\title{The MSCB bus - a field bus tailored to particle physics experiments}
\author{S.~Ritt$^{1}$, R.~Schmidt$^{1}$ }
% You can optionally add the proposal numer and institutes, e.g.
\collaboration{R-99-05}{PSI$^{1}$}
% \abstract{ put your abstract here, this is optional}

\begin{contribution}

Particle physics experiments require what is commonly referred to as "slow control". This includes the measurement and control of environment variables such as temperature, pressure and humidity as well as the control of high voltages for photomultipliers and wire chambers. While most experiment use an inhomogeneous mix of systems involving RS232, GPIB and PLCs, the MUEGAMMA experiment (R-99-05) will use a new slow control system developed at PSI, called MSCB (Midas Slow Control Bus). This system will be used for the 960 high voltage channels of the experiment, for the control of the liquid xenon calorimeter and for the superconducting solenoid. The integration of all these systems into the central data acquisition and control system is essential for the long-term stability of the experiment.

The MSCB system uses a field bus- like architecture, where a number of "nodes" are connected to a serial bus, which is controlled by a central PC. Each node contains ADCs, DACs and digital I/O for measurement and controlling tasks. For critical installations the control PC can be backed up by a secondary PC for redundancy. The PCs are connected to the Midas DAQ system~\cite{R99-05:midas}, which allows for remote operation through a Web interface, history display, automatic alarm notification and for logging of slow control variables to tape. 

The hardware of a MSCB node is designed around a new generation of microcontrollers, which contain a 8051- compatible microcontroller core, ADCs, DACs and digital I/O on a single chip. We currently use the ADuC812~\cite{R99-05:aduc} from Analog Devices and the C8051F000 from Cygnal~\cite{R99-05:cygnal}. The nodes are connected via an RS-485 bus running at 384 kBaud. A segment can contain 256 nodes, and with one layer of repeaters 65536 nodes can be connected and addressed on a single bus. Two versions of MSCB nodes have been developed. A stand-alone module (Fig.~\ref{R99-05:node}) which can be embedded directly on a sensor or on an electronics board is powered from the bus, which uses a 10-wire twisted pair flat ribbon cable for distances up to 500 m. The production cost of such a node is about 50 CHF. A development kit for the Cygnal controller which includes a C compiler costs 99 US\$.

\begin{figure}[htb]
   \centering
   \includegraphics[width=1.0\linewidth]{R99-05-fig1.eps}
   \caption{Stand-alone node with an RS-485 transceiver, eight channel 12-bit ADC, two channel 12-bit DAC, 16-bit digital I/O and a temperature sensor. The right connector is for the MSCB bus, the one at the back for an optional LCD display.}
\label{R99-05:node}
\end{figure}                                      

In addition to the stand-alone module, a 19" rack system which hosts cards containing a MSCB node and signal conditioning, has been designed. Cards were made for voltage, current and temperature measurements as well as to control 220V consumers such as heaters. The MSCB bus runs on the back plane of the crate.

Using the local intelligence of the node controller, regulation loops (PID) and interlock systems can be realized without intervention of the central control PC. The nodes run a simple framework for the communication with the host system, which guarantees real-time behaviour. User routines can be added to implement application- specific logic. The nodes can be reprogrammed over the RS-485 bus. 

The MSCB protocol was optimised for minimal overhead. A 16-bit value from a node can be read out by sending a request of three bytes and receiving an reply of four bytes, both including a code (CRC) to avoid data corruption. Depending on the number of nodes,  a MSCB system can either use 8-bit or 16-bit addressing. A node can contain up to 256 "channels" for reading and writing and up to 256 "configuration parameters", which are stored in the EEPROM of the node and can for example be used as constants for PID regulation. Each channel and parameter is described by a set of attributes such as name, physical units and status. These attributes are stored in each node and can be queried from the control PC, making the configuration of large networks very simple. A special repeat mode has been defined which allows the readout of a series of nodes in less than 300 $\mu s$ per node.

For the control PC a "C" library has been developed running under Windows and Linux. Based on this library, a LabView driver and a driver for the Midas DAQ system are available. Simple LabView applications such as a data logger with graphical display has been implemented.

A prototype of the MSCB rack system is currently used for the pressure and high voltage control of the new proportional chamber for the SINQ POLDI detector. 

