The M84 does not have support for knock sensors in the hardware and is not able to use an external device for this either as there is no support in the Firmware for any knock control.
Stephen Dean MoTeC Research Centre Melbourne, Australia
You can do a ignition trim based on a 0-5v from a external knock detection device (ex Phormula KS, etc) but is not very effective, as it's only momentarily, and you cannot set the severity of it's reaction... Might save the engine in one or two cases thow