diff --git a/lib/usb_cdcacm.c b/lib/usb_cdcacm.c
index 9f3d01e..3afae5c 100644
--- a/lib/usb_cdcacm.c
+++ b/lib/usb_cdcacm.c
@@ -206,10 +206,11 @@ static int cdcacm_control_request(usbd_device *usbd_dev, struct usb_setup_data *
 
 	switch (req->bRequest) {
 		case USB_CDC_REQ_SET_CONTROL_LINE_STATE: {
-			/*
 			bool dtr = (req->wValue & (1 << 0)) ? true : false;
 			bool rts = (req->wValue & (1 << 1)) ? true : false;
-			*/
+			if (dtr || rts) { // host opened serial port
+				usbd_ep_write_packet(usb_device, 0x82, NULL, 0); // start transmitting
+			}
 			/* this Linux cdc_acm driver requires this to be implemented
 			 * even though it's optional in the CDC spec, and we don't
 			 * advertise it in the ACM functional descriptor.