continuously-varying signal which cannot be read without quite complicated hardware.

If you are a real arcade fanatic you could use one of the 'quality' joysticks (these are also sold as Atari add-ons). They feature multiple fire-buttons and smoother response than the 'standard' sticks, but they cost more than twice as much. Quality joysticks are generally only available from specialist computer shops. Trade names include 'Point Master', 'Le Stick' and 'Command Control'.

The Making of Mice

If you want to build your own joystick you will need to be fairly skilled at construction - it can be rather annoying to have your home-made masterpiece collapse in ruins just as you're about to set a new high-score! If you decide to spurn Atari all you will need, besides patience and a steady hand, is five fairly sturdy momentary-action switches, a small box and some connecting wire.

Figure 4 shows how you can build a 'mouse' type joystick. The box rests on the four direction switches with the fire button projecting from the top of the box. The rubber foot at the centre of the base of the box makes it impossible for the user to select all four directions at once.

Figure 4. A 'mouse' is a device similar to a joystick which can be used to move a cursor or game token to any position on the screen.

The size of the box you use is not crucial, but it should obviously be small enough to fit comfortably in your hand. The position of the fire button is really a matter of personal preference - you may find it best to fit it on the front surface of the box, where you can work it with your finger, or on one side where your thumb can reach it. Finally, if you are feeling silly enough, you can cover the whole thing with fur-fabric or knit a mouse-cosy!

Getting Hooked

Whether you build or buy your joystick, you will need a few tools to fit the modification. You'll need a soldering iron of 25 watts or less, some solder and a cross-point screwdriver. If you want to do things properly it would be a good idea to buy a plug and a socket so that you don't end up with the joystick permanently fixed to your computer. The Atari joystick plug is rather an odd one, so we chopped it off and fitted a DIN plug in its place.

If you remove the joystick plug you will find six wires inside. Table 2 indicates the colour-code used. These colours are more-or-less standard for the PET and Atari joysticks, although red, orange or pink may be used for the 'fire' button. If you are in any doubt about the colour code then it is quite easy to check it by dismantling the joystick - it is not difficult to trace the wiring.

Table 2. Joystick colour-code.

BLACK Common wire. Terminal 2.
BLUE Joystick down. Terminal 20.
BROWN Joystick right. Terminal 19.
GREEN Joystick left. Terminal 16.
ORANGE/RED Fire button. Terminal 17.
WHITE Joystick up. Terminal 18.

There are six wires, so the obvious choice of connector is a 7-pin DIN plug. Unfortunately it is rather difficult to find 7-pin chassis sockets, so we decided to use a 5-pin plug. Five wires are connected to the pins of the plug and the sixth goes to the 'screen' around the outside of the plug. So long as you use a metal DIN socket, you can pick up the signal from the screen by connecting the appropriate wire to the frame of the socket.

Another acceptable approach is to use two stereo jack plugs and sockets to convey the six signals: three on each plug. The advantage of this scheme is that it is much easier to solder to jack plugs and sockets than to DIN connectors there isn't much room for error inside a DIN plug!

First Peel Your Genie

Fit the chosen plug to the end of the joystick lead, pull out all plugs connecting the computer to anything, and then dismantle the Genie. Figure 5 shows the position of the eight screws which must be undone before the top can be removed: don't undo any other screws or the computer's power supply will fall out!

Figure 5. Peeling the Genie is a simple matter of undoing eight screws. marked by crosses in the diagram above.

Figure 6 shows the inside view of the computer. Disconnect the 20 way cable which joins the keyboard to the main circuit-board on the left-hand side of the machine. The cable can be detached from the socket on the keyboard, but be careful to pull gently and evenly so that you don't bend the wines at the end of the cable. Now you have electrically isolated the keyboard from the rest of the machine.

Figure 6. Schematic diagram of the internal layout of the Genie: the keyboard is joined to the main printed circuit board by a 20-way plug and socket.

Eight screws around the perimeter of the board hold it in place; undo them and lift the keyboard assembly out of the computer. At this point the socket for the joystick can be fitted. Figure 6 shows the position of this; it is mounted in the lower part of the box, just below and to one side of the main circuit-board.

Six wires will run from this socket (ideally via another plug and socket pair) to the -keyboard assembly. Figure 7 shows where you should connect the wires to the keyboard.

Figure 7. Connection diagram for the joystick or 'mouse' - see also Table 2.

Turn the keyboard assembly so that the keys face downwards and the space bar is furthest from you. At the left-hand side of the closest edge of the board is a row of twenty terminals, and on the other side of the board these connect to the 20-way socket. Solder the six wires from the joystick socket to appropriate terminals on the row, using Figure 7 as a guide. Table 2 indicates which wire goes to which terminal, assuming that the terminals are numbered 1-20 from left to right (the Revision B Video Genie circuit board actually labels the terminals in this way).

