【正文】 configuration tools for the processing engine. The flexible naming scheme and the default display and alarm properties for each record ease the connection between the operator tools and the IOC?s. The flexible data acquisition supports the poll mode as well as the publish subscribe mode. The latter reduces the traffic drastically. PLC?s PLC?s provide nowadays the same rich functionality as it was known from stand alone control systems in the past. Besides the basic features like the periodic execution of a defined set of functions they also allow extensive munication over Ether including embedded servers and different sets of munication programs. Besides the munication processors, display processors can be linked to PLC?s to provide local displays which can be prised as touch panels for operator intervention and value settings. These kind of PLC?s are attractive for turn key systems which are missioned at the vendors site and later integrated into the customers control system. Intelligent I/O New developments in I/O devices allow to ?cluster? I/O in even smaller groups and connect theses clustered I/O channels directly to the control system. PLC?s are not any more necessary for distributed I/O. Simple munication processors for any kind of field buses or for Ether allow an easy integration into the existing controls infrastructure. Little local engines can run IEC 61131 programs. The differences between PLC?s and intelligent I/O subsystems fade away. FUNCTIONALITY The ever lasting question why control systems for accelerators and other highly specialized equipment are often home grown or at least developed in a collaboration but only in rare cases mercial shall not be answered here. We try to summarize here basic functionalities of different controls approaches. Frontend Controller One of the core elements of a control system is the frontend controller. PLC?s can be used to implement most of the functions to control the equipment. The disadvantage is the plicated access to the controls properties. For instance all of the properties of a control loop like the P, I and D parameter, but also the alarm limits and other additional properties must be addressed individually in order to identify them in the munication protocol and last not least in the display, alarm and archive programs. In addition any kind of modifications of these embedded properties is difficult to track because two or more systems are involved. This might be one strong argument why control loops are mainly implemented on the IOC level rather than PLC?s. 1 I/O and Control Loops Complex control algorithms and control loops are the domain of DCS alike control systems. The support for sets of predefined display and controls properties is essential. If not already available (like in DCS systems) such sets of generic properties are typically specified throughout a plete control system (see namespaces). 2 Sequence/ State programs Sequence programs can run on any processor in a control system. The runtime environment depends on the relevance of the code for the control system. Programs fulfilling watchdog functions have to run on the frontend processor directly. Sequence programs for plicated startup and shutdown procedures could be run on a workstation as well. The basic functionality of a state machine can be even implemented in IEC 61131. Code generators can produce ?C? code which can be piled for the runtime environment. 3 Supported Hardware The support for field buses and Ether based I/O is a basic functionality for SCADA type systems it is mercially available from any SCADA system on the market. The integration of specific hardware with specific drivers and data conversion is the hard part in a mercial environment. Open API?s or scripting support sometimes help to integrate custom hardware. If these tools are not provided for the control system it is difficult – if not impossible to integrate custom hardware. New industrial standards like OPC allow the munication with OPC aware devices and the munication between control systems. One boundary condition for this kind of functionality is the underlying operating system. In the case of OPC it is bound to DCOM which is a Microsoft standard. UNIX based control systems have a hard time to get connected. Only control systems supporting multiple platforms can play a major role in a heterogeneous environments. As a result the limited support for custom or specialized hardware may give reason for the development of a new control system. Display and Operation Besides the frontend system the operator interfaces play a major role for the acceptance of a control system. SCADA tools e with a homogeneous look and feel throughout their set of tools. Toolkits implemented in a collaboration might vary because the individual tools were developed by different teams. 1 Graphic Synoptic displays are the advertising sign for any control system. Commercial synoptic displays e with a rich functionality and lots of special features. Starting to make use of all these features one will find out that all individual properties of the graphic objects must be specified individually. Since SCADA systems must be generic they cannot foresee that an input channel does not only consist of a value but also consists of properties like display ranges and alarm values. Defining all of these properties again and again can be a pretty boring job. Some systems allow to generate prototypes of graphic objects. These prototype or template graphics are plex and need a specialist to generate them. DCS or custom synoptic display programs can make use of the mon set of properties each I/O point provides. This predefined naming scheme will fill in all standard property values and thus only require to enter the record – or