RFCS_0013 - Add sensor_index field to mavlink_scaled_imu_t typedef

text/0013-template_scaled_imu_msgs.md

@@ -0,0 +1,80 @@
+  * Start date: 2019-02-25
+  * Contributors: Mark Sauder <mcsauder@gmail.com>, ...
+  * Related issues: 
+
+# Summary
+
+This RFC proposes adding a struct member to identify the scaled imu instance, `uint8_t sensor_index` (or some similarly named variable).  This RFC would allow the three `mavlink_scaled_imu`, `mavlink_scaled_imu2`, and `mavlink_scaled_imu3` message definitions to be consolidated into a single definition, potentially freeing messages 116 and 129 for other uses.
+
+This RFC would impact the MAVLink protocol in two ways:
+  1) Impacting the `mavlink_scaled_imu` message defintion directly, and
+  2) Allowing `MAVLINK_MSG_ID_SCALED_IMU2 116` and `MAVLINK_MSG_ID_SCALED_IMU3 129` to be deprecated/re-purposed.
+
+# Motivation
+
+The primary motication of this RFC is to reduce code duplication in mavlink protocol and projects utilizing the protocol.
+
+By adding a sensor index/instance value to the message, the scaled_imu message will also more closely resemble the current usage of `mavlink_msg_actuator_control_target.h` and `mavlink_msg_servo_output_raw` messages.
+
+# Detailed Design
+
+The proposed implementation would add a struct member variable **`uint8_t sensor_index`** to the `mavlink_scaled_imu_t` type definition in `mavlink_msg_scaled_imu.h`.
+
+LOC added/impacted would look something like these:
+
+```c++ hl_lines=3
+MAVPACKED(
+typedef struct __mavlink_scaled_imu_t {
+...
+ uint8_t sensor_index; /*< IMU instance*/
+}) mavlink_scaled_imu_t;
+
+#define MAVLINK_MSG_ID_SCALED_IMU_LEN 23
+#define MAVLINK_MSG_ID_SCALED_IMU_MIN_LEN 23
+#define MAVLINK_MSG_ID_26_LEN 23
+#define MAVLINK_MSG_ID_26_MIN_LEN 23
+
+#define MAVLINK_MSG_ID_SCALED_IMU_CRC XXX
+#define MAVLINK_MSG_ID_26_CRC XXX
+
+
+#if MAVLINK_COMMAND_24BIT
+#define MAVLINK_MESSAGE_INFO_SCALED_IMU { \
+    26, \
+    "SCALED_IMU", \
+...
+         { "sensor_index", NULL, MAVLINK_TYPE_UINT8_T, 0, 21, offsetof(mavlink_scaled_imu_t, sensor_index) }, \
+         } \
+}
+#else
+#define MAVLINK_MESSAGE_INFO_SCALED_IMU { \
+    "SCALED_IMU", \
+...
+         { "sensor_index", NULL, MAVLINK_TYPE_UINT8_T, 0, 21, offsetof(mavlink_scaled_imu_t, sensor_index) }, \
+         } \
+}
+#endif
+```
+
+`_mav_put_uint8_t(buf, 21, sensor_index);`
+
+...
+
+`packet.sensor_index = sensor_index;`
+
+etc.
+
+
+
+# Alternatives
+The proposed change places the sensor_index field such that it does not impact offsets for all other values, however, a personal preference would have been to place the index at the beginning of the typedef prior to sensor values.
+
+There are likely alternatives to this RFC, please suggest additional potential solutions.
+
+# Unresolved Questions
+
+# References
+
+- Mavlink github issue [949](https://github.com/mavlink/mavlink/issues/949)
+- PX4/Firmware PR [#9556](https://github.com/PX4/Firmware/pull/9556)
+