When you solder the wires take care not to bridge the gaps between terminals. If you do accidentally do this you will have to remove the 'bridge' of solder, or the computer won't be able to distinguish between keys properly. The terminals are not directly connected to any sensitive electronics, but you should still avoid heating up any terminal for longer than a few seconds at a time, or you may damage the socket on the other side of the circuit-board.

If you have a later model Genie I computer with four arrow keys rather than the original ESC and CTRL keys you may find that the keyboard circuit board in your computer contains a set of un-used pads. These are intended for the numeric keypad added for the Genie II, but include all the same signals needed for the joystick. You may find it more convenient to make the new connections there, tracing connectivity from the locations shown in Figure 7 to the new pads, near the middle of the computer.

Another idea that has come up since the original article was written is that you may wish to leave the plug on the joystick and fit a 9 pin male D-type connector in the computer. As well as the Atari and Vic joysticks mentioned in 1983 you will find that keypads and joysticks made for Sega Megadrive, MSX, Commodore 64 and Amiga systems use a compatible circuit - unlike controllers made for Sony or other consoles, or the 15 pin and USB ones used on Apple and IBM-compatible PCs, or those for Dragon, Tandy Color Computer or BBC micro systems; those are not compatible with this circuit as they use analogue or serial rather than digital parallel signals.

Once all of the wires are safely connected you may re-assemble the computer, but before you do so you should look closely at all of the work you have done - make one final check of all of the connections.

Screw the keyboard assembly back in place, taking care not to do the screws up too tightly, since they bore directly into the plastic pillars of the box. Re-connect the 20-way cable between the main circuit board and the keyboard.

The trick here is to make sure that all of the wires are straight before you insert them and push them all into the socket at once, evenly. Don't force the wires - if they are reluctant to go in then pull them back, straighten them, and try again. Be careful not to plug in the cable out-of-step one place to the left or right of the correct position. It is easy to make a mistake and 'miss' with one wire, so that it does not enter the socket. Look above and below the socket to make sure that no wires have gone astray!

The last step is to close the computer case. Put the lid back and replace the eight screws which hold it in place. The shortest of these screws goes into the hole nearest to the mains-lead; the positions of the other screws are interchangeable.

A Trying Time

If all has gone well, you should have a working joystick modification. This project has been successfully carried out by many digital disaster areas, including some people who have trouble wiring a 13 amp plug the same way twice, so your joystick should work perfectly. However it is best to test the modification systematically.

Plug in your computer and TV, but don't connect the joystick yet. Likewise, if you've got a disk system then leave it disconnected. It is very unlikely that you will have done something wildly wrong, but it is best to connect up the minimum of hardware just in case.

When you power up the computer it should work as normal. If the usual display does not appear then you have accidentally altered the workings of the computer. No one who has fitted the modification so far has had this problem, so award yourself ten marks for ingenuity, then unplug the computer immediately and check all of the connections you made.

You may find that the display appears as normal but the keyboard does not work. If the red power light is not glowing then you have plugged in the 20-way cable wrongly. Likewise if some keys do not appear to work you may have fitted the cable in the wrong manner. Unplug the computer and check the position of the cable. It is very unlikely that you have damaged the computer in any way.

You might find that some keys generate the wrong codes - 'J' and 'R' might both produce the 'R' character, for example. If this has happened then you have accidentally shorted out some of the keyboard connections. Don't worry, this will not harm the machine but it will make it rather difficult to type programs until you cure the problem! Unplug your computer and check the wiring on the underside of the 20-way connector, looking for solder 'bridges' between the terminals. If you can't see any it may be that wires on the joystick socket are shorting together. Re-solder the wires so that they can't accidentally touch one another.

If all of the keys on your machine seem to work correctly then load Program 1. Connect up the joystick, and when you push the joystick forwards the message "UP" should appear. Likewise the fire button should produce "BANG" and the other directions should work sensibly. Make sure that you can generate 'diagonal' signals - it should be possible to produce "UP" and "LEFT" simultaneously by pushing the joystick away and to the left.

At this point you may find that some joystick directions don't register: this means that the connection between the joystick and the keyboard is not complete. First check the plug on the joystick lead. If that looks correct then dismantle the computer and ensure that the wires between the socket and the keyboard are intact and correctly placed.

The joystick may create multiple signals even when you only move it up, down, left or right. In this case there is a short circuit in the plug at the end of the joystick lead (a short inside the computer would stop the keys working properly).

There is one more possible problem. It might be that all of the directions produce a unique signal but you get the wrong message for a given direction. Perhaps "LEFT" appears when you move the stick right. You can correct this in a number of ways, but the easiest is probably to change the wiring inside the joystick. A standard Vic or Atari joystick will contain clips connecting coloured wires to the internal switches so you can swop over colours by exchanging the clips, Alternatively, swop over the soldered wires in the plug or inside the computer.


By now you should have full arcade-style joystick facilities on your Video Genie. At last you can look Atari owners straight in the eye. Just centre them in your Space Gunsights and press the 'fire' button...

Simon N Goodwin, simon (at) studio (dot) woden (dot) com