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5. Adding a GUI with Tcl/Tk

Tcl/Tk is the most convenient way to create GUIs that will work on both Windows and Linux. Even if you only use one system, you will benefit from the fact that Tcl/Tk is quite easy to learn and used by a lot of people. The particular mix of Tcl/Tk and C that we recommend here was the best solution we found; if you know of other ways to do it, let us know!

Now, take a look at ssc-tester.tcl. This program is very simple: it contains a loop that runs 16 times to create sixteen sliders, one for each of the servos that can be controlled by the servo controller. The critical command is

    scale .servo$i.scale -width 20 -length 200 -orient vertical
      -from 500 -to 5500 -command "stepTo 0 $i " -showvalue 0

This creates a slider with values from 500 to 5500, so that whenever you adjust the slider, the command

stepTo 0 slider position

will be run, with slider and position replaced by the slider number and position. This command is defined in ssctester.c by the line

  Tcl_CreateObjCommand(interp, "stepTo", &stepToProc, NULL, NULL);

The arguments are interpreted by the C function stepToProc and passed on to stepTo in ssc.c, where they are translated into commands to be sent to the serial port. When modifying the program for your own purposes, you will need to find the documentation for both commands in the Tcl/Tk language and special C functions used for interacting with Tcl. The Tcl/Tk manual will probably be your most useful reference.

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Free Circuit Cellar magazine October 2019
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