diff --git a/src/platforms/common/aux_serial.c b/src/platforms/common/aux_serial.c
index be80055..3d9708f 100644
--- a/src/platforms/common/aux_serial.c
+++ b/src/platforms/common/aux_serial.c
@@ -40,6 +40,7 @@
 static char aux_serial_transmit_buffer[2U][TX_BUF_SIZE];
 static uint8_t aux_serial_transmit_buffer_index;
 static uint8_t aux_serial_transmit_buffer_consumed;
+static bool aux_serial_transmit_complete = true;
 
 #ifdef DMA_STREAM0
 #define dma_channel_reset(dma, channel) dma_stream_reset(dma, channel)
@@ -285,6 +286,29 @@ static void aux_serial_receive_isr(const uint32_t usart, const uint8_t dma_irq)
 	nvic_enable_irq(dma_irq);
 }
 
+static void aux_serial_dma_transmit_isr(const uint8_t dma_tx_channel)
+{
+	nvic_disable_irq(USB_IRQ);
+
+	/* Stop DMA */
+	dma_disable_channel(USBUSART_DMA_BUS, dma_tx_channel);
+	dma_clear_interrupt_flags(USBUSART_DMA_BUS, dma_tx_channel, DMA_CGIF);
+
+	/*
+	 * If a new buffer is ready, continue transmission.
+	 * Otherwise we report the transfer has completed.
+	 */
+	if (aux_serial_transmit_buffer_fullness()) {
+		aux_serial_switch_transmit_buffers();
+		usbd_ep_nak_set(usbdev, CDCACM_UART_ENDPOINT, 0);
+	} else {
+		usbuart_set_led_state(TX_LED_ACT, false);
+		aux_serial_transmit_complete = true;
+	}
+
+	nvic_enable_irq(USB_IRQ);
+}
+
 #if defined(USBUSART_ISR)
 void USBUSART_ISR(void)
 {
@@ -318,6 +342,27 @@ void USBUSART2_ISR(void)
 }
 #endif
 
+#if defined(USBUSART_DMA_TX_ISR)
+void USBUSART_DMA_TX_ISR(void)
+{
+	aux_serial_dma_transmit_isr(USBUSART_DMA_TX_CHAN);
+}
+#endif
+
+#if defined(USBUSART1_DMA_TX_ISR)
+void USBUSART1_DMA_TX_ISR(void)
+{
+	aux_serial_dma_transmit_isr(USBUSART1_DMA_TX_CHAN);
+}
+#endif
+
+#if defined(USBUSART2_DMA_TX_ISR)
+void USBUSART2_DMA_TX_ISR(void)
+{
+	aux_serial_dma_transmit_isr(USBUSART2_DMA_TX_CHAN);
+}
+#endif
+
 #elif defined(LM4F)
 char *aux_serial_current_transmit_buffer(void)
 {
diff --git a/src/platforms/common/usbuart.h b/src/platforms/common/usbuart.h
index d465b39..d7f32a4 100644
--- a/src/platforms/common/usbuart.h
+++ b/src/platforms/common/usbuart.h
@@ -41,7 +41,6 @@ void debug_uart_run(void);
 #define RX_FIFO_SIZE (USART_DMA_BUF_SIZE)
 #define TX_BUF_SIZE (USART_DMA_BUF_SIZE)
 
-extern bool aux_serial_transmit_complete;
 /* RX Fifo buffer with space for copy fn overrun */
 extern char buf_rx[RX_FIFO_SIZE + sizeof(uint64_t)];
 /* RX Fifo out pointer, writes assumed to be atomic */
diff --git a/src/platforms/stm32/usbuart.c b/src/platforms/stm32/usbuart.c
index f953ddb..a971f6c 100644
--- a/src/platforms/stm32/usbuart.c
+++ b/src/platforms/stm32/usbuart.c
@@ -38,8 +38,6 @@
 #define DMA_CGIF DMA_IFCR_CGIF_BIT
 #endif
 
-/* TX transfer complete */
-bool aux_serial_transmit_complete = true;
 /* RX Fifo buffer with space for copy fn overrun */
 char buf_rx[RX_FIFO_SIZE + sizeof(uint64_t)];
 /* RX Fifo out pointer, writes assumed to be atomic */
@@ -78,51 +76,6 @@ void usbuart_set_led_state(uint8_t ledn, bool state)
 	}
 }
 
-#define USBUSART_DMA_TX_ISR_TEMPLATE(DMA_TX_CHAN) do {				\
-	nvic_disable_irq(USB_IRQ);						\
-										\
-	/* Stop DMA */								\
-	dma_disable_channel(USBUSART_DMA_BUS, DMA_TX_CHAN);			\
-	dma_clear_interrupt_flags(USBUSART_DMA_BUS, DMA_TX_CHAN, DMA_CGIF);	\
-										\
-	/* If new buffer is ready, continue transmission.			\
-	 * Otherwise report transfer completion.				\
-	 */									\
-	if (aux_serial_transmit_buffer_fullness())							\
-	{									\
-		aux_serial_switch_transmit_buffers();					\
-		usbd_ep_nak_set(usbdev, CDCACM_UART_ENDPOINT, 0);		\
-	}									\
-	else									\
-	{									\
-		usbuart_set_led_state(TX_LED_ACT, false);			\
-		aux_serial_transmit_complete = true;						\
-	}									\
-										\
-	nvic_enable_irq(USB_IRQ);						\
-} while(0)
-
-#if defined(USBUSART_DMA_TX_ISR)
-void USBUSART_DMA_TX_ISR(void)
-{
-	USBUSART_DMA_TX_ISR_TEMPLATE(USBUSART_DMA_TX_CHAN);
-}
-#endif
-
-#if defined(USBUSART1_DMA_TX_ISR)
-void USBUSART1_DMA_TX_ISR(void)
-{
-	USBUSART_DMA_TX_ISR_TEMPLATE(USBUSART1_DMA_TX_CHAN);
-}
-#endif
-
-#if defined(USBUSART2_DMA_TX_ISR)
-void USBUSART2_DMA_TX_ISR(void)
-{
-	USBUSART_DMA_TX_ISR_TEMPLATE(USBUSART2_DMA_TX_CHAN);
-}
-#endif
-
 #define USBUSART_DMA_RX_ISR_TEMPLATE(USART_IRQ, DMA_RX_CHAN) do {		\
 	nvic_disable_irq(USART_IRQ);						\
 										\