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\title{Eurogam HV Control Specification}
\author{John Cresswell}
\date{August 1990}
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EDOC028\par
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EUROGAM PROJECT\par
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NSF DATA ACQUISITION SYSTEM\par
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Eurogam HV Control Specification\par
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Edition 1.0\par
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August 1990\par
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Nuclear Structure Software Systems Group\par
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Department of Physics\par
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Liverpool University\par
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\maketitle
\section{Introduction}
 
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[{\it This is a first draft for discussion and will be amended
and improved following user comment.}]
 
The Eurogam project has specified that the High Voltage supplied
to Ge and BGO detectors is to be computer controlled
due to the large number of detector channels involved (approx. 770).
This enables the setup and control of the HV supplies to be
integrated into the general data acquisition system.
The aim is to provide a user friendly computer interface
in the same form as the rest of the system.
 
\section{Hardware}
 
The LeCroy 1440 series of HV mainframes has been chosen for this
application.
The HV outputs are provided by boards which slot into each
mainframe.
There is space for 16 boards per mainframe and upto 
16 mainframes may be daisy-chained together from one computer interface.
 
The BGO detectors will be fed from boards of type 1443.
These boards have 16 output channels of upto 2.5 kV each.
 
The Ge detectors will be fed from boards of type 1444.
These boards have 8 output channels of upto 5.6 kV each.
 
The computer interface will be the LeCroy 1131 VME interface board.
This module has a command list execution feature together with  
DMA and data memory which all helps to free the
controlling cpu from some of the more onerous data transfer tasks.
 
\section{Software}
 
It is proposed that the software controlling access to the HV
mainframes be a single task in the crate controller of one of
the VME crates in the data acquisition system.
Only one VME interface module and very little VME bandwidth
will be involved.
Suitable crates may be the ones primarily allocated to
autofill or histogramming.
 
There will be a user interface in the Unix workstation
allowing the user to control which channels are activated
and at what voltage.
The user interface will allow the allocation of individual channels
in each mainframe to particular detectors.
 
The control task will be able to interrogate each mainframe
to find out which board type is present in each slot.
This will allow any mix of board types within a mainframe.
This freedom of placement may not in practice be used, but
should be incorporated in the design.
 
The VME task will keep a local database of the current
setup state for interrogation by users.
This database will list all available supply channels
and indicate the state of each.
The state will include the following details.
 
\begin{itemize}
\item Demand voltage ... to be set if/when turned on
\item Present on/off status
\item Trip status ... whether turned off automatically
\item Name associated with channel
\end{itemize}
 
The control task will monitor the state of each channel to
check for any irregularities.
In particular, the Ge HV cards have a trip feature which allows
the voltage to be ramped down to zero on application of an
external signal.
This condition will be noticed and a warning message sent to the
user.
Warning messages may be required for more than one destination.
An audible warning in the control room may be desirable.
 
The Ge HV board (1444) allows the following values to be set ...
 
\begin{itemize}
\item Voltage for each channel
\item Ramp rate for all channels on board
\item DC current limit for all channels on board
\item AC current limit for each channel
\end{itemize}
 
The BGO HV boards (1443) allow the voltage to be programmed for each 
channel.
 
It is proposed to fix certain parameters to suitable values
agreed with the users in order to simplify the software interface.
Examples would be the voltage ramp rate and current trip levels.
 
\section{User Requirements}
 
Values provided by the user will be real voltages and not
internal register settings.
 
After a voltage drop on any channel due to the application of the
trip signal, that channel will be turned off and will only be
re-activated by user command.
 
A mode should be provided to allow the user control whilst the
voltage is applied to a particular channel. The control required
will involve quick commands (single key hit?) for stopping,
starting and direction of voltage change.
 
Some thought as to the graphical presentation of 770 channels of
HV is necessary.
Suggestions range from a message informing of a problem with a 
certain channel, to histograms of HV channel 'number' versus
deviation from set point.
 
For the BGO detectors there will be curves of HV versus gain
for all the tubes.
These curves can be parametrised and stored as a database
to allow specified gain changes to be evaluated in terms
of a voltage change.
Then, for example, all detectors could be increased in gain by a 
certain amount, and each one applied automatically via
differing changes in HV.
 
There will be the need to control HV in terms of BGO shields
as well as singly.
The 10 HV channels of one BGO shield should be controllable
together wherever the channels are located in the various mainframes.
 
Any more ?
 
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