Reads back latest voltmeter results.
Waits for next unread data if necessary.
Clears new data available status bit.
RMS:
6 data values separated by commas
Vrms,Arms,Vdc,Adc,Vac,Aac
MEAN:
6 data values separated by commas
Vrms,Arms,Vmean,Amean,Vff,Aff
SURGE:
8 data values separated by commas
Vrms,Arms,Vpk,Apk,Vcf,Acf,
Vsurge,Asurge