Tandy 1000SX and TX DIP Switches
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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