1. DART INTRODUCTION

DART consists of a set of Data Source modules including the Fastbus Smart Crate Controller (FSCC), Crate Trigger Interface/Readout Controller (CTI/RC) and Damn Yankee Controller (DYC). These Data Sources drive differential RS-485 data over a cable to a Data Destination module, the Access Dynamics DC2 (and it’s daughter board, the DM-115). A series of Data Sources may be linked together on the same cable (a "stream’) to in turn drive the Data Destination, and this cable may connect to multiple Data Destinations, each one of which is resident in a unique VME chassis.

Commonly, data flow is defined as progressing "downstream" (from data source towards data destination) or "upstream" (from destination back towards source). These terms will be found throughout this and other DART system documents.

The data flow is carried on a parallel differential cable using RS-485 signal levels. The internal data path is carried on two parallel ribbon cables, one of 34 pins and a second of 50 pins. The organization of the cables is as follows:

• Low 16 data bits plus control signals on the 50-pin connector.
  
• High 16 data bits on the 34-pin connector.

• DSTROBE (Data Strobe) is pulsed up and down once per data word sent from Data Source to Data Destination.

• EOR (End of Record) is pulsed up and down once per block of data words sent from one or more Data Sources to the Data Destination, and is used to delineate ‘events’ or ‘blocks’ of data.

• WAIT is asserted by the Data Destination to signal the Data Sources that transmission of data should be halted. The WAIT signal propagates backwards along the cable with respect to Data, DSTROBE and EOR.

In response to triggers generated from the Trigger System, the Data Sources take data from the front end Converters and pass it down a cable to the Data Destination. If required, Repeater modules are used to insure that the data arrives intact at the Data Destination. The Event Builder takes the data from the Data Destinations and uses it to build and store complete events.

Multiple Data Sources are allowed on a single cable. Data Sources use a Permit token signal to pass control of the data cable amongst themselves. For this Permit scheme to function, one Data Source must be the ‘first’ and one other must be defined as the ‘last’, with all others defined as ‘middle’. The ‘first’ will be the first to send data, and the ‘last’ carries the responsibility of sending the End of Record signal to the Data Destination. ‘Middle’ and ‘last’ Data Sources wait to send data until the receipt of a Permit token. The Repeater must be able to respond to the Permit token in similar fashion to the Data Sources as the cable position of the ‘first’, ‘middle’ and ‘last’ Data Sources is not predetermined.