9 Converters: Between Apple and Atari
If you or your friends have Atari game systems and an Apple computer, you might want to use your favorite Atari joystick to play games on the Apple. Or maybe you want to play games on your Atari using an Apple joystick. With the two converters described in this chapter you can have both these options.
There are some limitations, however. The Atari-type joysticks have a digital output, so you can use them on the Apple for only the simplest games, like Snoogle, for which the joystick directs the movement of a character. For the many arcade games that require only digital input, Atari joysticks are much faster than conventional Apple joysticks.
We will cover the construction of two small circuit boards that function as converters: the first one, an Atari-to-Apple converter, plugs into an Apple computer. You plug your Atari joystick into this converter to play fast-action arcade games on the Apple. The second converter plugs into an Atari system so that you can plug an Apple joystick into it. Now you can play Atari games with your Apple controllers.
The Atari-to-Apple converter will greatly increase your speed in playing arcade games on the Apple, especially if it is used with homebuilt Atari paddles like the Tipping Disk (see chapter 5). Consequently, we expect that there will be greater interest in this first type of converter. The second converter, the Apple-to-Atari, is included because there is educational value in making up the circuit. But even more important, newly developed flight simulator or race car games for Atari-type systems will greatly benefit from being played with the steering wheel and foot pedal designs given in this book. Then your Apple-to-Atari converter will provide an exciting new realism when you play these games.
Both of these boards are good beginning electronics projects. Each has one or two integrated circuits, a few components, and some cables and plugs. Each is easily assembled on a small piece of general purpose printed circuit board. The parts are inexpensive and easy to obtain.
CHARACTERISTICS OF APPLE AND ATARI JOYSTICKS
The Apple joystick and the Atari joystick are two completely different devices. The Apple joystick has two lever-controlled pots and a pushbutton, while the Atari joystick is a collection of five pushbuttons, four of which are mechanically connected in two pairs. The Atari-to-Apple converter must make pushbutton closures look like changing pot values. The Apple-to-Atari converter must make changing pot values look like switch closures. In addition, there are two other differences between these joysticks that may cause some confusion when you build these converters.
First, the Apple joystick has its common connection (one side of each pot and each pushbutton) connected to the +5 volt supply line. The common connection for the Atari joystick (one side of each switch) is normally connected to the ground wire. The easiest way to design each of these connectors was to make the common connection of the type required by the host computer. This means that pin 8 of the Atari connector, which normally would be connected to ground, is connected to the +5 volt supply (pin 1) of the Apple game plug on the Atari-to-Apple converter.
The second difference is that the connector on the end of an Apple joystick cable has pins on it, so it is defined as a plug. The connector on the end of the Atari joystick cable has holes in it, so technically it is a socket. We have used the correct terms in the hope that you will order the correct parts. If you get confused, blame Atari, since they have installed a socket where you normally expect to find a plug.
Each circuit is made up from one-third of a printed circuit board (Radio Shack #276-154). You can mount the board in a small plastic box like a cassette tape holder or inside a block of flexible packing foam. To cut the circuit board, score it deeply with an X-acto knife and break it over the edge of a table. Drill the holes for the wire ties with the end of the knife and file all the circuit board edges smooth. Mounted on the board are several resistors and capacitors and sockets for the chips. Both boards draw the small amount of power they need from the host computer.
Most of the connectors for both converters are mounted on short pieces of cable. The Apple-to-Atari converter does have one socket for the Apple joystick; this socket is mounted directly on the board rather than on a cable.
THE ATARI TO APPLE CONVERTER: CIRCUIT THEORY
Figure 9-1 is a sketch of the Atari-to-Apple Converter without an enclosing box. Since this simple circuit can handle two joysticks, giving you the option of playing two-person games, the sketch shows plugs for two Atari joysticks. If you won't be playing such games you can install only one connector and save a little money. The circuit requires two chips, one capacitor, and fourteen resistors.
Figure 9-2 is a simplified schematic of the circuit that will help you understand how the card works. As noted, two chips are used. The 74LS04, a digital chip, has six inverters. If the input to an inverter is high (+5 volts) then its output goes low (nearly 0 volts), and vice versa. The second chip, the 4066, is a special purpose CMOS chip. It acts like four small single-pole, single-throw relays. If one of its control lines is brought high (+5 to +15 volts), then one of the relays closes and conducts electricity. If the control line is low (0 volts), then the relay is open and does not conduct.
The Atari joystick has two internal normally-open switches
that correspond to the directions up and down. For the Apple to read these
two switch closures they must be converted to pot readings between 0 and
Depending on the position of the Atari joystick, the game control circuit may read 68K (R7), 0, or 168K (R7 + R12). These resistive values are read as 128, 0, and 255 by the Apple, and the program interprets them as center, up, and down.
With the handle in the center position, R7 is in the circuit but R12 is bypassed by one section of the 4066, so the reading is 128. If the handle is pushed up, the switch in the joystick bypasses R7 and the reading is 0. If the handle is pushed down, the section of the 4066 opens and the reading is 255.
