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High-Rate Encoder Touch-Base (Aug 22, 2023)

Questions:

• will send one number per shot?
  • keep it simple for now (Larry/Namit have all encoder signals hooked up for future fancy behavior)
• 32-bit or 64-bit number? (how many total ticks)
  • Zach thinks less than 4 billion ticks (maybe even less than 1 million)
  • Larry says that the firmware bandwidth usage doesn't change in either case
  • start with 32 bits for now to save daq disk space and network space
• can start daq-integration now?
  • rogue code GitHub repo?
• timestamping
• • don't need daughter code (need xilinx dev board kcu105) 
  • rogue code GitHub repo?  https://github.com/slaclab/high-rate-encoder-dev
• timestamping: done in kcu105 using the usual weaver/ruckman module
• use pgp4 6Gbit/s use pgp4 for 
  • increased trigger jitter ok?  Larry says only 20-30ps RMS, so that's fine
• handling limit/set-up/alarm signals
  when does rix need interpolating encoder?
  • data sheet: https://jira.slac.stanford.edu/secure/attachment/30151/L-9517-9426-03-D_Data_sheet_TONiC_UHV_EN.pdf 
  • Nick says set-up is least important (says encoder readout is aligned: says that read-head is at the right distance from the strip with the ticks).  this should be a one-time setup, requires breaking vacuum to fix.  we think this information comes from some of the X,E,P,Q,Z signals.
  • P,Q are limit (are these the end of the readout strip?  X is setup, E is alarm.  Z is reference-mark (like a "home" position?)
    • Georgi feels reference-mark is important (although we should probably use the companion absolute encoder for this and for limit-protection)
    • Alarm feels important too (corrupts data, either because going too fast or signal level too low, so should never happen)
      • don't use an IOC: the DAQ will crash if the alarm goes off and absolute encoder takes care of limits/reference-mark
      • Larry will include all the bit (and a latched error bit (counter?)) in the 64-bit DAQ data word, but DAQ will nominally only use alarm bit
    • Larry counts alarms and reference-mark
    • Not clear if we have a reference mark or not: ideally would check.  Nick will email axilon to try to find out.  Could check also check empirically.
    • the companion absolute encoder can be used instead of the reference-mark.
    • Georgi thinks absolute encoder should protect the "limits"
  • we guess that it's the same for limit and alarm
  • might need to latch some of these (and make them clearable)
  • when does rix need interpolating absolute encoder?
    • 3 months (november?)
  • when does rix need high-rate relative encoder?
    • spring 2024
  • metastability:
    • Larry reports that the way the encoder works this should not be an issue

  Encoder Simulator

  Chris Ford has an encoder simulator in lab3, with documentation here:  UDP Encoder Interface (rix Mono Encoder).  He also has committed lab3-caf-encoder.cnf to lcls2 GitHub in psdaq/psdaq/cnf/

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