Nowadays the most common analogue joystick type is PC analogue joystick. This joystick model was presented by IBM together with their first IBM PC computer. The joystick is just a basic analogue joystick with two buttons. The original joystick interface had circuit for connecting two joysticks, but had only one joystick connector. A special Y-cable was needed if there was need for two joysticks at the same time. Later time some manufacturers put two connectors to their interface card and some card manufacturers implemented only one joystick input. Fortunately most of the card nowadays have option for two joysticks like the original IBM joystick card.
The joystick interface card was designed to be as simple and cheap as possible. The card consisted only of bus interface electronics and four monostable multivibrators (all in on 558 chip). Those monostable multivibrators were simple timer circuits which put out a pulse with width directly proportional to the joystick resistance value. The pulse width was then measured using software loop. This has caused anormous amounts of problems to game programmers when computers have become faster and faster all the time. On faster machines, the joystick routine in the software does not read the joystick signal properly resulting in a timing problems. Some dedicated joystick cards are designed to vary the joystick signal so the software can properly detect the joystick and process the data.
|3||X1||Joystick 1 - X|
|4||GND||Ground (for switch 1)|
|5||GND||Ground (for switch 2)|
|6||Y1||Joystick 1 - Y|
|11||X2||Joystick 2 - X|
|12||GND||Ground (for switch 3,4)|
|13||Y2||Joystick 2 - Y|
|15||+5V||+5 VDC or N/C|
The joystick consists of two potentiometers with variable resistance value between 0 Ohm and 100 kohm (in some joysticks up to 150 kohm). The potentiometer resistances have the minimum values when the joystick is at the top left position. One end of the potentiometer is connected to +5V pin and the center pin is connected top the analogue input of the joystick. The other end of the potentiometer is left not connected to anywhere.
There are two commonly used ways how PC analogue joystick stick mechanism is constructed. Some joystick convert the stick position to linear motion, whcih then changes the position of the slider in about 100 kohm linear potentiometer. More popular construction is to use normal axial potentiometers and the joystick movement directly turns those potentiometers. Some joystick used special 100 kohm potentiometer which can only turn that 60..90 degrees which joytick can turn. The more common construction is to use the standard 470 kohm (lin) 270 degree potentiomer and use about one fourth of the scale from the beginning (in this way getting 0..120 kohm value range). Usually those potentiometers are normal carbon slider potentimeters which do not last long in intense gaming.
How PC joystick port hardware works
The joystick port is a very simple 8 bit I/0 card which resides in ISA bus I/O address 201h. The CPU can read and write to the joystick port I/O address 201h. Writing to that address starts joystick postition measurement. Joystick interface only uses the signal that somebody is writing to the I/O address to reset the multivibrators in the card. The data value is not stored anywhere, so it is really same what value is written to this address.
When you read one byte from I/O addess 201h, you get the status information of the joystick interface. The following table will show how the bits are mapped in the value you get.
Game port 201h byte: _______________________________________________________ | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | | but4 | but3 | but2 | but1 | stk4 | stk3 | stk2 | stk1 | |______|______|______|______|______|______|______|______|
The four most significant bits tell you the state of the joystick buttons. Four least significant bits tell the state of the multivibrators which are used for measuring the resistance value of the joytick position potentiometers. More accurate description of the bit meanings can be found at the table below:
7 6 5 4 3 2 1 0 * . . . . . . . Button B2 (pin 14), 0=closed, 1=open (default) . * . . . . . . Button B1 (pin 10), 0=closed, 1=open (default) . . * . . . . . Button A2 (pin 7), 0=closed, 1=open (default) . . . * . . . . Button A1 (pin 2), 0=closed, 1=open (default) . . . . * . . . Monostable BY (from pin 13), 1=timing, 0=timed-out . . . . . * . . Monostable BX (from pin 11), 1=timing, 0=timed-out . . . . . . * . Monostable AY (from pin 6), 1=timing, 0=timed-out . . . . . . . * Monostable AX (from pin 3), 1=timing, 0=timed-out
Resistive analogue inputs (joystick position)
Joystick position inputs are simply inputs where a variable 0-100 kohm joystick potentiometer is connected to. The potentiometers are wired between the +5V power pin and one potentiometer input pin, are connected between GROUND and 5V. The following picture tries show how the wiring is done:
+5 ________________ | stick1 ________ | | 100K | _///\_____| stick2 ________ | | 100K | _///\_____| stick3 ________ | | 100K | _///\_____| stick4 ________ | | 100K | _///\_____|
The resistance value of the joystick potentiometers are measured using very simple monostable multivibrator circuit, where a small capacitor is loaded through the joystick potentiometer to a certain voltage level. The joystick interface has four of this type of monostable multivibrators. Typically joystick interface multivibrators are all in one 558 IC (that IC is like four simplified 555 type timers in one IC) for two joystick ports and 556 (dual 555 timer) for one joystick only port.
Joystick Pot in >---+ (stick n) | R1 / ____________ 2.2K | | / | |>----------> data bit to ISA bus | | Monostable |out +-------| Multi- | | | vibrator | C1 --- | |<---------->write
NOTE: Reference design in Crystal Semiconductor CD4376B calls recommends C1 to be 5.6 nF and R1 to be 2.2 kohm for their soundcard reference design.
The multivibrator work on the following way:
- 1. Normally on idle state the capacitor C1 is fully charged (5V) and multivibator output gives out logic 1.
- 2. Computer writes to I/O address 201h to reset the multivibrators. The multivibrator get a triggger pulse from I/O write signal and it discharged capacitor C1. The multivibrator output goes to logic 0 because the capacitor is discharged (0V).
- 2. The capacitor starts to charge form the current which goes through R1 and the joystick potentiometer.
- 3. When the capacitor voltage reaches certain oltage threshold level, the multivibrator output goes back to logic 1.
The bigger the potentiometer resistance value is, the longer it takes for the capacitor to reach the threshold voltage. The time how long it takes for each multivibator to reach the logic 1 after the triggering is measued using software and that value tells the resistance of the potentiometer (and the stick position as well because the potentiometer is connected to the stick). This simple method for measuring the resistance value is quite cheap, but not very accurate.
This simple joystick port hardware implementation (originally designed for 4.7 MHz IBM PC) causes many headaches to game programmers (problems of different computer speeds, joycitck port differences, problems in multitasking operating systems etc.). Big tolerances in joysticks and joystick ports mean that the every game must have an option to cablibrate the joystick (which actually means that the program adapts to accept the values in th range which the joystick reading routine does and then converts the values to its internal format using the values measured in joystick calibration process).
Switch inputs (joytick buttons)
The joystick buttons are simple on/off inputs. Joystick button either leaves the pin floating or connect it to the ground.
___ button1 ______o o_______ ___ | button2 ______o o_______| ___ | button3 ______o o_______| ___ | button4 ______o o_______| | GND ________________|
The state of the buttons is directly fed to the joystick interface and through it to ISA bus data lines when somebody reads the joystick interface I/O address. The buttons are very easily wired. Every input has 1 kohm pull-up resistor R1 which ensires that the joystick button pins stay in +5V potential (logic 1) when no button is pressed down.
+5V | / R1 1K / Joystick | button in >--------+-------------------------> data bit to ISA bus (button n)
NOTE: Reference design in Crystal Semiconductor CD4376B calls for 4.7 kohm pullup resistor and 1000 pF capacitor from input to ground.