The incoming GPS message is echoed. When the incoming sentence in that frame is finished the DDM waits for 100msec then sends the B50 sentences. These must be finished before the next incoming sentence starts arriving. GPS messages which fill the frame and do not allow time for the 100msec wait and the B50 sentence time are unsuitable. Some units have a message frame every second and others every two seconds. Incoming sentences in one frame must not have gaps between them as a 100msec gap will trigger the B50 sentence.
If possible turn off any unnecessary sentences.
The Garmin 35 and Garmin 16 are suitable GPS engines as the messages and the GPS engines can be configured. Early handheld Garmin units are generally suitable. As we don't control what GPS manufacturers do we are not responsible if any particular unit has a problem with the DDM.
If no GPS is connected the B50 will transmit the B50 sentences every 2.5 seconds.
NMEA sentence structure:
Speeds, vario, Macready are in knots.
Leading zeroes are suppressed.
Sentence has following format:
AAA = TAS 0 to 150 knots
BBB.B = Vario, -10 to +15 knots, negative sign for sink
C.C = Macready 0 to 8.0 knots
DDDDD = IAS squared 0 to 22500
EE = bugs degradation, 0 = clean to 30 %
F.FF = Ballast 1.00 to 1.60
G = 0 in climb, 1 in cruise
HH = Outside airtemp in degrees celcius ( may have leading negative sign )
CHK = standard NMEA checksum
crlf = standard NMEA sentence delimiter
$PBSER, xxxx (4 digit serial number unique to every DDM)*CHK
The commas are important.
The B50 Flight generator program is available from our website. This lets a PC emulate a GPS/B50 and sends NMEA(or B57 mode) sentences out of the PC serial port. You can vary speed, altitude, heading, cruise/climb, turn rate etc. Glide computer developers will find this a very useful tool as we found during the B57 and B2000 development.