diff --git a/bacnet-stack/ports/bdk-atxx4-mstp/rs485.c b/bacnet-stack/ports/bdk-atxx4-mstp/rs485.c
index d546f292..c5e15423 100644
--- a/bacnet-stack/ports/bdk-atxx4-mstp/rs485.c
+++ b/bacnet-stack/ports/bdk-atxx4-mstp/rs485.c
@@ -214,6 +214,9 @@ bool rs485_baud_rate_set(
 static void rs485_usart_init(
     void)
 {
+    /* enable the internal pullup on RXD0 */
+    BIT_CLEAR(DDRD, DDD0);
+    BIT_SET(PORTD, PD0);
     /* enable Transmit and Receive */
     UCSR0B = _BV(TXEN0) | _BV(RXEN0);
     /* Set USART Control and Status Register n C */
diff --git a/bacnet-stack/ports/bdk-atxx4-mstp/serial.c b/bacnet-stack/ports/bdk-atxx4-mstp/serial.c
index fa61cd55..281517e3 100644
--- a/bacnet-stack/ports/bdk-atxx4-mstp/serial.c
+++ b/bacnet-stack/ports/bdk-atxx4-mstp/serial.c
@@ -165,6 +165,9 @@ bool serial_baud_rate_set(
 static void serial_usart_init(
     void)
 {
+    /* enable the internal pullup on RXD1 */
+    BIT_CLEAR(DDRD, DDD2);
+    BIT_SET(PORTD, PD2);
     /* enable Transmit and Receive */
     UCSR1B = _BV(TXEN1) | _BV(RXEN1);
     /* Set USART Control and Status Register n C */