Putting this in a post so that people can search and find the answer.

If you have connected your DL1 to the ECU's RPM ouput and the RPM readout on your stock dash no longer works properly, Race Technology can provide you with simple RPM isolator circuit. Some OE dashes that are very voltage sensitive (S2000, Integra, 1gen Miata are the ones I've heard about), only seems to be an issue with a particular batch of DL1s (no, I don't know the serial number range).

Contact RT directly or the dealer you got your equipment from to get the isolator circuit if needed.

 

RT told me to simply put a 100k-ohm resistor in-line with the rpm input. In my case, it resulted in a working RPM readout on the dash, but no data in the DL1. Others may have better luck.

 

ask for the isoator; no reason you should be without RPM in your dta logs