NSCL DAQ Software Documentation
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56.4. Writing device support software in Tcl

The addtcldriver command allows you to add a Tcl command ensemble as a module which can then be added to the -modules list of modules read out by a stack. In Tcl a command ensemble is a command that has subcommands. The addtcldriver command registers the base command of a command ensemble as a module. In turn, the command ensemble is required to provide at least two subcommands; Initialize and addReadoutList which perform functions analagous to methods with the same name in a C++ driver.

Possibly the simplest way to build command ensembles that can be re-used to support more than one module is to use a Tcl object oriented extension. When you do this, a driver is a class and instances of those classes are modules. Almost all Tcl object oriented extensions make objects (class instances) command ensembles where the base name is the object names and methods of the class are subcommands.

Driver modules will also need to access the VM-USB during initialization and create lists of VME operations in their addReadoutList method. This is accomplished by wrappgin the CVMUSB and CVMUSBReadoutList classes using the Simplified Wrapper and Interface Generator or SWIG. SWIG wrappers are provided as loadable Tcl modules in the lib directory of the NSCLDAQ software installation.

This chapter will look at two trivial drivers that put a marker in the buffer and, at initialization time, turn on the bottom yellow LED. One of these drivers is written using Incr-Tcl (itcl) the other using Snit Is Not Incr Tcl (snit). While trivial these examples illustrate most of the key concepts you need to understand when writing device support software in Tcl.

Finally, a configuration file fragment is shown that illustrates loading and using these two drivers.

56.4.1. An Incr-Tcl (itcl) driver

The example below is a complete itcl driver class. When the run is initialized, it lights to bottom yellow LED of the VM-USB. For each event it inserts a programmable marker (literal) value in the event.

Example 56-9. Itcl VM-USB device driver

lappend auto_path /usr/opt/daq/10.1/lib   
package require Itcl                      
package require cvmusb                    
package require cvmusbreadoutlist         

itcl::class marker-itcl {                 
    public variable value 0               

    constructor args {                    
        eval configure $args
    }

    public method Initialize driverPtr { 

        cvmusb::CVMUSB c -this $driverPtr;  

        set leds [c readLEDSource]          

        set leds [expr {$leds &  0xffff}]
        set leds [expr {$leds | 0x110000}]

        c writeLEDSource $leds             (11)

    }
    
    public method addReadoutList list {   (12)

        cvmusbreadoutlist::CVMUSBReadoutList l -this $list; (13)

        l addMarker $value                 (14)

    }
}

In order to locate the SWIG wrappers of the CVMUSB and CVMUSBReadoutList, the Tcl variable auto_path must be extended to include the lib subdirectory of the NSCLDAQ installation directory. In this case, NSCLDAQ is installed in /usr/opt/daq/10.1. You will need to check your installation and use the appropriate value here.

This loads the Incr-Tcl extension. Incr-Tcl provides object oriented constructs for Tcl.

The cvmusb package is the SWIG wrapping of the CVMUSB C++ class.

The cvmusbreadoutlist package is the SWIG wrapping of the CVMUSBReadoutList C++ class.

This command creates an Incr-Tcl class. A class itself is a command ensemble whose main purpose is to create instances (objects) of the class, which are also command ensembles whose subcommands are the methods of the class. The class name marker-itcl is used to generate instances of the class.

As with all object oriented languages, classes wrap behavior and data into a single package.

C++ device support modules provide a set of configuration options. We want our driver to do that as well. In Incr-Tcl, instance variables that are declared as public can be set both at object construction time, and via an object's built in configure method.

This makes Incr-Tcl objects very much like Tk widgets. The example below shows how you can set the value variable at both construction and configuration time:

marker-itcl obj -value 0x1234;  # Set value to 0x1234 at construction time.
obj configure -value 0x4321;    # use configure to modify value.

Constructor methods are analagous to C++ constructors. They are called when a class instance is created. the eval configure $args takes the arguments passed to the construtor and uses them to configure public variables.

This constructor does nothing except allow the use of configuration option settings when an object is constructed.

Initialize implements device initialization that is done at the start of the run. The driverPtr is a SWIG pointer that represents the address of the CVMUSB object normally passed to C++ Initialize driver methods.

This linexcomp contructs a SWIG wrapping of a CVMUSB object. The -this option tells SWIG to build its wrapping around an existing SWIG pointer. The end result of this line is that the object named c is created that talks to the same VM-USB as the object normally passed in to a C++ device support class/object.

Invokes the readLEDSource method of the CVMUSB object. This reads the current value of the VM-USB LED source register. The arithmetic that follows modifies the bottom Yellow LED selector to use the inverse of the Not Slot one state as the source of the LED.

(11)

Writes the new value back to the LED register. Using the inverse of the Not Slot one source ensures that as long as the VM-USB is being used as a slot 1 controller, it will have its bottom yellow LED lit.

(12)

The addReadoutList method is intended to provide a list of VME operations that are executed in response to each event trigger. The list is a SWIG pointer to the CVMUSBReadoutList normally passed to a C++ driver's addReadoutList method.

The first command in this method wraps the list in a SWIG object named l so that it can be used from within Tcl to manipulate the list.

(14)

Adds a marker to the list. A marker is a literal value. The value of the marker comes from the value object instance variable. As previously discussed, since this is a public variable it is hooked to the -value configuration option for the object.

In general you will need to look at the reference information on the SWIG wrappers for CVMUSB CVMUSBReadoutList

56.4.2. A Snit Is Not Incr Tcl (snit) driver

Snit is a pure Tcl object oriented extension to Tcl. In this section we will look at an annotated sample Snit Tcl driver. The sample driver will just turn on the VM-USB's bottom yellow LED at initialization time and inserts a configurable marker into each event in response to a trigger.

