190-00228-08 GPS 16x Technical Specifications Rev. CPage 26SynopsisAPPENDIX C: EPHEMERIS DATA DOWNLOAD (PROGRAMMING EXAMPLE)This section describes, using an example, how to download ephemeris information from a Garmin 15, 16, 17 or 18family GPS unit with the exception of the GPS 15-W and the GPS 15-F.Garmin Binary Format ReviewTo download the ephemeris data, you must first command the unit to output information in Garmin Binary Format(Garmin mode) instead of the default NMEA output format. To put the unit in Garmin mode, connect to the unitusing a terminal program and send the following NMEA sentence: $PGRMO,,G*hhThe checksum *hh is used for parity checking data and generally is not required in normal PC environments, but isrecommended for use in environments containing high electromagnetic noise. When used, the parity bytes (hh) arethe ASCII representation of the exclusive-or (XOR) sum of all the characters between the “$” and “*” characters,non-inclusive. Sentences may be truncated by after any data field and valid fields up to that point willbe acted on by the GPS sensor. See section 4 Software Interface. The unit will stay in Garmin mode until the nextpower cycle.Now that unit is in Garmin binary format, transmitted and received packets are structured as follows:Byte Description Name NotesPacket Delimiter DLE 0x10Packet ID (type) ID Packet typeData Size SIZE Number of bytes in data portion(notincluding escaped DLEs. See below)Data bytes DATA Not to exceed 256 bytes. . .. . .. . .Checksum CHKSUM 2’s complement of the arithmetic sum ofall the bytes from the Packet ID byte tothe last DATA byte(inclusive) notcounting escaped DLEs. See belowPacket Delimiter DLE 0x10End of Packet ETX 0x03The DLE (0x10) is a delimiter byte used in conjunction with the ETX byte to determine beginning and ending of apacket. However, a 0x10 could appear in the data itself; if this occurs, the byte is escaped with another DLE byte(sometimes referred to as DLE stuffing). In other words, if a DLE occurs in the data, another DLE is transmittedimmediately after to indicate that it is a data byte and it is not being used as a delimiter. Note that the size byte of thepacket does not count the second DLE byte in an escaped DLE pair in the data field. Since a DLE that is a part ofthe data will have a second DLE to escape it, a single DLE followed by an ETX byte means that the end of a packethas been reached.In order to interpret these packets properly, one must remove the escaped DLE bytes. This can be achieved using analgorithm similar to the Sample C Code fragment on the previous page.