The final system will be available from the PSI electronics pool in spring 2002 upon request. It can be concluded that the MSCB system is an attractive alternative to GPIB multimeters and to Programmable Logic Controllers with respect to cost and integration. For more information visit the MSCB home page at http://midas.psi.ch/mscb.

\begin{thebibliography}{9}
\bibitem {R99-05:midas} MIDAS home page http://midas.psi.ch
\bibitem {R99-05:aduc} http://products.analog.com/products/info.asp?product= AduC812
\bibitem {R99-05:cygnal} http://www.cygnal.com/products/C8051F000.htm
\end{thebibliography}

\end{contribution}

\title{MuEgamma Prototype Timing Counter Tests}
\author{
}

% You can optionally add the proposal numer and institutes, e.g.
\collaboration{R--99--05  }{MuEGamma Collaboration : BINP Novosibirsk -- ICEPP Tokyo -- INFN Pisa -- IPNS KEK\\ 
-- Nagoya -- PSI -- Waseda}

\begin{contribution}

The aim of the $\bf{MuEGamma}$ experiment is to search for the lepton-flavour violating decay $\mu \rightarrow {\mathrm{e}} \gamma$ to a sensitivity of $\sim 10^{-14}$, more than two orders of magnitude lower than the current search limit set by the MEGA-experiment at LAMPF \cite{mega99}. In order to achieve this goal, a detector capable of handling the intense surface muon rate, produced by the $\pi$E5 beam at PSI, and able to resolve the positron and photon from the decay with the best possible momentum/energy-, positional- and timing-resolutions, is required.

The main detector components, are shown in Fig.\ref{fig:muEgamma-fig1}. Surface muons of 28~MeV/c enter the thin-coil, superconducting solenoid and are stopped in a small, thin target. The subsequent decay of the muon leads to a Michel positron, which exhibits a spiral path with increasing pitch, the bending radius of which depends entirely on its total momentum and is 'independent' of its emission angle. These novel features are achieved by the gradient field of the solenoid magnet, with a maximum field strength of 1.27~T at the centre of the detector and decreasing axially. The positron momentum and angle are determined by tracking the particle with a set of azimuthally spaced, staggered-cell drift chambers, while the energy, timing and angle of the photon are obtained from a liquid Xenon calorimeter, viewed from all sides by some 800 photomultiplier tubes. The timing of the positron and hence the trigger condition for coincident back-to-back decay products, is given by a set of fast, double-layered, orthogonally placed, scintillator timing-counter arrays, positioned at opposite ends of the detector.

\begin{figure}[htb]
   \centering
   \includegraphics[width=0.85\linewidth=3.0]{muEgamma-fig1.eps}
\caption{\small\slshape Plan View of the MuEgamma Detector showing the main components, as well as an example of the decay trajectories of the positron and photon.}
\label{fig:muEgamma-fig1}
\end{figure}                                      

%\vspace{-2mm}

Several timing-counter prototypes were built and tested at the cosmic ray test facility CORTES at INFN Pisa.
The results reported below are for a (100 x 5 x 1) $cm^{3}$ Bicron 404 scintillator bar, wrapped with 50 $\mu$m of aluminized Mylar foil and coupled to Philips XP2020 UR photomultiplier tubes (PMTs) at either end. Tests were also done with Hamamatsu R5946 fine mesh tubes and variously shaped light guides.

The CORTES facility consists of a set of cosmic trigger counters, placed above and below the central tracking region, with two sets of four planes of Microstrip Gas Chambers (MSGCs) \cite{Bellazzini2001}. The dimensions of each chamber are 10 cm x 10 cm. Each set of the four planes of MSGCs provides two sets of orthogonal coordinates via two stereo planes inclined at 5.7$\,^\circ$. Two additional fast timing counters, placed behind each other and just below the upper trigger counter give a good timing reference signal for the scintillator bar to be tested, which is placed between the two sets of MSGCs. The track impact point along the  counter to be tested was reconstructed with a resolution better than 1 mm.

The cosmic muon timing, relative to the reference counters, is independently measured by each of the PMTs at either end of the counter, after corrections are made for "time-walk" effects. The resultant resolutions are shown in Fig.\ref{fig:muEgamma-fig2}. The weighted average of the two measurements is of the order of $\sim$ 60 ps, independent of the position along the counter. It was also checked that the timing resolution depends on 

\begin{figure}[h]
   \centering
   \includegraphics[width=0.9\linewidth=3]{muEgamma-fig2.ps}
\caption{\small\slshape Timing resolutions measured along the length of the counter.}
\label{fig:muEgamma-fig2}
\end{figure} 

%\vspace{-1mm}

\noindent                                     
the square-root of the total number of photoelectrons. A full Monte Carlo simulation, taking into account the measured values, shows that the required 40 ps timing resolution stated in the proposal, will be obtained with a suitable counter geometry, the engineering studies of which are in progress. 

\vspace{2mm}
\begin{thebibliography}{9}

\bibitem{mega99}
M.~L.~Brooks {\it et~al.},\\
Phys.~Rev.~Lett {\bf 83}, 1521 (1999).

\bibitem{Bellazzini2001}
R.~Bellazzini {\it et~al.},\\
Nucl.~Instr.~and Meth. {\bf A~457}, 22 (2001).
\end{thebibliography}

\end{contribution}