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I have an OEM touchpad I want to connect to a PC. The drivers for this touchpad are widely available. I asked the manufacturer for the original datasheet, and it says there, that the touchpad interface/protocol is PS2/SMBus.

On the device there is an 8 pin connector - of which 7 pins are used. They are laid out as follows: Pin assignment

I do know however that a PS/2 connector has only 6 pins (7 incl. the shield, but is it used at all?), and not 8. I can't see how this connector is compatible with PS/2.

FWIW, this is what the spec had to say about interface:

Interface

PS2

Refer to IBM PS2 specification. Internal pull-up resistors (5k typical) are provided on DATA and CLOCK lines.

All the commands specified by IBM are implemented, except for the following:

  • "Scaling" and "Set Resolution" commands generate an "ACK" response from the touchpad, but leave the touchpad resolution unchanged.

SMBus

Refer to the System Management Bus (SMBus) Specification. Version 2.0.

That's it.

So am I looking here at something that is technically possible?

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Your TouchPad is a modular embedded device that offers multiple protocols output. You can either wire the ps/2 pins internally or externally with a ps/2 cable, or wire the SMBUS pins (mainly internal only in practice). If the TouchPad is really nice, it might support both interfaces at the same time, since no pins share functions or overlap.

All pins are broken out to a single 8 pin connector so an OEM laptop builder can wire it as they want. Manufacturer convenience, nothing more.

Newer TouchPad also come with usb interfaces instead of ps/2. Those can be wired to a usb cable the same way.

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Only four of the six PS/2 pins are used in a real mouse - clock, data and power/gnd. Clock and data are open collector, hence the pullups.

From Wikipedia :

PS2 Mouse

There is a useful description of the PS/2 mouse protocol here, which I've waded through myself to get a PS/2 mouse talking to a mid-range PIC.

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  • \$\begingroup\$ That matches the first 4 pins of the touchpad, but what about pins 5-7 (SMbus)? \$\endgroup\$
    – pilau
    Commented Dec 17, 2014 at 12:25
  • \$\begingroup\$ @pilau I'm assuming that it's either PS/2 or SMBus, not both. \$\endgroup\$
    – Roger Rowland
    Commented Dec 17, 2014 at 12:27
  • \$\begingroup\$ What do you mean? From what I gather, it says the interface is PS/2 (physical layer) and the protocol is SMBus (logical layer) - or am I mistaken here? \$\endgroup\$
    – pilau
    Commented Dec 17, 2014 at 12:28
  • \$\begingroup\$ @pilau I can't imagine how you'd need both. The PS/2 protocol already has everything you'd expect. Have you tried anything yet? \$\endgroup\$
    – Roger Rowland
    Commented Dec 17, 2014 at 12:29
  • \$\begingroup\$ No, but I worry of burning the touchpad if I connect something the wrong way. \$\endgroup\$
    – pilau
    Commented Dec 17, 2014 at 12:31

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