Tandy 1000SX and TX DIP Switches

Some detail about the DIP switches in the Tandy 1000SX and Tandy 1000TX computers.

As for the switches in the SX and TX, there are four.  In the
SX the block is located at the end of the expansion slots.
In the TX, it is located between the expansion slots and the
power supply.  I have managed to figure out what they do,
using the tech manual, assorted calls to Fort Worth, and my
own experiments.

SW1:  Flipping this switch is supposed to redirect the video
      signals to the correct port for a monochrome video
      adapter.  It seems to have no effect with my Everex EGA
      board.   The system is supposed to automatically sense
      the presence of any color expansion board (CGA, EGA, or
      VGA), but I understand that it doesn't always work).
      Conclusion:  turn this switch off if you use any add-on
      video adapter.

SW2:  This switch decides whether the on-board video is
      synchronized internally, or is synchronized to an
      expansion device set to IRQ5.  Any device appears to
      work, since I've tested it both with Hard Drive
      Controllers set to IRQ5 and with a Logitech Bus Mouse
      set to IRQ5.  It may not have any effect if you use an
      EGA or VGA board.

      The intent was to allow industry standard Hard Drive
      Controllers using IRQ5 to work in the 1000's, "sharing"
      the interrupt with the video.  This setup prevents snow
      on the screen during drive access.  If you leave this
      switch on with a hard drive controller set to IRQ5, the
      system will hang during boot-up.

SW3:  This switch tells the system whether the on-board
      floppy controller uses IRQ6 alone, or must "share" IRQ6
      with an expansion board.  I've not tested this one, but
      it appears that this is to allow the installation of a
      2nd floppy drive controller to run a tape backup
      system.  The I/O summary in the tech manual lists a
      port address for a 2nd floppy controller.  Since I
      don't think it's possible for the CPU to access 2
      drives at the same time, this is probably the case.

SW4:  This switch tells the system whether the on-board
      printer port is the only device using IRQ7, or whether
      the printer must "share" IRQ7 with an expansion device.
      This is intended to allow the installation of a 2nd
      parallel port, for running a 2nd printer, or a plotter
      with a parallel input.  My Everex video board also has
      a parallel port on it, and yesterday I tested the setup
      with a 2nd printer.  The Everex board has 3 parallel
      port addresses, and the 1000 can recognize 2 of the
      three.  The highest priority isn't recognized at all,
      the 2nd priority (0378-037F) is used by the on-board
      port as PRN or LPT1, and the 3rd priority (0278-027F)
      is recognized as LPT2.  I set the board for IRQ7 at the
      3rd address, and turned SW4 "off".  Both printers
      worked perfectly.

I know this is long winded, but if I'd had this info a year
ago, my own setup would have been a lot easier.

Rich

See Also

• Tandy 1000SX
• Tandy 1000TX