SFI - Scanning Frequency Information

        1   2   3      4                     x
        |   |   |      |                     |
 $--SFI,x.x,x.x,xxxxxx,c .......... xxxxxx,c*hh<CR><LF>

 Field Number: 
  1) Total Number Of Messages
  2) Message Number
  3) Frequency 1
  4) Mode 1
  x) Checksum