Kantronics KPC-4 - Page 42

Speed of program execution is important! Generally, interpretive BASIC will be too
slow unless you want to limit reception to say one picture at a time. Most advanced
BASICs will allow the allocation of up to 32K of storage in the I/O buffer. Then, as you
are displaying incoming WEFAX lines, new data will be slowly accumulating in the
buffer. If you don't have your program try to do too much, interpretive BASIC can just
make it. Compiled BASIC, C, or assembly language programs, of course, are faster and
more preferable.
Some considerations for the RS-232 port. When the TNC is set in WEFAX Mode, it will
send pixel bytes to your computer continually until you send it a Ctrl-C or turn it off!
When in WEFAX Mode, the TNC will do nothing else; it is simply too busy handling all
that data. The bytes sent to your computer on the RS-232 receive data line include one
start bit, eight pixel bits, and one stop bit. The most significant bit is the left-hand bit
for your display. In effect then, if you specify WEFAX 1280, the TNC will send you
bytes every 8/1280 of a second or 160 bytes per second.
Now for printing. We recommend that you use a printer that is Epson graphics
- compatible with the PC or Commodore. If you write the program and your printer can
handle 600+ pixels per line (this is about 60 dots per inch or dpi) and at least 32 lines
per inch vertically, then it should be capable of printing WEFAX maps and charts. Note
that six lines per inch of standard text is roughly equivalent to 72 lines per inch of
graphics.
What about the structure of your program? We recommend that you include a
simple terminal program that would be used to set the basic parameters of your TNC
(ABAUD, MAXUSERS, etc.). In addition, we recommend that you have a software loop
that is interruptible by the arrival of pixel bytes. These bytes would then be stored
and/or displayed in the interrupt routine. Once the bytes are processed, the interrupt
routine would return to the main loop to await more pixel bytes from the TNC. If you
use BASIC, BASICA, or compiled BASIC, then the ON COM statement is useful. Look
in the manual under trapping.
34 WEFAX
: © Copyright 1989, Kantronics, Inc. All Rights Reserved.
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