While this driver is realtively trivial, it illustrates many of the key points you will need to understand to write Tcl drivers in snit. If you have looked at the Incr-Tcl driver in the previous section there will be very little that is new here other than Snit syntactical differences from Incr-Tcl.

Example 56-10. A Snit VM-USB driver.

lappend auto_path /usr/opt/daq/10.1/lib   

package require snit                      

package require cvmusb                   
package require cvmusbreadoutlist        

snit::type marker-snit {                 
    option -value 0;                     

    constructor args {                   
        $self configurelist $args        
    }

    method Initialize driverPtr {       

        cvmusb::CVMUSB v -this $driverPtr; 

        set leds [v readLEDSource]     
        
        set leds [expr {$leds & 0xf0ffffff}]; 
        set leds [expr {$leds | 0x08000000}]; 

        v writeLEDSource $leds         (11)
    }

    method addReadoutList list {       (12)
        cvmusbreadoutlist::CVMUSBReadoutList l  -this $list; (13)

        l addMarker $options(-value)   (14)

    }
}

: In order to load the packages that wrap the CVMUSB and CVMUSBReadoutList you must add the directory in which they are installed to the auto_path variable. This is the lib directory below the top level of your NSCLDAQ installation. You may need to change the directory in the example script to match your installation.
: This line loads the snit package. Snit provides definitions of the commands that make up the Snit is Not Incr Tcl package.
: Loads the cvmusb package. This package provides the Tcl wrapping of the CVMUSB class.
: Loads the cvmusbreadoutlist package. This package provides the Tcl wrapping of the CVMUSBReadoutList.
: Snit allows you to create three types of 'classes', types, widgets,and widgetadaptors. The latter two have to do with creating widgets in snit's Tk megawidget framework. The first, the type, is the most suited class type for a VMUSB driver.; This line creates a new snit::type named marker-snit. Each instance of this class can be registered as a module allowing it to be included in a stack.
: Snit objects all have a built in configure sub-command much like the one that Tk objects have. The snit option defines an option that is a target for the configure sub-command. Snit options are stord in an array accessible to all methods named options the indices of this array are the option names, the values of the array are the values of the options.; The purpose of the -value option is to hold the value of the marker that will be inserted by this driver into each event.
: constructor methods are called by snit when an object of a specified type is created. The body of this constructor invokes the configurelist built in sub-command (self is like the C++ this pointer).; The call to configurelist processes the parameters to the constructor as a set of option/value pairs. This allows objects to be constructed and configured in a single step (again like Tk widgets).
: The Initialize method of an object is called by the readout framework when a run is being started. It is expected to interact with the hardware to initialize the device it manages in accordance with its configuration.; The driverPtr is a SWIG Pointer. Swig pointers are text strings that provide a strongly typed pointer to a C++ object.
: This line creates a new SWIG object that wraps the C++ object represented by driverPtr. The resulting object is called v
: This invokes the readLEDSource method on the SWIG object v. That method reads the current value of the LED Source register, a register internal to the VM-USB that controls what makes the front panel LED's light.; The arithmetic that follows sets the field responsible for controlling the bottom yellow LED such that it will light on the inverse of the case when the VM-USB is not a slot one controller. This means that if the VM-USB is in slot one, it will have the bottom yellow LED lit.
(11): Once the new value of the LED source register is computed this line writes then ew value back into the LED source register.
(12): This method is normally called as a run is starting. list is a SWIG pointer to a CVMUSBReadoutList. The method is expected to add the entries to that list it needs to execute for each event trigger.
(13): Wraps the list parameter in a SWIG object named l in a mannner analagous to what was done in Initialize
(14): Adds a markter to the list of VM-USB operations that will be performed in response to an event trigger. The value of the marker word (options(-value)) is the value of the -value configuration option.

In general you will need to look at the reference information on the SWIG wrappers for CVMUSB CVMUSBReadoutList

56.4.3. Using a Tcl driver in a DAQ configuration script

This section assumes you are using a driver that has a generator of driver instances. The object oriented examples meet those criteria. To use a Tcl driver in a DAQ configuration file you must:

Incorporate the driver in the daqconfig file source code.
Create and configure an instance of the driver for the device(s) it manages in your physical configuration
Use the addtcldriver command to turn each driver instance into a module.
As with any module, incorporate it into a stack's -modules list.

Consider the drivers we described in the previous section. Suppose the source code for those driver files, tcldriver-itcl.tcl and tcldriver-snit.tcl are located in the same directory as the DAQ configuration file. The following configuration file fragment creates an instance of each and adds them to a stack containing other natively coded modules that is triggered on the NIM 1 input.

Example 56-11. USing a Tcl VM-USB driver.

source tcldriver-snit.tcl;                   # load a snit tcl driver.
marker-snit create snitmarker -value 0x5a5a; # Create an instance..
addtcldriver snitmarker;                     # Add it to the list of known modules.


source tcldriver-itcl.tcl;                   # load an incrtcl driver.
marker-itcl itclmarker -value 0xa5a5         # crate/configure an instance.
addtcldriver itclmarker                      # register it.

...
stack config event -modules [list test test2 test snitmarker itclmarker]

In the exmample above, the Tcl modules are highlighted in the stack configuration command.

Other approaches to packaging. If you rdriver is intended for re-use across several setups and even users, the method described above is not maintainable. In that case, it is better to stoere the driver sources in some central location, add package provide commands to each driver files and use pkg_mkIndex command to build a package index file.

If this is done, and the directory added to the Tcl search path, you could then use package require to load the driver file. Storing driver code centrally allows you to ensure that experiments are using up-to-date versions of your software.

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