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Plan for GEM-OTMB Data Format

This document outlines the data format plan for fiber communication between GEM and OTMB at Texas A&M University. Operating at 3.2 Gbps with two synchronized links per GEM, the protocol utilizes 8-bit and 10-bit formats to achieve minimized latency without elastic buffers. Each BX consists of four 16-bit frames, including alignment and trigger data. The design accounts for encoding the BC0 signal and error-check mechanisms, allowing for distinct signal identification across one or more GEM hits. This framework is critical for efficient data transmission in high-energy physics experiments.

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Plan for GEM-OTMB Data Format

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  1. Plan for GEM-OTMB Data Format J. Gilmore Forward Muon Workshop Texas A&M University 9 Feb 2014

  2. Fiber Links Between GEM and OTMB • Operate the fiber links at 3.2 Gbps • Use two links per GEM • Operate them synchronously with LHC clock • Use standard 8-bit 10-bit protocol • Constant (minimized) latency on the links • Do not use the usual elastic buffers • Net Result: four 16-bit frames per link for every BX • Initially using a simple structure • One alignment frame (comma) • Three trigger data frames • We can consider other options later • Provides 96 data bits per BX • ~10 different comma character possibilities • May be considered as 3 additional bits on each link • Essentially allows for 102 distinct signals from GEM every BX • Useful for BC0 sync monitoring or additional data GEM Trigger Frame Format 8b10b comma character GEM trigger data GEM trigger data GEM trigger data … …

  3. GEM-OTMB Fiber Communication Detail • Consider two links from GEM Opto-hybrid to OTMB • Link “A” and Link “B” • Data frames are arranged like this for each BX • Other considerations… • Encode a BC0 signal • Encode an error checking mechanism? • Would use up to ~8 bits per BX • Explore other options with comma frames • Send them only once per orbit? • Send them only when there is no data present Allow 8 choices for alignment characters --easy option to use as 3 “bits” per link 16 bits per link 16 bits per link 16 bits per link “Link A” comma character GEM trigger data GEM trigger data GEM trigger data “Link B” comma character GEM trigger data GEM trigger data GEM trigger data

  4. GEM-OTMB Data Encoding • Need to carry ID of triggered GEM strips to the OTMB • GEM has 3 columns in Phi, 8 sectors in Eta • 24 VFAT chips installed on GEM front end • Assign 5 bits for VFAT ID • Up to 64 trigger pads per VFAT chip • VFAT3 can be configured for 2 strips or 4 strips per trigger pad • Assign 6 bits for GEM Pad ID  Up to 11 bits required for unique trigger channel ID • Planned fiber protocol has 102 signals available per BX • Can carry up to 9 GEM hits/BX without loss to OTMB

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