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 CCUSB to perform the desired device changes. If, however the CCUSB 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 CCUSBReadout 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 example below shows how to a Phillips PH7106 leading edge discriminator:
The 3ccusb reference section describes the Module types available, the configuration options available, their control panels and the network protocol used to communicate with the controls server.