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Instruments are controlled from the VAX 4000, which runs the DEC standard VMS operating system together with VAX-ADAM. Each of the sub-systems (TAURUS, Cass A&G, the autoguider, the detectors, the DMS, and the telescop40 sub-system has an associated ADAM 'd-task' in the 4000 which is responsible for controlling that sub-system.
A control interface is provided through the Interactive Command Language (ICL). Individual commands can be sent to the sub-systems via the d-tasks, or alternatively a sequence of commands can be bundled into ICL procedures to allow more complex, multi sub-system operations to be carried out.
The All d-tasks communicate with the outside world using a common communications task resident in the VAX 4000 known as Utilnet. This handles all the low level communications functions such as packaging the commands, transmitting and receiving datagrams (each command is usually contained within a single datagram) and handling timeouts and retries.
Each instrument is controlled by a 4MS, a microprocessor system which is responsible for executing commands and returning status information for that instrument. Commands are sent from the 4000 to the 4MS systems over a Ethernet-based 'utility network'. The utility network uses an RGO-defined protocol which conforms to the requirements of the IEEE 802.2 and IEEE 802.3 standards. The physical, data--link, network and transport layers of the ISO Seven Layer Reference Model for Open Systems Interconnection are separately implemented in hardware data concentrators, the Sension Network Interface Units (NIU), each of which is able to connect up to 4 instruments to Ethernet. The remaining layers (session, presentation and application) are implemented in software in each of the attached instrument processors.
All instruments on the network have unique 4-letter identifiers which are transmitted on initialisation to the relevant NIU, allowing any instrument to communicate with any other by name. The protocol involves an attached processor announcing its presence by a broadcast message and then being able to communicate on a one--to--one basis with any other processor on the network. Note that, at present, only commands and status are interchanged in this fashion, image data transfer occurs over a private, parallel link.