The inverter in the 74LS04 and the pull-down resistor R3 make the section of the 4066 operate the opposite of the down switch in the joystick: when the down switch is open the 4066 section is closed and vice versa.
This circuit is repeated for the left and right switches. Since there are four switches in the 4066 chip and six inverters in the 47LS04 chip, you can control two joysticks with just these two chips simply by adding a few more resistors and a second connector.
THE ATARI-TO-APPLE CONVERTER: WIRING
Figure 9-3 is the complete schematic of the Atari-to-Apple converter. Once again, follow the procedures outlined in chapter 14 for checking your wiring by coloring in two copies of the schematic. The Apple plug is a 16-pin DIP header and is shown as it appears from the top. The Atari connectors are shown from the back as they will appear when you solder them.
A word of caution: the 4066 is a CMOS chip and these are easily damaged by static electricity. Leave the 4066 in its protective package until you have completed the circuit and thoroughly checked it out. It is imperative that you use a socket on the board for this chip.
We used #24 solid wire from a scrap of telephone cable for jumpers on the board. Ribbon cable was used for the cables to the connectors. The cable for the Apple plug requires eight conductors, but we doubled up the +5 supply and ground, so ten conductors were installed. Two 6-wire runs of cable were used for the Atari sockets.
Double check your work before applying power to the circuit. Color in the second copy of the schematic, using an ohms meter to trace the wires. When everything checks out, plug the two chips into their sockets on the board, making sure the #1 pins are in the correct places, and install the plastic hoods on the Atari connectors. The hoods have small plastic tabs that may have to be filed down. They also come with mounting bolts, which aren't needed for this project.
To test the converter, turn off your computer, plug in the new unit (without a joystick attached to it), and turn the computer back on. If the computer does not start up exactly as usual, turn it off immediately and recheck all your work. When the circuit checks out, turn off your computer, plug in an Atari joystick, and turn on the computer again. Now run the Controller Checkout program from the software section and check out the game paddle reading of your new adapter and joystick combination.
THE APPLE-TO-ATARI CONVERTER
Figure 9-4 is a sketch of the Apple-to-Atari converter. If an Apple-style joystick or set of paddles is plugged into the empty socket on this board and the Atari socket is plugged into an Atari game port, the Apple joystick will perform like an Atari joystick.
The mechanical construction of this converter is quite
similar to the one just described. It is built on one-third of a general-purpose
circuit board and has one chip, twelve components, a cable and socket to
connect to the Atari, and a simple DIP socket for the Apple joystick.
Figure 9-5, the simplified schematic, shows that this converter works on completely different principles than the first one. The pot in the Apple joystick is used in a resistive bridge with R1 so that the voltage on pin 1 varies with the pot setting. Capacitor C3 reduces electrical noise.
The LM339 chip contains four voltage comparators, each
with two inputs and one output. They compare the voltage on one of their
inputs with a reference voltage on the other input. They produce a digital
output to indicate whether or not the input voltage is greater than the reference.
The pull-up resistors R4 and R5 make the outputs look like the switch closures
of a standard Atari joystick.
The diodes D1 and D2, along with resistor R3, provide two reference voltages (0.7 volts and 1.4 volts) for the comparators. Here we are using a property of a real diode, its forward conductance voltage drop, which a theoretically ideal diode would not have. If we passed a small current through an ideal diode there would be no voltage drop. If we pass a current through a real silicon diode, however, there is a drop of about 0.7 volts, and this drop changes very little with fluctuations in temperature and current.
Two comparators in the LM339 compare the voltage from pot 1 with our two reference voltages and put out the correct digital signals for the joystick position: left, right, or center. A similar circuit checks pot 0 for the signals for up, down, or center.
You will have to adjust the circuit to use this converter with joysticks that have correction caps. The resistive bridge circuit measures resistance only, so if the pot is not 150K, resistors RI and R2 will have to be changed. The correct value is about one and a half times the value of the joystick pot.
Figure 9-6 is a detailed schematic for wiring the converter. Be sure you lay out the #1 pins on the chip and socket in the correct direction. The plus lead of capacitor C1 must go to the +5 supply. On the LM339, the pins used for the +5 and ground are not those commonly used for digital chips (the 74LS series), so double check them before applying power.
Double check your wiring, do not plug any electronic device into a live socket, and be quick to turn off the Atari if everything doesn't look absolutely right.
As mentioned earlier, the circuit boards should be mounted in small boxes to prevent the wires from shorting. You could use cassette tape boxes, cutting out holes for the cables with a coping saw and small files. Alternatively, you could encase the circuit in a foam block, as was described for the Multiple Socket Extension (see chapter 3). Or you could purchase a small electronics box, fit the circuit board into the box before mounting the components, and drill and file out holes for the cables. Depending on the box you use, you may have to cut the PCB and arrange the components differently from those of the prototype.
From the standpoint of their electronics, these two converters are the most sophisticated projects in this book. A little care in their construction, however, will give you an excellent lesson in electronics and a device that will greatly increase your enjoyment of your other controllers.
Atari to Apple Converter
Apple to Atari Converter
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