Also, are there any general conventions that govern the usage of symbols in schematics?
I'll hazard a guess that these are test points. These provide an convenient access to a signal. Test points are useful for debugging, quality control testing, calibration, troubleshooting. A signal can be probed at a test point. Or, a test signal may be injected into a test point to exercise the downstream circuitry.
On the PCB, a test point can manifest itself differently, depending on the expected type of test.
- It can be a throughole of surface-mount component for clipping a probe to.
- It can be throughole for sticking an oscilloscope into.
- It can be a surface-mount pad for automated test equipment.
As far as I know, there is no single standard schematic symbol for test points. Here are a few symbols that come with schematic software.
The standard designator for test point it