10 Hostmode
150
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
loaded
.
This is due to the overflow of (sometimes non-existant) buffers and also
timing problems on the RS-232 interface itself.
The CRC-hostmode solves all these problems in that every hostmode data packet has a
highly secure integrated CRC check sequence included. Errors can therefore be easily
detected. The CRC-hostmode
protocol also allows the request for and the automatic re-
transmission of packets recognized as containing errors.
10.9.1
Extended CRC-hostmode
The command JHOST5 enables the extended CRC-Hostmode. This is compatible to
JHOST4, but allows data packets to be transferred with a lengh of max 1024 bytes on the
FAX-channel 252 in the direction PTC to PC. The added 2 length bits are bit 4 and bit 5
of the code-byte.
JHOST5 especially enhances weather FAX receiption with the use of the Bluetooth
interface by compensating the probably occuring higher latency time. Certainly, also with
the use of the USB or RS232 interface the FAX receiption timing becomes less critical
with the use of JHOST5. (With slower PC’s, the probability of “gaps“ in received pictures
decreases.)
10.9.2
Basic principles
The term
send packet
or
receive packet
has nothing to do with the data actually
transmitted or received via the HF link. They concern themselves
only
with data present
on the RS-232 interface. The # character means
Binary byte
.
The protocol is based on the
extended hostmode
. A number of data packets are built up
according to this sub-protocol.
The HOST (PC) is, as in
WA8DED
mode, the MASTER. That means that every action is
initiated by the PC. The TNC/PTC (SLAVE) may never send data with-out being first
requested by the PC.
For every action by the MASTER, exactly one reaction from the SLAVE must follow.
The Master must wait for this reaction before it starts any other new action. There is of
course, a timeout allowed for this waiting time (see below).
Every
WA8DED
data packet is expanded by a (unique) header, consisting of #170 -
#170.
