diff --git a/ports/atmel-samd/common-hal/busio/UART.c b/ports/atmel-samd/common-hal/busio/UART.c
index 70857de6de..c4e70787a8 100644
--- a/ports/atmel-samd/common-hal/busio/UART.c
+++ b/ports/atmel-samd/common-hal/busio/UART.c
@@ -52,9 +52,12 @@ static void usart_async_rxc_callback(const struct usart_async_descriptor *const
 }
 
 void common_hal_busio_uart_construct(busio_uart_obj_t *self,
-        const mcu_pin_obj_t * tx, const mcu_pin_obj_t * rx, uint32_t baudrate,
-        uint8_t bits, uart_parity_t parity, uint8_t stop, mp_float_t timeout,
-        uint16_t receiver_buffer_size) {
+    const mcu_pin_obj_t * tx, const mcu_pin_obj_t * rx,
+    const mcu_pin_obj_t * rts, const mcu_pin_obj_t * cts,
+    const mcu_pin_obj_t * rs485_dir, bool rs485_invert,
+    uint32_t baudrate, uint8_t bits, uart_parity_t parity, uint8_t stop,
+    mp_float_t timeout, uint16_t receiver_buffer_size) {
+
     Sercom* sercom = NULL;
     uint8_t sercom_index = 255; // Unset index
     uint32_t rx_pinmux = 0;
@@ -62,6 +65,10 @@ void common_hal_busio_uart_construct(busio_uart_obj_t *self,
     uint32_t tx_pinmux = 0;
     uint8_t tx_pad = 255; // Unset pad
 
+    if ((rts != mp_const_none) || (cts != mp_const_none) || (rs485_dir != mp_const_none) || (rs485_invert == true)) {
+        mp_raise_ValueError(translate("RTS/CTS/RS485 Not yet supported on this device"));
+    }
+
     if (bits > 8) {
         mp_raise_NotImplementedError(translate("bytes > 8 bits not supported"));
     }
diff --git a/ports/cxd56/common-hal/busio/UART.c b/ports/cxd56/common-hal/busio/UART.c
index 40ae3b7614..688a355424 100644
--- a/ports/cxd56/common-hal/busio/UART.c
+++ b/ports/cxd56/common-hal/busio/UART.c
@@ -53,11 +53,17 @@ STATIC busio_uart_dev_t busio_uart_dev[] = {
 };
 
 void common_hal_busio_uart_construct(busio_uart_obj_t *self,
-    const mcu_pin_obj_t *tx, const mcu_pin_obj_t *rx, uint32_t baudrate,
-    uint8_t bits, uart_parity_t parity, uint8_t stop, mp_float_t timeout,
-    uint16_t receiver_buffer_size) {
+    const mcu_pin_obj_t * tx, const mcu_pin_obj_t * rx,
+    const mcu_pin_obj_t * rts, const mcu_pin_obj_t * cts,
+    const mcu_pin_obj_t * rs485_dir, bool rs485_invert,
+    uint32_t baudrate, uint8_t bits, uart_parity_t parity, uint8_t stop,
+    mp_float_t timeout, uint16_t receiver_buffer_size) {
     struct termios tio;
 
+    if ((rts != mp_const_none) || (cts != mp_const_none) || (rs485_dir != mp_const_none) || (rs485_invert == true)) {
+        mp_raise_ValueError(translate("RTS/CTS/RS485 Not yet supported on this device"));
+    }
+    
     if (bits != 8) {
         mp_raise_ValueError(translate("Could not initialize UART"));
     }
diff --git a/ports/nrf/common-hal/busio/UART.c b/ports/nrf/common-hal/busio/UART.c
index d79b65ff81..c422a187f6 100644
--- a/ports/nrf/common-hal/busio/UART.c
+++ b/ports/nrf/common-hal/busio/UART.c
@@ -128,10 +128,17 @@ void uart_reset(void) {
     }
 }
 
-void common_hal_busio_uart_construct (busio_uart_obj_t *self,
-                                      const mcu_pin_obj_t * tx, const mcu_pin_obj_t * rx, uint32_t baudrate,
-                                      uint8_t bits, uart_parity_t parity, uint8_t stop, mp_float_t timeout,
-                                      uint16_t receiver_buffer_size) {
+void common_hal_busio_uart_construct(busio_uart_obj_t *self,
+    const mcu_pin_obj_t * tx, const mcu_pin_obj_t * rx,
+    const mcu_pin_obj_t * rts, const mcu_pin_obj_t * cts,
+    const mcu_pin_obj_t * rs485_dir, bool rs485_invert,
+    uint32_t baudrate, uint8_t bits, uart_parity_t parity, uint8_t stop,
+    mp_float_t timeout, uint16_t receiver_buffer_size) {
+
+    if ((rts != mp_const_none) || (cts != mp_const_none) || (rs485_dir != mp_const_none) || (rs485_invert == true)) {
+        mp_raise_ValueError(translate("RTS/CTS/RS485 Not yet supported on this device"));
+    }
+  
     // Find a free UART peripheral.
     self->uarte = NULL;
     for (size_t i = 0 ; i < MP_ARRAY_SIZE(nrfx_uartes); i++) {
diff --git a/ports/stm32f4/common-hal/busio/UART.c b/ports/stm32f4/common-hal/busio/UART.c
index bfa541c17d..3c19092591 100644
--- a/ports/stm32f4/common-hal/busio/UART.c
+++ b/ports/stm32f4/common-hal/busio/UART.c
@@ -71,10 +71,12 @@ void uart_reset(void) {
     uart_clock_disable(ALL_UARTS);
 }
 
-void common_hal_busio_uart_construct(busio_uart_obj_t* self,
-        const mcu_pin_obj_t* tx, const mcu_pin_obj_t* rx, uint32_t baudrate,
-        uint8_t bits, uart_parity_t parity, uint8_t stop, mp_float_t timeout,
-        uint16_t receiver_buffer_size) {
+void common_hal_busio_uart_construct(busio_uart_obj_t *self,
+    const mcu_pin_obj_t * tx, const mcu_pin_obj_t * rx,
+    const mcu_pin_obj_t * rts, const mcu_pin_obj_t * cts,
+    const mcu_pin_obj_t * rs485_dir, bool rs485_invert,
+    uint32_t baudrate, uint8_t bits, uart_parity_t parity, uint8_t stop,
+    mp_float_t timeout, uint16_t receiver_buffer_size) {
 
     //match pins to UART objects
     USART_TypeDef * USARTx;
@@ -84,6 +86,10 @@ void common_hal_busio_uart_construct(busio_uart_obj_t* self,
     bool uart_taken = false;
     uint8_t uart_index = 0; //origin 0 corrected
 
+    if ((rts != mp_const_none) || (cts != mp_const_none) || (rs485_dir != mp_const_none) || (rs485_invert == true)) {
+        mp_raise_ValueError(translate("RTS/CTS/RS485 Not yet supported on this device"));
+    }
+    
     //Can have both pins, or either
     if ((tx != mp_const_none) && (rx != mp_const_none)) {
         //normal find loop if both pins exist