Q: Will pyads notice and update symbol changes (offset, type) while running or on reconnect?

[tvogel]

This is more a question: Today, the company that built and services our PLC (heating system) updated the control logic because some hardware components changed (electricity counters via Modbus RTU).

After the rollout, some scripts that I have written to fine-tune the behavior of the systems, suddenly read awkward values. I am not sure whether the connection was dropped in between, I guess so. Could it be that pyads will keep symbol info (offsets, types) cached across connections such that reading a symbol will still use the outdated offset? (I hope, my scripts didn't write to unintended addresses...!)

Anyway, restarting the scripts fixed the issues.

[chrisbeardy]

If you are using read_by_name, or the get_symbol then it may cache the symbol info e.g. https://github.com/stlehmann/pyads/blob/54a34185ed963f8d9928a1877ab2d99fd38c9ef1/src/pyads/connection.py#L514C5-L536C82 for as long as the connection class instance is the same. If you restart the script (or recreate the connection class I believe) then this should refresh. I always just monitor for symbol changes and just restart my scripts if one changes as I'm not 100% sure of the recreating the connection class behaviour. You can monitor for symbol changes using something like the following:

self._plc.add_device_notification(
                (int("0xF100", 16), int("0x0000", 16)),  # ADSIGRP_DEVICE_DATA, ADSIOFFS_DEVDATA_ADSSTATE
                pyads.NotificationAttrib(ctypes.sizeof(pyads.PLCTYPE_INT)),
                self._on_plc_status_change,
            )
self._plc_symbol_changes: int = 0
self._plc.add_device_notification(
    (int("0xF008", 16), 0),  # ADSIGRP_SYM_VERSION
    pyads.NotificationAttrib(ctypes.sizeof(pyads.PLCTYPE_BOOL)),
    self._on_symbol_table_change,
)

def _on_plc_status_change(self, notification, _):
        """Notification callback for a change in PLC state (run to config etc)."""
        *_, value = self._plc.parse_notification(notification, pyads.PLCTYPE_INT)  # type: ignore
        if value != pyads.ADSSTATE_RUN:
            self._plc_fatal_com_error_message = "PLC exited run mode"

def _on_symbol_table_change(self, *_):
    """Notification callback for plc heartbeat."""
    self._plc_symbol_changes += 1
    if self._plc_symbol_changes > 1:
        self._plc_fatal_com_error_message = "PLC symbol table has changed"