56.5. The slow controls subsystem

There is nothing to stop you from creating device support that does not do anything in its addReadoutList. You could do this to implement static controls devices. That is non data taking devices whose configuration is set up at the start of a run and cannot be dynamically modified.

There are cases, however when you'd like to have some control panel associated with a device, that would provide a virtual knob allowing you to dynamically modify settings in the device. The slow controls subsystem supports this by offering a TCP/IP server which can accept commands to set and query devices.

If the run is halted, the slow controls server can directly manipulate the VMUSB to perform the desired device changes. If, however the VMUSB is in data taking mode, the slow controls server pauses the run, makes the setting and then resumes the run. You should avoid working with control panels during production runs for that reason.

The philosophy of the slow controls subsystem is similar to that of the readout subsystem. A controls configuration file defines the control modules present in the system and their unchanging configuration. Note that the controls configuration file is read only as VMUSBReadout starts, not once per run as for the data acquisition configuration file.

The Module command is used to create configure and query the configuration of slow controls devices known to the server. The following fragment from a controls configuration file creates and configurea a CAEN V812 CFD.

Module create caenv812 cfd
Module config cfd -base 0x1245000 \
                  -file [file join [file dirname [info script]] cfd1.cofig]
            

The slow controls network protocol allows clients to connect to a Tcp/IP port and perform requests that are dispatched to specific drivers. The --port option specifies the port number on which this Tcip/IP server is listening.

Drivers have entry points to get and set values as well as to update their understanding of the state of the device. In addition a mechanism exists that allows drivers to periodically poll devices for status without pausing data taking. This is done by providing a monitor list which is programmatically triggered on a periodic basis.

Future releases of this documentation will describe how to write slow contrrols drivers and integrate them with the VM-USB readout framework.

Normally device drivers for slow controls modules also have control panels associated with them. For more information about the slow controls subsystem and for the protocol control panels use to communicate with the control server, see the 3vmusb reference section.