Readout Electronics

Readout of 38 k pickup strips is accomplished with the use of custom-made VME-based discriminator/time-multiplexing boards developed by Princeton University and Osaka City University groups. The system consists of the signal discrimination and multiplexing boards, crate controller boards (one per crate) for crate-wide control and data processing, string controller boards for downloading and controlling multiple readout crates, and FASTBUS time-to-digital converters.
The discriminator boards are 6U-size VME boards with 96 input channels per board. A comparator (MAX908CPD) is used to generate a logic signal if the voltage on the input channel exceeds the threshold voltage. This threshold can be selected via a programmable digital-to-analog converter to be any value from - 250 mV to + 250 mV. A time multiplexer scheme combines hit information from 12 RPC channels into a single high-speed serial data stream that is passed to a LeCroy 1877 pipelined TDC. The multiplexing is accomplished with a Xilinx XC4005E FGPA. A schematic diagram of the readout electronics is shown in Fig. . In addition, the logical OR of the hits for each 12-channel group is generated and is available for use as a fast trigger signal.

Figure: Schematic diagram of the KLM readout electronics.

Each VME crate has a crate-controller board which transmits control data from the string controller to the discriminator boards via the dedicated VME backplane. A 10 MHz clock signal from the crate-controller board is distributed throughout the crate for the discriminator boards in time sequencing the RPC hits. The string controller is a multi-function VME-compatible board using a Xilinx 4013 programmable gate array to allow downloading and control of a string of up to 8 RPC readout crates. Once the discriminator board is programmed, the time sequenced hit information travels directly from each 96 channel board to 8 TDC channels residing in a FASTBUS crate. In this manner, 38 k RPC channels are reduced to 3200 FASTBUS TDC channels, resulting in significant cost savings. It is also possible to read RPC strip-hit data directly through the string controllers, as was done during system commissioning when the production TDC system was not yet available.