RS-485 fail-safe bias resistors appear reversed on ISL83485 A/B lines

## Description

The RS-485 fail-safe bias network around the ISL83485 transceiver appears to be connected with the wrong polarity in the Rodent schematic.

The current circuit appears to have:

- **B/Z connected to 3.3 V through a 20 kΩ resistor**
- **A/Y connected to GND through a 20 kΩ resistor**

According to the ISL83485 documentation:

- **A/Y is the non-inverting RS-485 line**
- **B/Z is the inverting RS-485 line**
- The receiver output is high when `A > B`
- The typical operating circuit pulls **A/Y toward 3.3 V** and **B/Z toward GND**

The bias connections should therefore be:

- **A/Y → 3.3 V through 20 kΩ**
- **B/Z → GND through 20 kΩ**

## Possible impact

With the current connections, the inactive RS-485 bus is biased toward a logic-low differential state instead of the normal logic-high UART idle state.

This may cause problems such as:

- False start-bit detection
- UART framing errors
- Unreliable RS-485 or Modbus RTU communication
- Incorrect data being received while no transmitter is actively driving the bus

## Suggested fix

Please verify the schematic and manufactured PCB revisions. If the connections are not intentional, the two bias resistor connections should be swapped so that A/Y is pulled up and B/Z is pulled down.

It would also be useful to document which Rodent hardware revisions are affected.

## References

Rodent repository and schematics:

https://github.com/bigtreetech/Rodent/tree/master

Renesas ISL83485 product page and datasheet:

https://www.renesas.com/en/products/isl83485?tab=overview

The relevant information is in the pin descriptions and the “Typical Operating Circuits” section of the datasheet.

Start with the Rodent repository schematics and compare the ISL83485 bias connections against the datasheet’s pin descriptions and Typical Operating Circuits. Verify the affected manufactured PCB revisions, then confirm that A/Y is pulled to 3.3 V and B/Z to GND, documenting which hardware revisions are affected.

Written by the indexing model from the issue text.