Can somebody help me understand the ASi (AS-Interface) power supply?
I think I understand how the data decoupling works (because of the induction voltage of the inductor).
I don't understand the purpose of the capacitor and resistor in parallel that is used for symmetrization. Can somebody help me here, or give me a link with resources about this topic?
The resistors perform a weak pull from each side to ground. I don't know why they've labeled it as "shield" as AS-i cable doesn't have a shield. In the systems I'm familiar with the centre of the divider is connected to the machine frame / ground / earth. As you have pointed out the result of the resistors is that the lines are ±15 V with respect to ground.
The fact that there is a ground reference means that the AS-i gateway can monitor the + and - lines and detect a ground fault. I had to trace one in the past few weeks and we used a Fluke Scopemeter to monitor the lines relative to the machine frame and could see the two traces jump from ±15 V to 0 - -30 V intermittently. We eventually realised that the AS-i + wire inside a pad printer was being hit by the moving slide, the insulation had worn away and was making intermittent ground contact. This was easily fixed.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Simple AS-i earth fault indicator.
To assist in future fault tracing we're building a simple tester as shown in Figure 1.
D3 and D4 provide reverse polarity protection. R1 and R2 are 1 W.
The hope is that thes can be clamped anywhere on the AS-i cable and the lights positioned for easy correlation between the indicators and the moving equipment.
The resistors and capacitors have identical values, so they keep the shield at half voltage between ASi+ and ASi- voltages.
The resistors keep the DC level bias of the shield at DC at low frequencies. The resistances must be high enough not to waste too much DC current, so the impedance can be rather high. That's why the capacitors are there. They have high impedance at DC so they don't do much, but low impedance at high frequencies.
