H.L. Groenstege, A.N.M. Zwart, NIKHEF, Amsterdam, The Netherlands.

 
A.C. Konig, Th.A.M. Wijnen, University of Nijmegen, Nijmegen, 
The Netherlands. 



!!
In 1996 we started to define the functionality of the NIMROD (NIKHEF
MDT Read Out Driver). The NIMROD interfaces between the serial output
links of the TDCs on the MDT chambers and the main ATLAS trigger and
DAQ system. It basically consists of two parts. One part distributes
the trigger, clock and control (TTC) signals to the on-chamber TDC
chips. The other part receives the data from the TDCs and builds the
events which are then sent to a so called Read Out Buffer (ROB) of the
ATLAS trigger and DAQ system.

Level one trigger signals will be forwarded by the NIMROD to all its
TDCs. Each TDC will respond by sending back the data which belong to
a well defined time window relative to the timing of the trigger
itself. Data from subsequent triggers is queued while data transfer is
in progress. As a consequence, the data coming in from different TDCs
at a given moment may belong to different events.

The NIMROD stores the TDC data in buffers. It starts to build the
event as soon as the data from all TDCs for the event currently being
processed is complete. It adds some extra information like possible
error flags and word counts, and sends the event to the ROB. In the
process headers from "empty" TDCs can be suppressed, thus reducing the
total event size. The connection between the NIMROD and the ROB (ROL)
has an effective capacity of 1 Gbit/s.

Even though the NIMROD functionality was still evolving and not
completely finalized, it was two years ago decided to build a
downscoped prototype version. The prototype NIMROD allows for 16
serial input links running at 40 Mbit/s and it is being read out
through VME (rather than via a 1 Gbit/s optical ROL). The prototype
allowed us to test our ideas on NIMROD functionality as well as to
gain experience with implementing it by means of large FPGAs (mainly
Altera 10K series), which were introduced in our institute at that
time. Moreover, we had a direct application in mind for the prototype:
the cosmic-ray extension to the L3 detector at CERN.

The L3+Cosmics extension includes a fully independent read-out of the
L3 muon drift chambers. Behind the existing L3 pre-amplifiers and
discriminators the chamber signals are split off and fed into
32-channel TDC chips (TDC32 from CERN-EPC/MIC). The signals of 3 TDCs
on a card are serialized to emulate the future ATLAS TDC-link. The
physical connection to the NIMROD is an FTP cable. Lacking the ATLAS
TTC system, the timing and trigger parts have been implemented in the
L3+Cosmics trigger module. The 40 MHz clock is derived from a GPS
receiver, which also provides us with the world time of the events.
The prototype NIMROD is a standard size (6U) VME module and is VME64
compatible.  The NIMROD is now in use at L3+Cosmics, as well as in
LHCb (inner tracker) and ATLAS (DATCHA) test stands. L3+Cosmics is
demanding in the sense of large events during calibration periods.
Buffer size is an issue here, since the data transfer via VME is not
as fast as it would be via a ROL. Actual data taking is much less
demanding.

!!
The NIMROD (NIKHEF MDT Read Out Driver) is a VME module which
interfaces between the serial output of the TDCs from the ATLAS MDT
precision muon chambers and a single Read Out Link (ROL). The ROL
connects to a Read Out Buffer (ROB) of the main ATLAS trigger and data
acquisition system. The TDC-links will be operated at 40 Mbit/s; the
Read Out Links run at a speed of 1 Gbit/s. In addition to multiplexing
the upstream data, the NIMROD distributes the clock and trigger
information to its TDCs. Moreover, it checks the data integrity,
builds the events and, as an option, performs zero suppression. A
prototype NIMROD module has been designed and tested. Sixteen modules
are now operational in the L3+Cosmics experiment at CERN.
!!