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10 Hostmode
149
change of the status has occurred in the meantime. The output could thus be:
"255,01,255,0". This means that new status information is available on channel 254. The
channel 254 in the G poll list of channel 255 (
extended hostmode)
is cleared after polling
of channel 254. (When any status change occurs, channel 254 is added to the list again).
The normal G-poll of channel 254 is always possible, independent from new information
in the status. Even is the auto status is active, the actual value can always be obtained by a
regular poll.
10.7
TRX Control Channel on Hostmode
Virtual hostmode channel 253 serves as transparent data channel between PC and TRX
port. Arbitrary data can be exchanged between PC and transceiver on channel 253. This
enables, for example, direct transceiver remote control from the PC side. There is one
length limitation: If more than 1000 bytes of data sent from the transceiver are already
buffered, the PTC does not accept more data until the buffer is flushed (data fetched by
the PC application).
10.8
NMEA Channel
On hostmode channel 249 the PTC-IIIusb provides
all
NMEA sentences of a connected
GPS receiver. Except the terminating <CR> (ASCII13) the data is exactly matching with
the one the GPS receiver sends. The PTC-IIIusb buffers 32 sets of NMEA data internally.
This NMEA channel is also included in the
extended hostmode
channel 255, which means
that the usual poll on channel 255 is sufficient to determine if there is new NMEA data
available on channel 249.
10.9
CRC hostmode
The expanded
WA8DED
hostmode (
extended hostmode
) has established itself as a de-
facto standard for communication between TNC/PTC and PC control programs.
Although containing a very well thought out structure, which considerably eases
compatibility of extensions, the
WA8DED
hostmode has two basic weak points, which
can cause serious problems with data corruption or loss during operation:
1.
There is no reasonable possibility for a resynchronization of the hostmode operation if
through any reason the synchronization is broken. (Even if a
new synchronization
does occur, it is highly probable that it will cause defective data transfer during the
synchronization phase).
In the worst case, it only needs one destroyed bit in the data flow between
the PC and
the TNC to cause a hostmode crash.
2.
Defective data cannot be uniquely identified, and it is not possible to request
repetition of destroyed data. The data security in the link between the PC and the TNC
has a weak point here. A correct transfer of sensitive data (such as program data in
7PLUS formator direct binary data) via an
insecure
WA8DED
-
hostmode cannot
therefore be fully guaranteed.