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The structure and operation of the programs invoked by DAS commands should be completely transparent to observers: you do not need to read this section to use the system. However, if you are curious, or if you are trying to cure a problem in the system, the following information may be useful.
ICL is a program running on the system computer, which displays its output on the user-interface VAXstation by means of the X-windows system. The mimic runs in a similar fashion.
Commands given to ICL cause messages to be passed to a set of programs which also run on the system computer. These programs are `ADAM tasks' (a.k.a. `D-tasks', `C-tasks' etc.) and it is the protocol by which the messages are exchanged that defines this part of the WHT software as an `ADAM system'.
The ADAM tasks extract data from the incoming ADAM messages and send out commands to the computers controlling the CCDs and instruments. The latter computers are mounted on the telescope and the ADAM tasks talk to them over a local-area network, the Utility Network.
The control computers send status messages back over the utility network to the ADAM tasks. The tasks process this information into a human-readable form and transmit it to the mimic display. Finally, at the end of a particular operation or `action', the ADAM tasks send a summary status back to ICL, which reports the outcome of the action to the command terminal.
Tasks particularly concerned with data-acquisition (as opposed to instrument control or telescope control) are the NDF-writing Task (NWT), the Header-Collection Task (HCT) and the Run-Administration Tasks (RATs). Of these, the NWT is concerned with writing data to disk on the system computer, the HCT collects header information from other ADAM tasks and the RATs coordinate the rest of the software in order to perform the observations. There is one RAT per active detector channel and progress messages prefixed by the name of the channel are coming from the RAT.