diff --git a/src/machine/machine_rp2040_uart.go b/src/machine/machine_rp2040_uart.go
index 1d927df128..ea73175d4b 100644
--- a/src/machine/machine_rp2040_uart.go
+++ b/src/machine/machine_rp2040_uart.go
@@ -9,7 +9,9 @@ import (
 
 // UART on the RP2040.
 type UART struct {
-	Buffer    *RingBuffer
+	Buffer     *RingBuffer
+	rxCallback func(byte) bool
+
 	Bus       *rp.UART0_Type
 	Interrupt interrupt.Interrupt
 }
diff --git a/src/machine/uart.go b/src/machine/uart.go
index eeeb7d6a0b..c452926066 100644
--- a/src/machine/uart.go
+++ b/src/machine/uart.go
@@ -99,8 +99,17 @@ func (uart *UART) Buffered() int {
 	return int(uart.Buffer.Used())
 }
 
+func (uart *UART) SetRXCallback(cb func(byte) bool) {
+	uart.rxCallback = cb
+}
+
 // Receive handles adding data to the UART's data buffer.
 // Usually called by the IRQ handler for a machine.
 func (uart *UART) Receive(data byte) {
+	if uart.rxCallback != nil {
+		if uart.rxCallback(data) {
+			return
+		}
+	}
 	uart.Buffer.Put(data)
 }
diff --git a/src/machine/usb/cdc/usbcdc.go b/src/machine/usb/cdc/usbcdc.go
index 5b5ffbf7c4..8459f0ee66 100644
--- a/src/machine/usb/cdc/usbcdc.go
+++ b/src/machine/usb/cdc/usbcdc.go
@@ -59,17 +59,27 @@ func (usbcdc *USBCDC) Buffered() int {
 	return int(usbcdc.rxBuffer.Used())
 }
 
+func (usbcdc *USBCDC) SetRXCallback(cb func(byte) bool) {
+	usbcdc.rxCallback = cb
+}
+
 // Receive handles adding data to the UART's data buffer.
 // Usually called by the IRQ handler for a machine.
 func (usbcdc *USBCDC) Receive(data byte) {
+	if usbcdc.rxCallback != nil {
+		if usbcdc.rxCallback(data) {
+			return
+		}
+	}
 	usbcdc.rxBuffer.Put(data)
 }
 
 // USBCDC is the USB CDC aka serial over USB interface.
 type USBCDC struct {
-	rxBuffer *rxRingBuffer
-	txBuffer *txRingBuffer
-	waitTxc  bool
+	rxBuffer   *rxRingBuffer
+	txBuffer   *txRingBuffer
+	rxCallback func(byte) bool
+	waitTxc    bool
 }
 
 var (