Furuno RD-50 Operator's Manual page 78

APPENDIX 4 DIGITAL INTERFACE
HDG - Heading, deviation and variation
$ HDG, x.x, x.x, a, x.x, a*hh<CR><LF>
--
Magnetic sensor heading, degrees
HDM - Heading magnetic
$ HDM, x.x, M*hh<CR><LF>
--
Heading, degrees magnetic
HDT - Heading true
$ HDT, x.x, T*hh<CR><LF>
--
Checksum
Heading, degrees true
AP-8
Checksum
Magnetic variation, degrees E/W
Magnetic deviation, degrees E/W
Checksum