If you're using an UART to RS485 conversion chip like 75176 or anything similar and using one of the RPi GPIO to switch rx - tx functionality of the chip.

In addition to standard libmodbus API, addtional APIs are included to support GPIO based rx-tx switching

• modbus_enable_rpi : Sets an enable Rpi integer in ctx data structure.
• modbus_configure_rpi_bcm_pin : Configures GPIO BCM pin to be used for rx-tx switching (if the RE and DE are the same)
• modbus_configure_rpi_bcm_pins : Configures GPIO BCM pins (RE & DE) to be used for rx-tx switching (if the RE and DE are not on the same pin)
• modbus_rpi_pin_export_direction : export configured GPIO, configures it as an output.
• modbus_rpi_pin_unexport_direction : unexport configured GPIO, configures it as an input.

• modbus_enable_rpi(ctx);
• modbus_configure_rpi_bcm_pin(ctx,18); // configures BCM pin # 18 for DE & RE used for switching
• modbus_configure_rpi_bcm_pins(ctx,17,18); // configures BCM pin # 17 for DE and # 18 for RE used for switching
• modbus_rpi_pin_export_direction(ctx); // exports configured BCM pin #

• modbus_rpi_pin_unexport_direction(ctx); //unexport configured BCM pin #

See test.c in rpi-test folder for template.