Hi,
what you can do is a so-called loopback-test: connect a patch wire with proper polarity, between e.g. the tail sensor port and the RPM input port.
This should instantly show a low but stable rpm reading (the servo frequency, calculated to rpm by pole count and gear ratio).
This is to check if the input port is working properly.
To the best of my knowledge, all sensors connected with proper polarity work on the NEO, so if the NEO is not reading anything at all, it's probably wrong polarity or a bad sensor, if it's creating checksum errors I'd assume you connect to the sensor port but not to the RPM port (which could be the problem all from the beginning ..?).
The sensor port would be for the external (optional) gyro sensor only, or for the boot plug (to update a NEO).
Kind regards
Eddi
Born to fly ...
forced to work.