SCS PTC-IIex, Руководство пользователя

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10 Hostmode
"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 NMEA Channel
On hostmode channel 249 the PTC-IIex 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-IIex 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.
10.8 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.
3. Data transmission via multiple nodes (e.g. WINLINK forwarding) increases the
potential for error due to corruption via the RS-232 links.
Possible sources of error:
•
During active transmitter operation, HF can corrupt data.
•
Short spikes due to heavy loads being switched on or off on the AC power line, or
through close lightning strikes etc. They can all cause
error bursts, especially on
longer RS-232 lines
.
•
Slow or wrongly configured multitasking systems (WINDOWS) tend to swallow a
character or even complete pieces of text, especially when the computer is heavily
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