The main function of the VRB is to provide Level 2 buffering for the
SVX silicon readout system. A set of VRB modules reside in a VME subrack
along with one System Controller or Controller Fanout Module. The System
Controller communicates with the VRB through a special J3 backplane, used
to send the buffer number for event readout and scan and to monitor operational
status.
Data received on each link is stored in the buffer pointed to by the READOUT
BUFFER number supplied by the System Controller. The beginning and end of
data transfer is application dependent.
For the SVX system, the events stored by the VRB are events which have been
accepted by the Level 1 trigger and are waiting for a Level 2 accept or reject.
For a reject, the buffer is re-used when the System Controller requests an
overwrite (i.e. re uses the buffer number). Events that are accepted by the
Level 2 trigger are accessed via the VME interface with the System Controller
supplying the SCAN BUFFER number.
For non-SVX applications, either of the VRB ports may be programmed to operate
in FIFO mode. In this mode the readout and/or scan buffer numbers are incremented
automatically. Buffer numbers supplied by the System Controller are
ignored.
Use of the System Controller module is optional. Information normally supplied
by the System Controller through the P5/6 connector can also be provided
through VME if the control traffic will not interfere with data transfer.
The specifications for the VRB module are based on the SVX silicon detector
readout rates specified by DO and CDF. For SVX applications, the VRB must
input data at the L1 Accept Rate, within the given SVX readout time, and
must output data at the L2 Accept Rate. The following table shows typical
operating parameters.
| L1 Trigger Decision Time | 2.5 microseconds |
| L1 Accept Rate | 50 KHz |
| L1 (SVX) Readout Time | 10 microseconds |
| L2 Accept Rate | 1-2 KHz |
| L3 Accept Rate | 100 Hz |
| Event Size (average per VRB channel) | 300 Bytes |
| Event Size (average per VRB) | 3 KBytes |
Each VRB channel receives a stream of event data from the front-end system
and buffers that data for VME readout to the Level 3 trigger system. The
VME connection is shared by all VRB modules in a subrack.