Fix equipment readings 404 errors and implement BK Precision protocol support

[X9X0]

Summary

Fixes equipment readings returning 404 Not Found errors and implements proper protocol support for BK Precision 1900B series power supplies.

Problems Fixed

1. Equipment 404 Errors

• Equipment IDs were randomly generated using UUID, causing them to change on every reconnection
• Resulted in 404 Not Found errors when trying to access equipment readings after reconnection

2. Invalid VISA Session Errors

• Discovery scans would invalidate active equipment VISA sessions
• No auto-reconnection mechanism to recover from invalid sessions
• Resource manager would also become invalid, preventing reconnection

3. BK Precision Communication Failures

• BK Precision 1900B series doesn't support SCPI commands
• All commands (*IDN?, MEAS:VOLT?, etc.) timed out after 10 seconds
• Required switching to proprietary BK Precision protocol

4. Serial Port Resource Leaks

• Old instruments weren't properly closed before reconnection
• Caused file descriptor corruption and NoneType errors

5. Response Buffer Contamination

• PyVISA read_termination stopped at first \r, leaving OK\r in buffer
• Subsequent reads got contaminated data instead of actual responses

Changes Made

Deterministic Equipment IDs

• Generate equipment IDs using SHA256 hash of resource string
• Same resource always produces same ID (e.g., ASRL/dev/ttyUSB0::INSTR → ps_56fdd3df)
• Updated all equipment classes to use deterministic ID generation

Auto-Reconnection System

• Added _is_instrument_valid() to detect invalid VISA sessions
• Added _ensure_connected() to auto-reconnect on invalid sessions
• Added _is_resource_manager_valid() to detect invalid resource managers
• Added _refresh_resource_manager() to create new resource manager when needed
• Integrated into all _write(), _query(), and _query_binary() operations

BK Precision Protocol Support

• Switched from SCPI to BK Precision proprietary protocol
• Changed line endings from \r\n to \r only
• Implemented correct command format:
  • GMAX - Get max voltage/current (format: VVVCCC)
  • GETS - Get voltage/current settings (format: VVVCCC)
  • GETD - Get display readings (format: VVVVIIIIIM)
  • GOUT - Get output state (0=OFF, 1=ON)
  • VOLT{vvv} - Set voltage (voltage × 10)
  • CURR{ccc} - Set current (current × 10)
  • SOUT0/SOUT1 - Set output (0=ON, 1=OFF)
• All responses end with \rOK\r which is properly parsed

Proper Resource Cleanup

• Close old instruments before opening new ones
• Prevents file descriptor leaks and corruption
• Properly handles exceptions during cleanup

Response Buffer Handling

• Read full BK responses byte-by-byte until OK\r terminator
• No longer relies on read_termination for BK protocol
• Eliminates buffer contamination issues

Testing

Tested with BK Precision 1902B power supply:

• ✅ Equipment connects successfully
• ✅ Deterministic ID generated correctly (ps_56fdd3df)
• ✅ Readings return valid data (voltage, current, output state, CV/CC mode)
• ✅ Auto-reconnection works after discovery scans
• ✅ Resource manager refresh works
• ✅ No timeouts or buffer errors

Commits

• 2140b60 - Generate deterministic equipment IDs to prevent 404 errors
• 27a6e9a - Add automatic VISA session recovery to prevent InvalidSession errors
• ea3b8a5 - Refresh resource manager during auto-reconnection
• d6a9191 - Call _refresh_resource_manager() in BK power supply connect method
• 09da3e5 - Properly close old instrument before reconnection
• 3bf6cf7 - Update BK Precision driver to use proprietary protocol instead of SCPI
• 408a45c - Read full BK Precision response including OK terminator

Breaking Changes

None - all changes are backward compatible improvements to existing functionality.