EPICS Execution Flow

Table of Contents

IOC Initialization
Record Processing: Common Part
Process: Synchronous I/O
Process: Asynchronous I/O
Channel Access
ezca (Easy Channel Access)

IOC Initialization

• ld (vxWorks command)
  
  • iocCore
    
  • drvSup
    
  • recSup
    
  • devSup
    
• dbLoad("default.dctsdr")
  
  • Loads definitions of menus, record types, device types, driver types, etc.
    
• dbLoad("xxx.database"), dbLoadRecords, dbLoadTemplates
  
  • Loads record instances
    
• iocInit (main steps)
  
  • Dynamically link to record, device, and driver support (support entry tables)
    
  • Start general purpose tasks (log server, task watchdogs, callback, etc)
    
  • Initialize drivers
    
  • Initialize records (two passes) and devices
    
  • Start scan tasks
    
  • Start access security
    
  • Start Channel Access Server
    

Record Processing: Common Part

• Decide to process a record (Database Scanning)
  
  • Periodic
    
  • I/O Interrupt
    
  • Event
    
  • Passive
    
• Skip if record is already active
  
• Skip if record is disabled
  
• Call record support process routine
  
• Linked Records (input, output, forward) (Process Passive)
  
  • Process routines are called recursively.
    

Process: Synchronous I/O

• Set PACT true (processing active)
  
• Perform I/O operation
  
  • For hardware link access via device support
    
  • For database link, recursive processing may be invoked
    
  • For channel access link
    
    • inputs are monitored - just retrieve latest value
      
    • outputs are handled by separate task
      
• Check for record specific alarm conditions
  
• Raise channel access monitors for any fields modified while processing
  
• Request processing of forward link
  
• Set PACT false
  

Process: Asynchronous I/O

• Asynchronous Start (Called by iocCore)
  
  • Set PACT true (processing active)
    
  • Start I/O operation (normally device support does this)
    
  • return leaving PACT true
    
• Asynchronous Completion (record/device support directly calls process)
  
  • Complete I/O operation
    
  • Check for record specific alarm conditions
    
  • Raise channel access monitors for any fields modified while processing
    
  • Request processing of forward link
    
  • Set PACT false
    
  • return
    

Channel Access

• Search
  
  • Perform broadcast (normally local subnet) of PV name
    
  • Each ioc has CA broadcast listener.
    
  • IOC that contains PV sends UDP message back to client
    
  • For each IOC containing PVs for client a TCP connection is established
    
  • For each client IOC establishes following tasks
    
    • Server task (handles all communication except monitors)
      
    • Event task (handles monitors raised by record support or iocCore)
      
• Get
  
  • Issue request and wait for reply
    
• Put
  
  • Issue put and complete without waiting for response
    
  • May cause record processing
    
• Add event
  
  • Ask for monitors to be set on PV (record.field)
    
• put notify
  
  • Issue put and wait for completion
    
  • Completion is all records completing asynchronous phase of processing
    
• Connection management, Access security, Error handling, etc.
  

ezca (Easy Channel Access)

• Automatic search on first reference
  
• Simplified request types
  
• Only PV names - No Channel Ids
  
• Simplified error handling