There are some small problems in LPC43xx RS485 mode configuration. In particular: 1. UART0,2,3 do not have DTR pins (different from UART1), so, Kconfig needs to be adjusted. 2. lpc43_uart.c in RS485 mode only configures DIR pin, but doesn't enable pin output for UART0,2,3. 3. should be option to reverse DIR control pin output polarity. 4. lpc43xx/chip/lpc43_uart.h doesn't have USART3 definitions. NOTE: I didn't modified and didn't tested USART1, as it has different hardware. From Vytautas Lukenskas.

arch/arm/src/lpc43xx/Kconfig

@@ -638,11 +638,16 @@ config USART0_RS485MODE
 	---help---
 		Enable RS-485 interface on USART0.
 
-config USART0_RS485_DTRDIR
+if USART0_RS485_MODE
-	bool "USART0 DTR for DIR"
+	config USART0_RS485DIROIN
+	bool "Invert direction control pin polarity"
 	default n
 	---help---
-		Use the USART DTR pin for the DIR pin
+		If disabled, control pin will be driven to logic 0 when the
+		transmitter has data to be sent.  It will be driven to logic 1 after
+		the last bit of data has been transmitted.
+
+endif # USART0_RS485MODE
 
 endif # LPC43_USART0
 
@@ -670,11 +675,15 @@ config USART2_RS485MODE
 	---help---
 		Enable RS-485 interface on USART2.
 
-config USART2_RS485_DTRDIR
+if USART2_RS485MODE
-	bool "USART2 DTR for DIR"
+	config USART0_RS485DIROIN
+	bool "Invert direction control pin polarity"
 	default n
 	---help---
-		Use the USART DTR pin for the DIR pin
+		If disabled, control pin will be driven to logic 0 when the transmitter has data to be sent.
+		It will be driven to logic 1 after the last bit of data has been transmitted.
+
+endif # USART2_RS485MODE
 
 endif # LPC43_USART2
 
@@ -686,11 +695,15 @@ config USART3_RS485MODE
 	---help---
 		Enable RS-485 interface on USART3.
 
-config USART3_RS485_DTRDIR
+if USART3_RS485MODE
-	bool "USART3 DTR for DIR"
+config USART3_RS485DIROIN
+	bool "Invert direction control pin polarity"
 	default n
 	---help---
-		Use the USART DTR pin for the DIR pin
+		If disabled, control pin will be driven to logic 0 when the transmitter has data to be sent.
+		It will be driven to logic 1 after the last bit of data has been transmitted.
+
+endif # USART3_RS485MODE
 
 endif # LPC43_USART3
 endmenu # RS-485 Configuration

arch/arm/src/lpc43xx/chip/lpc43_uart.h

@@ -168,6 +168,28 @@
 #define LPC43_USART2_SYNCCTRL        (LPC43_USART2_BASE+LPC43_USART_SYNCCTRL_OFFSET)
 #define LPC43_USART2_TER             (LPC43_USART2_BASE+LPC43_USART_TER_OFFSET)
 
