Sferalabs Iono Pi Max v2 3+ Lite User Manual page 26

The RS-485 port is galvanically isolated using a dedicated isolated differential line
transceiver and a high-efficiency DC-DC converter. This configuration should prevent
ground loops between devices connected through this port.
The IGND terminal is the isolated ground reference for the RS-485 serial lines. Always use
this line, not GND, for the RS-485 connection.
The RS-485 TX/RX switching is implemented automatically in the Iono Pi Max
microcontroller, based on the USB serial device speed and number of bits settings, and is
completely transparent to the software on Raspberry Pi.
The RS-485 port can be alternatively used locally by custom firmware running on the
microcontroller, or routed to the Raspberry Pi standard UART TX/RX pins on the GPIO
connector, instead of the RS-232, using the serial/rs232_rs485_inv sysfs kernel module
file (I2C control register 26). When the RS-485 is routed to the Pi's UART, the RS-485 TX/
RX switching requires explicitly setting the serial configuration parameters or manually
controlling the TX-enable line via serial/rs485_txe sysfs file (I2C control register 27). If not
configured nor manually controlled it will work in passive mode.
The RS-485 line has 620 Ohm pull-up and pull-down resistors on lines A and B. The
biasing resistors ensure that, during idle periods the data lines are kept at a stable voltage
level and prevent false triggering of receiver input. These resistors can be disabled by
removing jumpers JP4 and JP6 respectively.
A 100 Ohm termination resistor between A and B is connected when the JP5 jumper is
inserted.
Jumpers JP4, JP5 and JP6 are located on the base circuit board, to the left of the RS-485
terminal block.
RS-485 JUMPERS SCHEMATIC
26 Iono Pi Max User Guide