I don't have a pic of the underside of my board handy and I'm not keen to go out to my gar in the rain and remove the hardtop to then remove the side of my roll cage to then access the Megasquirt in the footwell and then pull it apart to get a picture.
Go back to the diyautotune page and follow the written instructions.
"Getting the Tach Input Signal:
Run MSnS-E and the "2nd Trigger" Wheel Decoder option (where the missing tooth on the wheel decoder comes from a second sensor, usually cam position). The CAS has a CMP and CKP signal, so you take the outputs of each and run them into the MegaSquirt. Take the CKP signal (4 holes in the Optical sensor wheel, the "outer" ring of the CAS), and run it to the primary tach input on the MS with a 12v pullup resistor - then you take the inner ring of the CAS (1 hole in the sensor wheel) and run it to the secondary tach input on MS (it's pin 11 on the MS-I cpu) with a 5v pullup resistor.
If you follow my documentation for the mods below you'll externally wire the tach inputs like this:
The Primary tach input (CKP) (Reported to be White on Miatas) is connected to pin 24 on the MegaSquirt's DB37
The Secondary tach input (CMP) (Reported to be Yellow on Miatas) is connected to pin 25 on the MegaSquirt's DB37"
quoted from http://www.diyautotune.com/tech_arti...azda_miata.htm