+#define LPC43_USART3_RBR             (LPC43_USART3_BASE+LPC43_UART_RBR_OFFSET)
+#define LPC43_USART3_THR             (LPC43_USART3_BASE+LPC43_UART_THR_OFFSET)
+#define LPC43_USART3_DLL             (LPC43_USART3_BASE+LPC43_UART_DLL_OFFSET)
+#define LPC43_USART3_DLM             (LPC43_USART3_BASE+LPC43_UART_DLM_OFFSET)
+#define LPC43_USART3_IER             (LPC43_USART3_BASE+LPC43_UART_IER_OFFSET)
+#define LPC43_USART3_IIR             (LPC43_USART3_BASE+LPC43_UART_IIR_OFFSET)
+#define LPC43_USART3_FCR             (LPC43_USART3_BASE+LPC43_UART_FCR_OFFSET)
+#define LPC43_USART3_LCR             (LPC43_USART3_BASE+LPC43_UART_LCR_OFFSET)
+#define LPC43_USART3_LSR             (LPC43_USART3_BASE+LPC43_UART_LSR_OFFSET)
+#define LPC43_USART3_SCR             (LPC43_USART3_BASE+LPC43_UART_SCR_OFFSET)
+#define LPC43_USART3_ACR             (LPC43_USART3_BASE+LPC43_UART_ACR_OFFSET)
+#define LPC43_USART3_ICR             (LPC43_USART3_BASE+LPC43_USART_ICR_OFFSET)
+#define LPC43_USART3_FDR             (LPC43_USART3_BASE+LPC43_UART_FDR_OFFSET)
+#define LPC43_USART3_OSR             (LPC43_USART3_BASE+LPC43_USART_OSR_OFFSET)
+#define LPC43_USART3_HDEM            (LPC43_USART3_BASE+LPC43_USART_HDEN_OFFSET)
+#define LPC43_USART3_SCICTRL         (LPC43_USART3_BASE+LPC43_USART_SCICTRL_OFFSET)
+#define LPC43_USART3_RS485CTRL       (LPC43_USART3_BASE+LPC43_UART_RS485CTRL_OFFSET)
+#define LPC43_USART3_ADRMATCH        (LPC43_USART3_BASE+LPC43_UART_ADRMATCH_OFFSET)
+#define LPC43_USART3_RS485DLY        (LPC43_USART3_BASE+LPC43_UART_RS485DLY_OFFSET)
+#define LPC43_USART3_SYNCCTRL        (LPC43_USART3_BASE+LPC43_USART_SYNCCTRL_OFFSET)
+#define LPC43_USART3_TER             (LPC43_USART3_BASE+LPC43_USART_TER_OFFSET)
+
 /* Register bit definitions *****************************************************************/
 
 /* RBR (DLAB =0) Receiver Buffer Register */

arch/arm/src/lpc43xx/lpc43_config.h

@@ -73,7 +73,6 @@
 #ifndef CONFIG_LPC43_USART0
 #  undef CONFIG_USART0_SERIAL_CONSOLE
 #  undef CONFIG_USART0_RS485MODE
-#  undef CONFIG_USART0_RS485_DTRDIR
 #endif
 
 #ifndef CONFIG_LPC43_UART1
@@ -85,13 +84,11 @@
 #ifndef CONFIG_LPC43_USART2
 #  undef CONFIG_USART2_SERIAL_CONSOLE
 #  undef CONFIG_USART2_RS485MODE
-#  undef CONFIG_USART2_RS485_DTRDIR
 #endif
 
 #ifndef CONFIG_LPC43_USART3
 #  undef CONFIG_USART3_SERIAL_CONSOLE
 #  undef CONFIG_USART3_RS485MODE
-#  undef CONFIG_USART3_RS485_DTRDIR
 #endif
 
 /* Is there a serial console? There should be at most one defined.  It could be on

arch/arm/src/lpc43xx/lpc43_serial.c

@@ -169,9 +169,6 @@ static struct up_dev_s g_uart0priv =
   .parity         = CONFIG_USART0_PARITY,
   .bits           = CONFIG_USART0_BITS,
   .stopbits2      = CONFIG_USART0_2STOP,
-#if defined(CONFIG_USART0_RS485MODE) && defined(CONFIG_USART0_RS485_DTRDIR)
-  .dtrdir         = true,
-#endif
 };
 
 static uart_dev_t g_uart0port =
@@ -239,9 +236,6 @@ static struct up_dev_s g_uart2priv =
   .parity         = CONFIG_USART2_PARITY,
   .bits           = CONFIG_USART2_BITS,
   .stopbits2      = CONFIG_USART2_2STOP,
-#if defined(CONFIG_USART2_RS485MODE) && defined(CONFIG_USART2_RS485_DTRDIR)
-  .dtrdir         = true,
-#endif
 };
 
 static uart_dev_t g_uart2port =
@@ -274,9 +268,6 @@ static struct up_dev_s g_uart3priv =
   .parity         = CONFIG_USART3_PARITY,
   .bits           = CONFIG_USART3_BITS,
   .stopbits2      = CONFIG_USART3_2STOP,
-#if defined(CONFIG_USART3_RS485MODE) && defined(CONFIG_USART3_RS485_DTRDIR)
-  .dtrdir         = true,
-#endif
 };
 
 static uart_dev_t g_uart3port =

arch/arm/src/lpc43xx/lpc43_uart.c

@@ -364,7 +364,31 @@ void lpc43_usart0_setup(void)
 
