surprising news to me the V.24 modem FW does not actually set byte 5 of the GET_STATUS opcode, so will never properly flag transmit status, as such I have implemented some faux handling in the host itself to emulate the behavior, this is done by flagging Tx once we start successfully writing data over V.24 (this can be misleading if the data is sent to our V.24 modem but the V.24 modem never sends it to a Quantar);

src/host/modem/ModemV24.cpp

@@ -413,6 +413,11 @@ void ModemV24::clock(uint32_t ms)
         LogError(LOG_MODEM, "Failed to write to serial port!");
     }
 
+    // if both queues are empty -- unset the Tx flag since the modem won't be transmitting anything
+    if (m_txImmP25Queue.isEmpty() && m_txP25Queue.isEmpty()) {
+        m_tx = false;
+    }
+
     // clear an RX call in progress flag if we're longer than our timeout value
     now = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now().time_since_epoch()).count();
     if (m_rxCallInProgress && (now - m_rxLastFrameTime > m_callTimeout)) {
@@ -489,8 +494,10 @@ int ModemV24::writeSerial(RingBuffer<uint8_t>* queue)
     std::lock_guard<std::mutex> lock(m_txP25QueueLock);
 
     // check empty
-    if (queue->isEmpty())
+    if (queue->isEmpty()) {
+        m_tx = false;
         return 0U;
+    }
 
     // get length
     uint8_t length[2U];
@@ -504,6 +511,7 @@ int ModemV24::writeSerial(RingBuffer<uint8_t>* queue)
     // this ensures we never get in a situation where we have length bytes stuck in the queue by themselves
     if (queue->dataSize() == 2U && len > queue->dataSize()) {
         queue->get(length, 2U); // ensure we pop bytes off
+        m_tx = false;
         return 0U;
     }
 
@@ -554,6 +562,10 @@ int ModemV24::writeSerial(RingBuffer<uint8_t>* queue)
             // only remove an entry once it was written successfully
             DECLARE_UINT8_ARRAY(discard, len + 11U);
             queue->get(discard, len + 11U);
+
+            // bryanb: because m_tx may not be set by the V.24 modem -- we will indicate "Tx" state here based on 
+            // whether we're actually writing data to the modem or not
+            m_tx = true;
         }
 
         return ret;