Hi

I was using read_by_name before and wanted to switch to get_symbol(...).read() because of the nicer syntax and not having to pass around the plc object. So I discovered, that the latter failed for monitored properties.

With monitored properties I mean properties that have the monitoring pragma.

I couldn't find any documentation on accessing monitored properties over ADS, neither here nor on TwinCAT, so I experimented a bit.

Windows 10
TwinCAT 3.4024.10

With read_by_name

❌ is a ADSError: symbol not found (1808)
Same for write_by_name however with {attribute 'monitoring' := 'variable'} writing has not effect.

With get_symbol(...).read() or .write(...)

Mostly the same as above but function block with call fails with ADSError: objects do not match (1806)
Again .write(...) has no effect.

I was mostly interested in the 1806 case so I looked deeper and found that for the affected case adsGetSymbolInfo returns an index_group of 0xF019 and an index_offset of 0x0.
I could not find any documentation on the index group 0xF019.
As a workaround I tried to get a handle with plc.get_handle(...) for the case that index_group was 0xF019 and use that instead. That worked.

I did not look deeper into the case of the 1808 error.

In summary

I think

• get_symbol should not fail, where read_by_name succeeds. Thus a workaround should be implemented.
• it would be worth documenting that monitored properties can be read and written through ADS, with the limitations listed above.