 #ifdef CONFIG_USART0_RS485MODE
   lpc43_pin_config(PINCONF_U0_DIR);
-#endif
+
+  /* Enable direction output pin */
+
+  regval = getreg32(LPC43_USART0_RS485CTRL);
+  regval |= UART_RS485CTRL_DCTRL;
+  putreg32(regval, LPC43_USART0_RS485CTRL);
+
+#ifdef CONFIG_USART0_RS485DIROIN
+
+  /* Invert direction control output pin polarity */
+
+  regval = getreg32(LPC43_USART0_RS485CTRL);
+  regval |= UART_RS485CTRL_OINV;
+  putreg32(regval, LPC43_USART0_RS485CTRL);
+
+#else
+
+  /* Do not invert direction countrol output pin polarity */
+
+  regval = getreg32(LPC43_USART0_RS485CTRL);
+  regval &= ~(UART_RS485CTRL_OINV);
+  putreg32(regval, LPC43_USART0_RS485CTRL);
+
+#endif /* CONFIG_USART0_RS485DIROIN */
+#endif /* CONFIG_USART0_RS485MODE */
 
   leave_critical_section(flags);
 };
@@ -467,7 +491,31 @@ void lpc43_usart2_setup(void)
 
 #ifdef CONFIG_USART2_RS485MODE
   lpc43_pin_config(PINCONF_U2_DIR);
-#endif
+
+  /* Enable direction output pin */
+
+  regval = getreg32(LPC43_USART2_RS485CTRL);
+  regval |= UART_RS485CTRL_DCTRL;
+  putreg32(regval, LPC43_USART2_RS485CTRL);
+
+#ifdef CONFIG_USART2_RS485DIROIN
+
+  /* Invert direction control output pin polarity */
+
+  regval = getreg32(LPC43_USART2_RS485CTRL);
+  regval |= UART_RS485CTRL_OINV;
+  putreg32(regval, LPC43_USART2_RS485CTRL);
+
+#else
+
+  /* Do not invert direction countrol output pin polarity */
+
+  regval = getreg32(LPC43_USART2_RS485CTRL);
+  regval &= ~(UART_RS485CTRL_OINV);
+  putreg32(regval, LPC43_USART2_RS485CTRL);
+
+#endif /* CONFIG_USART2_RS485DIROIN */
+#endif /* CONFIG_USART2_RS485MODE */
 
   leave_critical_section(flags);
 };
@@ -516,7 +564,31 @@ void lpc43_usart3_setup(void)
 
 #ifdef CONFIG_USART3_RS485MODE
   lpc43_pin_config(PINCONF_U3_DIR);
-#endif
+
+  /* Enable direction output pin */
+
+  regval = getreg32(LPC43_USART3_RS485CTRL);
+  regval |= UART_RS485CTRL_DCTRL;
+  putreg32(regval, LPC43_USART3_RS485CTRL);
+
+#ifdef CONFIG_USART3_RS485DIROIN
+
+  /* Invert direction control output pin polarity */
+
+  regval = getreg32(LPC43_USART3_RS485CTRL);
+  regval |= UART_RS485CTRL_OINV;
+  putreg32(regval, LPC43_USART3_RS485CTRL);
+
+#else
+
+  /* Do not invert direction countrol output pin polarity */
+
+  regval = getreg32(LPC43_USART3_RS485CTRL);
+  regval &= ~(UART_RS485CTRL_OINV);
+  putreg32(regval, LPC43_USART3_RS485CTRL);
+
+#endif /* CONFIG_USART3_RS485DIROIN */
+#endif /* CONFIG_USART3_RS485MODE */
 
   leave_critical_section(flags);
 };