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Virtual Control Panels


Traditional machine control panels are large sheets of steel with pushbuttons, knobs, lights and sometimes meters mounted on them. They have many advantages - the buttons are far more rugged than a computer keyboard, and large enough that you can usually operate the correct one by feel while looking elsewhere, for example at the tool. However, they also have disadvantages. The occupy a lot of panel space, they are expensive, and wiring them into the PC can use up a lot of I/O pins. That is where Virtual Control Panels come in.

A Virtual Control Panel (VCP) is a window on the computer screen with buttons, meters, switches, etc. When you click on a VCP button, it changes the state of a HAL pin, exactly as if you had pressed a physical button wired to an input pin on an I/O card. Likewise, a VCP LED lights up when a HAL pin goes true, just like a physical indicator lamp wired to an output pin on an I/O card. Virtual control panels are not intended to replace physical panels - sometimes there is just no substiture for a big rugged oil-tight pushbutton. But virtual panels can be used for testing or monitoring things that don't require physical buttons and lights, to temporarily replace real I/O devices while debugging ladder logic, or perhaps to simulate a physical panel before you build it and wire it to an I/O board.

Currently there are two VCP implementations included with EMC2: The older, simply named VCP, which used GTK widgets, and the newer, called pyVCP, which uses Tkinter widgets. VCP is deprecated and should not be used - it may be removed in the future.


The layout of a pyVCP panel is specified with an XML file that contains widget tags between <pyvcp> and </pyvcp>. For example:

        <text>"This is a LED indicator"</text> 

Image pyvcp_mypanel

If you place this text in a file called tiny.xml, and run

pyvcp -c mypanel tiny.xml
pyVCP will create the panel for you, which includes two widgets, a Label with the text ``This is a LED indicator'', and a LED, used for displaying the state of a HAL BIT signal. It will also create a HAL component named ``mypanel'' (all widgets in this panel are connected to pins that start with ``mypanel.''). Since no <halpin> tag was present inside the <led> tag, pyVCP will automatically name the HAL pin for the LED widget mypanel.led.0

For a list of widgets and their tags and options, see the widget reference below.

Once you have created your panel, connecting HAL signals to and form the pyVCP pins is done with 'halcmd linksp' as usual. If you are new to HAL, the HAL Tutorial[*] is recommended.

Using pyVCP with AXIS

Since AXIS uses the same GUI toolkit (Tkinter) as pyVCP, it is possible to include a pyVCP panel on the right side of the normal AXIS user interface. A typical example is explained below.

Place your pyVCP XML file describing the panel in the same directory where your .ini file is. Say we we want to display the current spindle speed using a Bar widget. Place the following in a file called spindle.xml:

        <text>"Spindle speed:"</text> 

Here we've made a panel with a Label and a Bar widget, specified that the HAL pin connected to the Bar should be named ``spindle-speed'', and set the maximum value of the bar to 5000 (see widget reference below for all options). To make AXIS aware of this file, and call it at startup, we need to specify the following in the [DISPLAY] section of the .ini file:

PYVCP = spindle.xml
To make our widget actually display the spindle-speed it needs to be hooked up to the appropriate HAL signal. A .hal file that will be run once AXIS and pyVCP have started can be specified in the [HAL] section of the .ini file:

POSTGUI_HALFILE = spindle_to_pyvcp.hal
This change will run the HAL commands specified in ``spindle_to_pyvcp.hal''. In our example the contents could look like this:

linksp spindle-rpm-filtered  pyvcp.spindle-speed
assuming that a singlal called ``spindle-rpm-filtered'' already exists. Note that when running together with AXIS, all pyVCP widget HAL pins have names that start with ``pyvcp.''.

Image pyvcp_AXIS

This is what the newly created pyVCP panel should look like in AXIS.

pyVCP Widget reference

HAL signals come in two variants, BIT and FLOAT. pyVCP can either display the value of the signal with an indicator widget, or modify the signal value with a control widget. Thus there are four classes of pyVCP widgets that you can connect to a HAL signal. A fifth class of helper widgets allow you to organize and label your panel.


Each widget is described briefly, followed by the markup used, and a screenshot. All tags inside the main widget tag are optional.


A LED is used to indicate the status of a BIT signal. The LED color will be on_color when the BIT signal is true, and off_color otherwise.


Image pyvcp_led

<halpin> sets the name of the pin, default is ``led.n'', where n is an integer
<size> sets the size of the led, default is 20
<on_color> sets the color of the LED when the pin is true. default is ``green''
<off_color> sets the color of the LED when the pin is false. default is ``ref''


A button is used to control a BIT pin. The pin will be set True when the button is pressed and held down, and will be set False when the button is released.


