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Serial Interface Specifications
Apollo SL30 Installation Manual
71
Example message:
$PMRRV20<chksm><CR><LF>
SL30 is running and ready to accept serial input.
CDI, GSI, AND RELATED FLAGS
This message outputs the current values of the CDI, GSI, and their related flags. After power up
this message will be output at a 10 Hz rate.
Message format:
“V” ................ Message class. This is a VHF NAV message.
“21” ............... Message identifier.
cc ................... CDI deflection. An eight-bit value indicating the amount of deflection of
the CDI needle, represented as two encoded hex
4
digits. The CDI
deflection is a twos complement signed integer in the range of –100 to
100. –100 indicates full left deflection, 0 indicates no deflection, and 100
indicates full right deflection.
gg................... GSI deflection. An eight-bit value indicating the amount of deflection of
the GSI needle, represented as two encoded hex
4
digits. The CDI
deflection is a twos complement signed integer in the range of –100 to
100. –100 indicates full deflection upwards, 0 indicates no deflection, and
100 indicates full deflection downwards.
ff .................... Flags. Eight bits for HNAV and VNAV related flags, represented as two
encoded hex digits.
Bit 1 (lsb) Back Course enable (1 = enabled)
Bit 2 Localizer detect (1 = using localizer)
Bit 3 FROM flag (1 = From)
5
Bit 4 TO flag (1 = To)
Bit 5 GSI superflag (1 = valid)
Bit 6 GSI valid (1 = valid)
Bit 7 NAV superflag (1 = valid)
Bit 8 (msb) NAV valid (1 = valid)
Example message:
$PMRRV21817??:<chksm><CR><LF>
This message indicates a full left CDI deflection (-127), a full up GSI deflection (127), both the
GSI and NAV flags/superflags are valid, TO flag set, FROM flag not set, using a localizer, with
no back course enable.
4
Encoded hex: each character consists of 4 bits of data placed in the low order nibble +30h. For example, the 8-bit
value 5Fh would be encoded as two characters with values of 35h and 3Fh, which map to the ASCII characters “5”
and “?”, respectively.
5
The TO and FROM flag can not both be 1, indicating that they are both valid. They can both be zero, indicating
that neither is valid. This situation will occur whenever the receiver determines that it is within the “cone of
confusion” directly over a VOR, or when no signal is being received.