The system ground (signal ground), which is the zero point used for reference by various A/D IO points on the PLC, must be at zero potential like high-voltage equipment (such as transformers), that is, good grounding (grounding resistance <: 4 Ω), so that the collected signals of A/D can be at the same reference point, and PLC can correctly handle external signals.
Shielded ground, that is, cables connected to PLC are generally shielded cables, that is, the outermost sheath is covered with a layer of interwoven metal mesh (copper wire), and the core wire is inside. That layer of shielding metal mesh is used to prevent the external power supply from interfering with the weak current signal transmitted on the core wire. Grounding the shielding layer can introduce the induced current generated by the external changing magnetic field into the earth (the charge is induced on the surface of the shielding layer, and the electric field inside the shielding layer is zero), so that the external changing magnetic field will not be induced on the core wire to generate the induced current and cause inaccurate signals.
protective ground, which is easy to understand, is the ground wire on ordinary electrical appliances to protect PLC from leakage.
AC ground, which is for some DC-driven PLCs, is actually the same thing as protection ground.
in fact, in engineering, the system (signal ground) is grounded separately, but the grounding resistance is <: 4 ohms is often not very strict. Shielded land is usually with protected land.