diff --git a/src/shearwater_common.c b/src/shearwater_common.c
index b809a3c..eabed11 100644
--- a/src/shearwater_common.c
+++ b/src/shearwater_common.c
@@ -35,7 +35,7 @@
 #define ESC_END   0xDC
 #define ESC_ESC   0xDD
 
-#define EXITCODE(n) ((n) < 0 ? (n) : 0)
+#define EXITCODE(n) ((n) < 0 ? DC_STATUS_IO : DC_STATUS_TIMEOUT)
 
 dc_status_t
 shearwater_common_open (shearwater_common_device_t *device, dc_context_t *context, const char *name)
@@ -141,7 +141,7 @@ shearwater_common_decompress_xor (unsigned char *data, unsigned int size)
 }
 
 
-static int
+static dc_status_t
 shearwater_common_slip_write (shearwater_common_device_t *device, const unsigned char data[], unsigned int size)
 {
 	int n = 0;
@@ -206,12 +206,12 @@ shearwater_common_slip_write (shearwater_common_device_t *device, const unsigned
 		return EXITCODE(n);
 	}
 
-	return size;
+	return DC_STATUS_SUCCESS;
 }
 
 
-static int
-shearwater_common_slip_read (shearwater_common_device_t *device, unsigned char data[], unsigned int size)
+static dc_status_t
+shearwater_common_slip_read (shearwater_common_device_t *device, unsigned char data[], unsigned int size, unsigned int *actual)
 {
 	unsigned int received = 0;
 
@@ -237,7 +237,7 @@ shearwater_common_slip_read (shearwater_common_device_t *device, unsigned char d
 			// packets generated by the duplicate END characters which
 			// are sent to try to detect line noise.
 			if (received)
-				return received;
+				goto done;
 			else
 				break;
 		case ESC:
@@ -267,16 +267,25 @@ shearwater_common_slip_read (shearwater_common_device_t *device, unsigned char d
 		}
 	}
 
-	return received;
+done:
+
+	if (received > size)
+		return DC_STATUS_PROTOCOL;
+
+	if (actual)
+		*actual = received;
+
+	return DC_STATUS_SUCCESS;
 }
 
 
 dc_status_t
 shearwater_common_transfer (shearwater_common_device_t *device, const unsigned char input[], unsigned int isize, unsigned char output[], unsigned int osize, unsigned int *actual)
 {
+	dc_status_t status = DC_STATUS_SUCCESS;
 	dc_device_t *abstract = (dc_device_t *) device;
 	unsigned char packet[SZ_PACKET + 4];
-	int n = 0;
+	unsigned int n = 0;
 
 	if (isize > SZ_PACKET || osize > SZ_PACKET)
 		return DC_STATUS_INVALIDARGS;
@@ -292,13 +301,10 @@ shearwater_common_transfer (shearwater_common_device_t *device, const unsigned c
 	memcpy (packet + 4, input, isize);
 
 	// Send the request packet.
-	n = shearwater_common_slip_write (device, packet, isize + 4);
-	if (n != isize + 4) {
+	status = shearwater_common_slip_write (device, packet, isize + 4);
+	if (status != DC_STATUS_SUCCESS) {
 		ERROR (abstract->context, "Failed to send the request packet.");
-		if (n < 0)
-			return DC_STATUS_IO;
-		else
-			return DC_STATUS_TIMEOUT;
+		return status;
 	}
 
 	// Return early if no response packet is requested.
@@ -309,15 +315,10 @@ shearwater_common_transfer (shearwater_common_device_t *device, const unsigned c
 	}
 
 	// Receive the response packet.
-	n = shearwater_common_slip_read (device, packet, sizeof (packet));
-	if (n <= 0 || n > sizeof (packet)) {
+	status = shearwater_common_slip_read (device, packet, sizeof (packet), &n);
+	if (status != DC_STATUS_SUCCESS) {
 		ERROR (abstract->context, "Failed to receive the response packet.");
-		if (n < 0)
-			return DC_STATUS_IO;
-		else if (n > sizeof (packet))
-			return DC_STATUS_PROTOCOL;
-		else
-			return DC_STATUS_TIMEOUT;
+		return status;
 	}
 
 	// Validate the packet header.