APRS Digipeatersoftware DIGI_NED
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Send- and Receive Problems

The used PC should in the first place be capable of handling the large number of interrupts. When your PC is not able to handle this it will run very slowly, or your PC will lock-up or crash. From experience the following table may used as a guideline (without guarantee):

PC
XTXT
286
386
MHz
5
8
12
20
Baud




300
*
*
*
*
1200
?
?
*
*
2400
-
?
*
*
4800
-
-
?
*

* Operation possible
? Operation probably possible (with Limitations)
- Operation impossible

As you see, a modern PC with at least a 486 or Pentium processor should not cause any problems with processing speed any more.

There are, however, also problems with PCs which are in principle fast enough. Most of the time the installation suffers from unstable reception, which leads to many REJects, which in turn will result in many retransmissions. This is in most of the cases caused by some residential program, driver or additional hardware which delay the interrupts handling too long. When the timer interrupt handling is delayed for more than 200ęs during the reception of a frame the whole frame will be lost (even if it occurs only once). When you experience these kind of problems you could start AX25_MAC using the '-D' option. When you hear interruptions of the tone you hear from the PC speaker, you experience this problem.

Common sources of problems:

  • Usage of Extended (XMS) or Expanded Memory (EMS) as buffer for the terminal program and disk-caches (especially on a 286 PC).
    To avoid this you should prevent usage of this RAM as long as AX25_MAC is used.
  • A driver, which enables resident programs to be loaded in high memory (e.g. EMM386)
    If the previous solution did not work for you, you could disable the whole EMM386 driver while using AX25_MAC.
  • Slow Keyboard-Driver (KEYB)
    If frames are always lost when a key is pressed you could try another keyboard driver.
  • VGA-cards and HD-controllers
    Many VGA-Card disable interrupts for some time when running in graphic mode. I also heard there are HD controllers which give the same kind of problem. When the controller was removed everything worked again. It is a problem for me to give a general solution for this. You could try a program which does not run in graphic mode.
  • Running under Windows
    Although you can not start AX25_MAC in a DOS session under Windows for use with a serial modem, you can start AX25_MAC in DOS before starting windows. Some people have reported this works for them, but most users will get nothing but trouble using this setup.
    Windows will delay the timer interrupts to much to make a reliable transmission. The only solution is to quit using AX25_MAC this way.
Sometimes you have to live with a compromise to use AX25_MAC. If you are not willing to accept this then AX25_MAC is not the right way to go. If you wonder why AX25_MAC stopped working all of a sudden you have to question if you loaded a new driver or changed anything else in the setup of your PC. Something I do NOT want is somebody using AX25_MAC on a Digi QRG with a setup which only receives one out of 4 frames correctly each time.

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