P. Hanke, E. Kluge, K. Meier, U. Pfeiffer K. Schmitt, C. Schumacher, 

Institut fuer Hochenergiephysik der Universitaet Heidelberg, Germany
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Raw trigger data from 7200 trigger tower channels distributed over 128 modules
have to be collected for the readout of the Pre-Processor system of the ATLAS
Level-1 Calorimeter Trigger. For each event accepted by the level-1 trigger the
corresponding raw data, on which the trigger decision is based, has to be read
out. Additional data is required for checking and analysis of the performance of
the Pre-Processor system. The readout has to cope with the maximum level-1
trigger rate of 100 kHz.

The readout system is based on a custom bus system, which connects several VME
boards by parallel point-to-point links to a ring-like structure. Data is
shifted through the bus by a synchronous protocol in a pipelined fashion.
Therefore the bus is called PipelineBus. The different nodes, which are
connected by a bus ring, consists of a master node, which has the responsibilty
to control the bus by injecting commands and data streams into the pipeline and
to process responses coming back through the ring. Readout nodes provide data
read out from the devices pre-processing the input data to the level-1 trigger.
A S-Link node receives this data and sends it after formatting via the standard
S-Link transmitter to ATLAS DAQ. PipelineBus master and S-Link node form the
level-1 Pre-Processor ROD.

The format of the data and control words on the bus is chosen in a way to make
it simple to interface to the S-Link. It consists of 32 bits of data and two
control bits indicating data, control or empty words. A parity bit is used for
error-checking. Only minimum formatting and some buffering is needed to form
standard ATLAS readout blocks. In a way, the PipelineBus can be seen as a bus
extension to the S-Link. 

On the module level a special ASIC is used to collect data from 64 trigger tower
channels via synchronous serial links. It forms readout blocks, which are
injected into the PipelineBus on request of the master node. In addition it
provides facilities for data compression in order to use the provided bandwidth
in an optimal way. A prototype of this ASIC, the RemAsic, implements run-length,
Huffman and a differential encoding.

For setting up a test environment for the PipelineBus and other Pre-Processor
system components a modular test system was developed. It is based on a general
purpose VME motherboard providing CMC slots for specialized daughterboards. An
object-oriented software framework provides flexible access to the hardware. We
will present results of test performed on a PipelineBus configuration based on
this test system.

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The input to the ATLAS Level-1 Calorimeter trigger consists of 7200 trigger
tower channels. The Pre-Processor system of the Level-1 calorimeter trigger
provides facilities to read out this raw data on which the trigger decision is
based. A high-bandwidth custom bus system, build from parallel point-to-point
links, is used to collect data from several VME modules. This data is fed to the
S-Link transmitter used for sending the read out data to the standard ATLAS DAQ
system. The architecture of this bus system and results from first
tests, performed within a modular test system, are presented.

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