Lecture Notes: EPICS Channel Access and Python Integration

Introduction

• Presenter: [Name not provided]
• Topic: Usage of EPICS Channel Access and Python
• Content Scope: Basics of libraries, developed applications, ongoing projects related to data collection

Why Use Python?

• Widely recognized at APS (Advanced Photon Source) for scripting, control, and data analysis
• Key Features:
  • Elegance and readability
  • Cross-platform compatibility
  • Free and open-source with many scientific libraries (e.g., NumPy, SciPy)

pyEPICS Channel Access Library

Three Layers:

1. Low-Level: Direct mapping to C-based Channel Access shared object library
   • Close to C code but in Python
   • Uses EPICS base DLL
2. PV Object Layer: Process variable (PV) with methods and attributes
   • More Pythonic and object-oriented
   • Ideal for significant programming involving Channel Access
3. Procedural Layer: Functions like caget, caput, camonitor, cainfo
   • Simplest interface
   • Uses a global cache of process variables

Procedural Interface

• Easy to use (e.g., CA put, CA get)
• Options for put (waiting for processing completion) and get (raw or string formatted values)
• CA info displays control fields and CA monitor for value change monitoring

PV Object

• Creation: PV(name)
• Methods/Attributes: Get, put, value
• Handles connection management using events/callbacks
• Event Callbacks: Functions run on PV value changes

Handling Waveforms and Strings

• Waveforms: Use NumPy arrays for dense data storage and efficiency
• Long Strings: Character waveforms for strings longer than 40 characters
• Methods provided for getting/setting these values as strings or arrays

Implementation with C Types Library

• C Types: Python interface to C/Fortran DLLs
• Benefits:
  • Simplified implementation (no additional C code required)
  • Easy installation across platforms (Windows, Mac, Linux)
  • Supported threading
• Channel Access Calls: (e.g., create_channel, create_subscription)
• Preemptive Callbacks: Default option for handling epics callbacks in a separate thread

High-Level Devices with pyEPICS

• Analog Input Device Example: Managed using fields and initializing attributes
• Motor Device: Complex handling of motor fields; supports movement, checking limits, tweaks, etc.

GUI Controls with WXPython

• Python GUIs using WX Python (a common toolkit)
• Widgets: Text widgets, choices, numeric controls, etc.
• Example Application: Displaying and controlling an area detector image

Advanced Applications

• Motor GUI Controls: Enhanced MEDM-like screens
• Instrument Management Application: Organizes PVS into instruments with savable states

Alarms and Event Management

• Simple Alarms: Scripts for monitoring PV values and triggering callbacks for events
• Useful for sending alerts or changing system states

Data Collection Tools

• Scanning Software (in-progress): Collects data using step scanning methods
• Positioners & Detectors: Capable of handling complex scan patterns

Moving to Scriptable and Network-Based Scanning

• GUI for Scans: Interface for configuring and running scans
• Database Integration: Using SQL databases for managing scan parameters and results in real-time

Future Goals and Challenges

• Standardizing HDF5 Data Storage: Ensuring robust data conversion and storage
• Improving Interactivity: Incorporating live plots and more responsive interfaces

Conclusion

• Summary: Comprehensive exposure to pyEPICS, its capabilities, and ongoing developments
• Acknowledgments: Special mention to contributors including Angus Gratton from ANU