Image pyvcp_button


A checkbutton controls a BIT pin. The pin will be set True when the button is checked, and false when the button is unchecked.

An unchecked checkbutton: Image pyvcp_checkbutton1 , and a checked one: Image pyvcp_checkbutton2


A radiobutton will set one of a number of BIT pins true. The other pins are set false.


Image pyvcp_radiobutton

Note that the HAL pins in the example above will me named, my-radio.two, and my-radio.three


The number widget displays the value of a FLOAT signal.


Image pyvcp_number

<font> is a Tkinter font type and size specification. Note that on Ubuntu 6.06 'Helvetica' is not available in sizes above ca 40 or 50. One font that will show up to at least size 200 is 'courier 10 pitch', so for a really big Number widget you could specify:

<font>('courier 10 pitch',100)</font>
<format> is a 'C-style' format specified that determines how the number is displayed.


A bar widget displays the value of a FLOAT signal both graphically using a bar display and numerically.


Image pyvcp_bar


Meter displays the value of a FLOAT signal using a traditional dial indicator.


Image pyvcp_meter


Spinbox controls a FLOAT pin. You increase or decrease the value of the pin by 'resolution' by either pressing on the arrows, or pointing at the spinbox and rolling your mouse-wheel.


Image pyvcp_spinbox


Scale controls a FLOAT pin. You increase or decrease the value of the pin be either dragging the slider, or pointing at the scale and rolling your mouse-wheel.


Image pyvcp_scale


Jogwheel mimics a real jogwheel by outputting a FLOAT pin which counts up or down as the wheel is turned, either by dragging in a cricular motion, or by rolling the mouse-wheel.


Image pyvcp_jogwheel


Use a Hbox when you want to stack widgets horizontally next to eachother.

    <label><text>"a vbox:"</text></label> 

Image pyvcp_hbox


Use a Vbox when you want to stack widgets verticallyon top of eachother.

    <label><text>"a vbox:"</text></label> 

Image pyvcp_vbox


A label is a piece of text on your panel.

    <text>"This is a Label:"</text> 

Image pyvcp_label

VCP: A small example

NOTE: VCP is deprecated, and will most likely not be getting any new development or additional widgets. We strongly recommend using pyVCP. Both pyVCP and VCP are included in EMC version 2.1, but it is likely that only pyVCP will be included in EMC 2.2. 1.1

Place the following in the file tiny.vcp:

vcp {

  main-window {

    box {

      button {

        halpin = vcp.pushbutton

        label { text = "Push Me" }


      LED {

        halpin = vcp.light





The above file describes a tiny Virtual Control Panel, with one push button, and one light. To see what it looks like, we need to start HAL:



Next we load halvcp, and give it the name of our .vcp file:

halcmd: loadusr halvcp tiny.vcp


There may be some text printed as halvcp parses the tiny.vcp file, but when it finishes, there should be a small window on your screen, with a button and an LED. It will look something like figure [*].

Figure: tiny.vcp on the screen
Image tinyvcp

So, we have a button and an LED, but they aren't connected to anything, so nothing happens when you push the button. However, the LED and the button both have HAL pins associated with them:

halcmd: show pin

Component Pins:

Owner  Type  Dir     Value      Name

 03    bit   IN      FALSE      vcp.light

 03    bit   OUT     FALSE      vcp.pushbutton


To make something happen, we can connect a HAL signal between the button and the light:

halcmd: newsig jumper bit

halcmd: linksp jumper vcp.pushbutton

halcmd: linksp jumper vcp.light

halcmd: show sig


Type      Value      Name

bit       FALSE      jumper

                         ==> vcp.light

                         <== vcp.pushbutton


Now push the button, and the the LED should light up!

VCP: Another small example with EMC

Place the following in the file estop.vcp:

vcp {  
   main-window {  
      toggle { halpin = vcp.estop }  
In your .hal file, remove any existing signal linked to iocontrol.0.emc-enable-in and add the following lines:

loadusr -W halvcp estop.vcp 
newsig estop bit 
linkps vcp.estop => estop 
linkps estop => iocontrol.0.emc-enable-in
Now, when running your machine, the ESTOP button in the GUI is disabled, and the ESTOP button in the VCP window is used instead.

VCP Syntax


A block's format is:

tag { contents }
The contents can consist of attributes that describe the block, or other blocks that nest inside it.

A attribute's format is

name = value
The attribute names that are acceptable for each block depend on the block tag, and will be listed later.


A .vcp to .xml translator that takes a vcp file and turns it into one that pyVCP can use is on my to-do list. That would enable VCP users to easily switch over to pyVCP. If such a translator is written, VCP may be removed from the version 2.2 release.