Kempston Mouse

This mouse is supported by The OCP Art Studio, The Advanced OCP Art Studio and Carrier Command.

Pictures

• Kempston Mouse Interface
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  Back (showing mouse connector) - faces away from CPC

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  Front (showing edge connector) - faces CPC

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  Side

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  Other side

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  From above

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  From below

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  PCB (joystick connector side)

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  PCB (edge connector side)

• Kempston compatible mouse
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  Kempston Mouse

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  Kempston Mouse (under)

Technical

The interface described here was loaned to arnoldemu by yoshi.doshi.

PCB has a 50-way edge connector. There is no through connector. The mouse connects via a 9-pin female D-sub (standard joystick socket) which is on the PCB.

The board is labelled "(C) KEMPSTON MICRO AMSTRAD MOUSE I/F" and "ISS B".

The ICs are labelled and are:

IC1 DM74LS00N  QUAD 2-INPUT NAND 
IC2 DM74LS191N SYNCHRONOUS UP/DOWN COUNTERS WITH DOWN/UP MODE CONTROL (4-bit binary counter)
IC3 DM74LS191N SYNCHRONOUS UP/DOWN COUNTERS WITH DOWN/UP MODE CONTROL (4-bit binary counter)
IC4 74LS125APC QUAD 3-STATE BUFFER
IC5 DM74LS191N SYNCHRONOUS UP/DOWN COUNTERS WITH DOWN/UP MODE CONTROL (4-bit binary counter)
IC6 DM74LS191N SYNCHRONOUS UP/DOWN COUNTERS WITH DOWN/UP MODE CONTROL (4-bit binary counter)
IC7 DM74LS138N 1-OF-8 DECODER/DEMULTIPLEXER
IC8 DM74LS257BN 3-STATE QUAD 2-DATA SELECTORS/MULTIPLEXERS
IC9 DM74LS257BN 3-STATE QUAD 2-DATA SELECTORS/MULTIPLEXERS

The mouse that comes with the interface has 2 buttons. It is a ball mouse. Unknown brand. Green label reads "Q.C. PASSED. MADE IN TAIWAN. R. O. C.". Mouse has 9-pin DIN like a joystick. Unknown if compatible with Atari ST or Amiga or neither.

The schematics and PCB are 90% the same as the Spectrum's. They differ in the port decoding.

NOTE: Counters are reset to 0 at power on. It's not yet confirmed if they are reset to 0 from a bus reset.

I/O decoding

Decoding seems to be (some testing to confirm this):

I/O port for X position is decoded as xxxx x0x1 xxx0 xxx0 (FBEE is normally used)
I/O port for Y position is decoded as xxxx x0x1 xxx0 xxx1 (FBEF is normally used)
I/O port for Buttons is decoded as    xxxx x0x0 xxx0 xxxx (FAEF is normally used)

Confirmed connected: A10, A0, A4, A8.

In Spectrum interface A10 chooses X,Y. On CPC interface A0 chooses X,Y. In Spectrum interface A8 chooses buttons or directions. On CPC interface A8 also chooses this. For CPC A4 must be 0. For CPC A10 must be 0.

Movement

Port FBEEh READ: Kempston Mouse 8 bit X position (left: -ve, right: +ve)
Port FBEFh READ: Kempston Mouse 8 bit Y position (up: +ve, down: -ve)

NOTE:

• Movement is accumulated so you only need to read the mouse status when you want e.g. once per frame.
• Speed is 1 for slow movement of the mouse and 12 for very fast movement.
• The position values are not reset after reading.
• The position values do not change if the mouse is not moving
• Position values wrap around e.g. when moving left with movement speed of 1, 0 wraps around to ff (0-1 -> ff) and when moving right with movement speed of 1, ff wraps around to 0 (ff+1->0)
• To calculated the distance moved since last read subtract the new value read from the previous value read and AND with ff. The value is the amount of movement.

0 means no movement. -ve means moving left/down, +ve means right/up.

  dist=new-old, old=new
whereas,
  dist=00h : no move
  dist=+01h..+7Fh moved right/up
  dist=-01h..-80h moved left/down

Buttons

Port FAEFh READ: Kempston Mouse Buttons
bit 0: Right Button (active low)
bit 1: Left Button (active low)
bit 2..7: not used. report as high (1)

Number of supported mouse buttons is unknown. It is believed to be 2. The mouse that comes with the interface has two.

Review (German)

From an article about the "Amstrad Consumer Show" in German magazine "Happy Computer 12/1986":

• "Bewundern Sie auch die Maussteuerung eines Atari ST oder eines Schneider PC? Bei Kempston gibt es für 69,95 Pfund auch für Ihren CPC oder Joyce so eine komfortable Benutzeroberfläche. Die Maus entspricht dabei völlig dem Standard mit zwei unabhängigen Tasten. Das mitgelieferte Desktop-Programm braucht sich vor den Brüdern auf 16-Bit-Computern nicht zu verstecken."

Get mouse position on screen

Calculating the mouse position from the information given on the Kempston mouse ports is easy.

We will proceed in 2 steps:

1. Calculate mouse pointer location on a virtual screen that has a 640*400 resolution.
2. Translate position from the virtual screen to the current CPC screen mode.

Here is the algorithm in pseudo-code:

// initMouse initializes variables and centers the mouse pointer on screen
function initMouse() {
 maxX = 639;
 maxY = 399;
 // centering the mouse pointer on the screen
 virtualX = maxX >> 1; // virtualX = maxX/2
 virtualY = maxY >> 1; // virtualY = maxY/2
 // store raw mouse values
 oldX = inp(&FBEE);
 oldY = inp(&FBEF);
 // get mouse pointer position
 refreshMouse();
}

// refreshMouse has to be called before you redraw the mouse pointer (and ideally on every frame)
function refreshMouse() {
 // get raw mouse values
 rawX = inp(&FBEE);
 rawY = inp(&FBEF);
 // get the relative mouse displacement since last call
 deltaX = rawX - oldX;
 deltaY = rawY - oldY;
 // store raw mouse values
 oldX = rawX;
 oldY = rawY;
 // calculate new unclipped virtual position
 virtualX = virtualX + deltaX;
 virtualY = virtualY - deltaY; // Kempston mouse Y-axis is inverted compared to screen coordinates!
 // perform clipping
 if (virtualX < 0) then virtualX = 0;
 else if (virtualX > maxX) then virtualX = maxX;
 if (virtualY < 0) then virtualY = 0;
 else if (virtualY > maxY) then virtualY = maxY;
 // now we translate position from the virtual screen to the current CPC screen mode
 mouseY = virtualY >> 1; // mouseY = virtualY/2
 if (mode2) then mouseX = virtualX;
 else if (mode1) then mouseX = virtualX >> 1; // mouseX = virtualX/2
 else if (mode0) then mouseX = virtualX >> 2; // mouseX = virtualX/4
}