Difference between revisions of "Atari-ST mouse adapter"
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The low-level XA/XB/YA/YB signal must be watched by software (the 300Hz interrupt is probably too slow to sense fast motions, so there's little chance to have the mouse driver running in background). | The low-level XA/XB/YA/YB signal must be watched by software (the 300Hz interrupt is probably too slow to sense fast motions, so there's little chance to have the mouse driver running in background). | ||
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| + | === CPC Plus Compatibility === | ||
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| + | The circuit reportedly doesn't work on CPC Plus computers. Probably simply because of the missing Fire3 signal on the joystick port of the CPC Plus (this could be fixed easily, by connecting the missing pin inside of the computer). There are also additional diodes in the CPC Plus, which may drop the voltages a little, but probably do not cause problems in this case. | ||
== Timings == | == Timings == | ||
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XB _____-------------------------------____ | XB _____-------------------------------____ | ||
In the above case, checking only raising XA would miss the direction change on falling XA (if that happens repeatedly, then the mouse would disappear towards the screen border, eg. if the mouse is in an unstable position, without actually being moved). | In the above case, checking only raising XA would miss the direction change on falling XA (if that happens repeatedly, then the mouse would disappear towards the screen border, eg. if the mouse is in an unstable position, without actually being moved). | ||
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== Software supporting the ST-Mouse == | == Software supporting the ST-Mouse == | ||
Revision as of 08:15, 4 April 2010
This adapter allows to connect Atari ST mouse and trackball.
Contents
Schematic
The schematic was released in german magazine "Schneider Magazin, 07/1988", apparently using 5 diodes and a 4.5V battery.
Aside from Atari-ST mice, the circuit should also work for Amiga mice, which are basically the same, but with XA/XB/YA/YB connected to other pins, which could be resolved at software side (or at hardware side).
CPC Joy Port Atari-ST Mouse Amiga Mouse JoyUp Pin1 --|>|------------- Pin1 XB Pin1 YA JoyDown Pin2 --|>|------------- Pin2 XA Pin2 XA JoyLeft Pin3 --|>|------------- Pin3 YA Pin3 YB JoyRight Pin4 --|>|------------- Pin4 YB Pin4 XB JoyFire2 Pin6 ------------------ Pin6 Left Button Pin6 Left Button JoyFire1 Pin7 ------------------ Pin9 Right Button Pin9 Right Button JoyFire3 Pin5 --|>|------------- Pin8 GND Pin8 GND JoySel1 Pin8 ------------------ Pin8 GND Pin8 GND Batt4.5V (-) ------------------ Pin8 GND Pin8 GND Batt4.5V (+) ------------------ Pin7 +5V Pin7 +5V
The low-level XA/XB/YA/YB signal must be watched by software (the 300Hz interrupt is probably too slow to sense fast motions, so there's little chance to have the mouse driver running in background).
CPC Plus Compatibility
The circuit reportedly doesn't work on CPC Plus computers. Probably simply because of the missing Fire3 signal on the joystick port of the CPC Plus (this could be fixed easily, by connecting the missing pin inside of the computer). There are also additional diodes in the CPC Plus, which may drop the voltages a little, but probably do not cause problems in this case.
Timings
The Atari/Amiga mice do output low-level signals. Ideally, software should check the signals more than 500 times per second. Using the CPCs 300Hz interrupt to check the signals might be a bit slow; it would work, but may miss some signals when the mouse if moved too fast.
Reading the mouse more often than 300 times per second would be better, but it'd require more CPU load since it can't be done via interrupts. Possibly best solution would be combination of IRQ and non-IRQ handling: Always check the mouse on 300Hz interrupts, and - if the program isn't busy with other tasks - additionally check it via polling.
Motion Signals
The XA/XB indicate horizontal motion (and YA/YB vertical motion), like so:
One Direction Opposite Direction XA _______----____ _____----______ XB _____----______ _______----____
The signal timings depend on how fast the mouse is moved (ie. if the mouse is not moved, then it may stop anywhere in the above diagram, eg. while XA and XB are both high, both low, or one high and one low). The simpliest approach is to wait for raising edge on XA, and then to check XB (either XB is already high, or XB is still low) and increase/decrease the software's mickey counter accordingly. In situations where the direction changes, it may be also recommended to handle "half mickeys" on falling edges on XA, eg.:
One Direction Stop Opposite Direction XA _______---------------------------______ XB _____-------------------------------____
In the above case, checking only raising XA would miss the direction change on falling XA (if that happens repeatedly, then the mouse would disappear towards the screen border, eg. if the mouse is in an unstable position, without actually being moved).
