Signal monitoring and command is performed by the IOCs. At the heart of an IOC is a memory resident database describing each of the signals to be monitored and controlled by the IOC. Each database entry (record) corresponds to a signal. When a given record executes, it accesses the appropriate I/O module to retrieve/update the signal value. The origin of this record execution request can be local to the IOC (i.e. another database record) or remote (i.e. operator intervention through an OPI or a record located in another IOC). In all cases, access to the record is by name and not by IOC location. The general EPICS mechanism to access a record consists of broadcasting the record name in the network. Upon receiving the broadcast, every IOC in that network searches its database to determine if it contains the record name being sought. The IOC holding the requested record responds to the query establishing a connection with the querying process.
The interface provided by the IOCs to access their record database is geared towards efficiency and not human friendliness. Two reasons dictate this choice: minimization of CPU overhead due to interface management and the fact that the system is distributed. Operator access to the record database of a given IOC is through a Graphical User interface (GUI) process executing in a UNIX based computer. The standard distribution of EPICS comes with a Motif based implementation of such GUI, the so-called MEDM. It is possible to also implement a GUI to the IOCs using the Tool Command Language (TCL) with X11 extensions (TK) (i.e. TCL/TK). Such implementation is used, for example, by the Hall A cryotarget system. There can be many instances of these GUIs executing in the same computer as well as in several different computers (i.e. the GUI processes are also distributed). Each of these GUI processes can access multiple IOCs simultaneously and within each IOC, all or a subset of the database records. The computers where these GUIs execute are referred to as the OPI.
Figure 6.1 shows a schematic view of the present Hall A controls layout. Exchange of signals between Hall A and other JLab control systems takes place through the main Jefferson Lab network (JLab LAN). Presently, the system is in a state of flux as management of various portions of the Hall A controls system is being transferred to the Accelerator Division Controls Group (ADCG). This might entail future changes in the organization of the Hall A control system like boot servers and IOC task re-arrangement. A brief description of the present state of the system follows.
There are three IOCs in Hall A which at the present time are directly managed by the ADCG. These IOCs are iocse10, iochla and iochla2. They are located in the row of racks next to the beam line. Iocse10 performs the readout of the Beam Position Monitors (BPMs) located in Hall A (i.e. those located at Compton region and before the target). Iochla handles Hall A beam line tasks like Fast Shut Down (FSD) logic, Møller target motion, Beam Loss monitors and ionization chambers. Hall A cryogenics distribution and magnet filling/level are controlled by iochla2 through a GPIB based network of CAMAC crates. This IOC also monitors all spectrometer magnet temperatures during a magnet cooldown. The iochla2 signals are available in the Hall A controls network through a dedicated IOC (hallasc9) located in the electronics room (middle room) of the Hall A Counting House. Only Accelerator Operations personnel are allowed to reboot and/or perform any hardware/software changes in iocse10, iochla and iochla2 and the associated equipment that they control. If necessary, hallasc9 can be rebooted by pressing the ``RESET'' push button located in the IOC front panel. Such operation does not have any effect on the Hall A cryogenics distribution itself (hallasc9 does not control any hardware, it is simply a signal repeater). If a magnet cryogenics problem develops in Hall A, the Hall A on-call technical staff should be notified.
The controls layout of each High Resolution Spectrometer (HRS) is similar; each consists of three IOCs which monitor and control the detector package infrastructure, general spectrometer functions and, spectrometer motion. In the case of the hadron HRS, these IOCs are hallasc4, hallasc16 and hallasc7. Hallasc4 and hallasc16 are located inside the detector hut. Hallasc4 is in the second floor of the detector electronics racks while hallasc16 is located under the Box Beam supporting the detectors and detector electronics. Access to hallasc16 is through a ``manhole'' located at the back of the detector hut. The IOC in charge of the hadron HRS motion is located right at the back of the dipole (power supplies level). In the case of the electron HRS, the IOCs are hallasc11, hallasc14 and hallasc18 respectively. Their location is similar to their hadron spectrometer counterparts.
The tasks assigned to IOCs hallasc7 and hallasc18 are well defined and unique: spectrometer motion. Hallasc4 monitors/controls the Vertical Drift Chambers (VDCs) High Voltages (HV), discriminator threshold levels and the low voltage power supplies for the discriminator cards as well as the Focal Plane Polarimeter (FPP) tracking chamber's HVs, discriminator levels and low voltage power supplies. Hallasc4 also controls the operation of the FPP carbon doors as well as monitoring the gas flow to the FPP tracking chambers and VDCs. In the case of the electron HRS, the corresponding IOC (hallasc11) monitors/controls similar quantities for the VDCs only (there is no FPP system). The general spectrometer infrastructure IOCs (hallasc14 and hallasc16) monitor/control magnets power supplies, field probes, hardware interlock systems and power leads cryogenic cooling, collimator motion, magnet/spectrometer vacuum and spectrometer horizontal/vertical angles. If a problem arises, all these IOCs can be hardware reset from the Hall A Counting House through the green buttons panel located in the middle room.
There is an extra IOC, hallasc22, located on the second floor of the electron HRS detector electronics racks (inside the detector hut). This IOC monitors and controls, through a private network, various high voltage power supplies. Two of these supplies are located in the electron HRS detector hut, one in the hadron HRS detector hut and one along the beam line. These power supplies are used by various systems like the spectrometer scintillator planes (trigger), gas Cerenkov and Aerogel counters, electron HRS calorimeter, FPP, hadron HRS calorimeter (when installed) and various beam line systems like the ARC energy measurement system.
The beam line IOC hallasc12 is reserved, at this time, for Hall A application testing and development. There are other specific purpose IOCs located in the group of electronics racks next to the beam line. The reader is referred to the specific description of those systems and related instructions elsewhere in this OPS manual.
There are several Hall A controls related IOCs and computer systems outside the hall. Some of them are specific purpose systems described elsewhere in this manual like the 9th magnet Bdl IOC and associated ARC workstation located inside the 9th magnet shed, and they are described elsewhere in this manual. The others are briefly described below.
Hallasc17 is located in the gas shed. It monitors the supply of gas used by the tracking chambers (i.e. VDCs and FPP chambers). If necessary, hallasc17 can be rebooted by pressing the ``RESET'' push button located in the IOC front panel. Hallasc5 is located in the electronics room of the Hall A Counting House (middle room) in the same VME crate than hallasc9. Hallasc5 monitors signals associated with Hall A beam current. It can be reset by pressing the ``RESET'' push button located in the IOC front panel. The remaining computers and X-terminals associated with the Hall A controls are used as OPI and/or boot servers for the IOCs. One of the X-terminals in the counting house is dedicated to the display of accelerator MEDM screens (it is automatic, no user involvement is required).
The ``hac'' computer is used to gain access to the Hall A control systems. Once logged into this computer, issue the command ``HAC_hp'' if logging was accomplished through a Hewlett Packard computer console or ``HAC_xt'' if through an X-terminal. The Hall A controls main screen shown in Figure 6.2 will appear. Follow then the instructions given in the system of your interest.