Please verify, but I believe that the _S option is for the "Stand alone" meter function.? This uses a second Nano to do the metering as well as other functions.? An S meter using the spare analog input is included in the base CEC software.? The _S version also requires a modification to the I2C bus to connect the second Nano.
Again, I am going from memory here, so it may be incorrect.? I am sure that it is described on the Hams Key (CEC) site somewhere.? Finding information on the site can be a challenge.? I would also check ubitx.net.? Often there is better information on that site.
73
Evan
AC9TU
