diff --git a/libraries/AP_RCProtocol/AP_RCProtocol_FPort.cpp b/libraries/AP_RCProtocol/AP_RCProtocol_FPort.cpp
index a19b51d8ec..8f15407435 100644
--- a/libraries/AP_RCProtocol/AP_RCProtocol_FPort.cpp
+++ b/libraries/AP_RCProtocol/AP_RCProtocol_FPort.cpp
@@ -42,6 +42,12 @@ extern const AP_HAL::HAL& hal;
 
 #define FPORT_TYPE_CONTROL 0
 #define FPORT_TYPE_DOWNLINK 1
+#define FPORT_PRIM_NULL 0x00
+#define FPORT_PRIM_DATA 0x10
+#define FPORT_PRIM_READ 0x30
+#define FPORT_PRIM_WRITE 0x31
+
+#define MAX_FPORT_CONSECUTIVE_FRAMES 2
 
 struct PACKED FPort_Frame {
     uint8_t header; // 0x7E
@@ -80,6 +86,18 @@ struct PACKED FPort_Frame {
     };
 };
 
+struct {
+    bool available = false;
+    uint32_t data;
+    uint16_t appid;
+    uint8_t frame;
+} telem_data;
+
+// receiver sends 0x10 when ready to receive telemetry frames (R-XSR)
+bool rx_driven_frame_rate = false;
+// if the receiver is not controlling frame rate apply a constraint on consecutive frames
+uint8_t consecutive_telemetry_frame_count;
+
 static_assert(sizeof(FPort_Frame) == FPORT_CONTROL_FRAME_SIZE, "FPort_Frame incorrect size");
 
 // constructor
@@ -130,10 +148,30 @@ void AP_RCProtocol_FPort::decode_control(const FPort_Frame &frame)
 void AP_RCProtocol_FPort::decode_downlink(const FPort_Frame &frame)
 {
 #if !APM_BUILD_TYPE(APM_BUILD_iofirmware)
-    if (frame.downlink.prim != 0x10 && frame.downlink.prim != 0x00) {
-        // only respond to frame types 0x00 or 0x10
-        return;
+    switch (frame.downlink.prim) {
+        case FPORT_PRIM_DATA:
+            // we've seen at least one 0x10 frame
+            rx_driven_frame_rate = true;
+            break;
+        case FPORT_PRIM_NULL:
+            if (rx_driven_frame_rate) {
+                return;
+            }
+            // with 0x00 and no rx control we have a constraint
+            // on max consecutive frames
+            if (consecutive_telemetry_frame_count >= MAX_FPORT_CONSECUTIVE_FRAMES) {
+                consecutive_telemetry_frame_count = 0;
+                return;
+            } else {
+                consecutive_telemetry_frame_count++;
+            }
+            break;
+        case FPORT_PRIM_READ:
+        case FPORT_PRIM_WRITE:
+            // do not respond to 0x30 and 0x31
+            return;
     }
+
     /*
       if we are getting FPORT over a UART then we can ask the FrSky
       telem library for some passthrough data to send back, enabling
@@ -146,6 +184,21 @@ void AP_RCProtocol_FPort::decode_downlink(const FPort_Frame &frame)
         return;
     }
 
+    /*
+      get SPort data from FRSky_Telem or send a null frame.
+      We save the data to a variable so in case we're late we'll
+      send it in the next call, this prevents corruption of
+      status text messages
+     */
+    if (!telem_data.available) {
+        if (!AP_Frsky_Telem::get_telem_data(telem_data.frame, telem_data.appid, telem_data.data)) {
+            // nothing to send, send a null frame
+            telem_data.frame = 0x00; 
+            telem_data.appid = 0x00; 
+            telem_data.data = 0x00;
+        }
+        telem_data.available = true;
+    }
     /*
       check that we haven't been too slow in responding to the new
       UART data. If we respond too late then we will corrupt the next
@@ -158,54 +211,45 @@ void AP_RCProtocol_FPort::decode_downlink(const FPort_Frame &frame)
         // we've been too slow in responding
         return;
     }
+    uint8_t buf[10];
 
-    /*
-      get the SPort data from FRSky_Telem and send it as an uplink
-      packet
-     */
-    uint8_t frametype;
-    uint16_t appid;
-    uint32_t data;
-    if (AP_Frsky_Telem::get_telem_data(frametype, appid, data)) {
-        uint8_t buf[10];
-
-        buf[0] = 0x08;
-        buf[1] = 0x81;
-        buf[2] = frametype;
-        buf[3] = appid & 0xFF;
-        buf[4] = appid >> 8;
-        memcpy(&buf[5], &data, 4);
-
-        uint16_t sum = 0;
-        for (uint8_t i=0; i<sizeof(buf)-1; i++) {
-            sum += buf[i];
-            sum += sum >> 8;
-            sum &= 0xFF;              
-        }
-        sum = 0xff - ((sum & 0xff) + (sum >> 8));
-        buf[9] = sum;
+    buf[0] = 0x08;
+    buf[1] = 0x81;
+    buf[2] = telem_data.frame;
+    buf[3] = telem_data.appid & 0xFF;
+    buf[4] = telem_data.appid >> 8;
+    memcpy(&buf[5], &telem_data.data, 4);
 
-        // perform byte stuffing per FPort spec
-        uint8_t len = 0;
-        uint8_t buf2[sizeof(buf)*2+1];
+    uint16_t sum = 0;
+    for (uint8_t i=0; i<sizeof(buf)-1; i++) {
+        sum += buf[i];
+        sum += sum >> 8;
+        sum &= 0xFF;              
+    }
+    sum = 0xff - ((sum & 0xff) + (sum >> 8));
+    buf[9] = (uint8_t)sum;
 
-        if (rc().fport_pad()) {
-            // this padding helps on some uarts that have hw pullups
-            buf2[len++] = 0xff;
-        }
+    // perform byte stuffing per FPort spec
+    uint8_t len = 0;
+    uint8_t buf2[sizeof(buf)*2+1];
 
-        for (uint8_t i=0; i<sizeof(buf); i++) {
-            uint8_t c = buf[i];
-            if (c == FRAME_DLE || buf[i] == FRAME_HEAD) {
-                buf2[len++] = FRAME_DLE;
-                buf2[len++] = c ^ FRAME_XOR;
-            } else {
-                buf2[len++] = c;
-            }
-        }
+    if (rc().fport_pad()) {
+        // this padding helps on some uarts that have hw pullups
+        buf2[len++] = 0xff;
+    }
 
-        uart->write(buf2, len);
+    for (uint8_t i=0; i<sizeof(buf); i++) {
+        uint8_t c = buf[i];
+        if (c == FRAME_DLE || buf[i] == FRAME_HEAD) {
+            buf2[len++] = FRAME_DLE;
+            buf2[len++] = c ^ FRAME_XOR;
+        } else {
+            buf2[len++] = c;
+        }
     }
+    uart->write(buf2, len);
+    // get fresh telem_data in the next call
+    telem_data.available = false;
 #endif
 }
 
@@ -306,6 +350,8 @@ bool AP_RCProtocol_FPort::check_checksum(void)
     uint16_t sum = 0;
     for (uint8_t i=0; i<len; i++) {
         sum += b[i];
+        sum += sum >> 8;
+        sum &= 0xFF;            
     }
     sum = (sum & 0xff) + (sum >> 8);
     return sum == 0xff;