The profusion of recording systems described in the last section is further complicated by the the possibility of mixing elements of the different systems. There are three elements of each system that must be specified in order to make proper schedules. These are specified in the SCHED station catalog with the paramters CONTROL, DAR, RECORDER and DISK. The first element is the control system -- what software (and hardware) is used to control the recording system. The options that SCHED can handle for wide band VLBI observations are VLBA and VEX. Formally, these actually refer to the control file type, although each type currently implies a specific computer and software system. Note that the CONTROL variable has other options, but they imply one of the above two, plus some specific other telescope control files.
The original VLBA control system runs on VME computers using software mainly written for the VLBA. A few other sites (VLA, Green Bank, and one of the systems at Effelsberg) have such systems to control their VLBI hardware. Systems controlled by the VLBA software always consist of VLBA data aquisition racks (DAR -- BBCs, formatter etc) and Mark5 disks. During 2010, new Digital Backends (RDBE) and Mark5C recorders began to be introduced on the VLBA. These major changes are being used as an opportunity to upgrade the telescope control systems. The new system is based on the EVLA Executor control system and runs on a standard Linux computer. Information is passed to this system from SCHED via the VEX file. SCHED now writes VEX files for all observations as a result. The new control system will initially just control the new data aquisition system and recorder. A new C-band (6cm) receiver is under construction and it, along with all of the LO/IF switching will be placed under control of the new computer. Over the course of the next few years, all VLBA systems will be moved to the new control. In the meantime, both crd files for the old system and VEX files for the new system will be required.
The VEX option for CONTROL causes SCHED to generate a schedule file in the VEX format, as does the DOVEX main schedule input parameter (now the default). This is the schedule distribution file format developed mainly for the PC Field System (PCFS) and the Mark IV correlators. It is now used by the DiFX software correlator in use at the VLBA and other places, so it is now needed by most if not all projects. It also is needed to control all new hardware being installed on the VLBA.
The PCFS, running on Linux PCs, is used to control VLBA, VLBA4, Mark IV, S2, and MARK5 VLBI hardware at many non-VLBA stations including all of the EVN. It is the descendent of, and replacement for, the control system for Mark III systems. Sometimes this system is refered to as FS9, for ``Field System 9'', refering to the earliest version that could handle VEX input.
Systems controlled by the PCFS can consist of a VLBA, VLBA4 or Mark IV DAR connected to a VLBA4, VLBA, Mark IV, S2, or Mark5 recorder. Each combination has slightly different capabilities and requirements. SCHED understands a considerable amount about what these capabilities and requirements are and attempts to insure that the requested observations are possible. However, when doing a large observation with many diverse telescopes, it is best to stick to standard, well tested, observing modes and use the frequencies selected using the setup file parameter BAND.
During the transitions between recording systems, it may not be obvious when the schedules are made which system a station will have. Or stations may have both and want to run observations on one or the other depending on the supply of media. Because of this situation, a separate input, called DISK can be specified in addition to RECORDER in the station catalog. It tells SCHED that the station has a disk system and schedules can be made for it. There is another new parameter, MEDIADEF, which can be TAPE or DISK, that can be used to set the default medium for the station while both are available. The MEDIA parameter in the TAPEINI section can be used to override the default. From this description, it is clear that this scheme was designed for the transition from tape to disk which was finished a few years ago. As new varieties of disk or real-time systems are implemented, changes are likely.