diff --git a/src/com.cpp b/src/com.cpp
index d77c303..91f2982 100644
--- a/src/com.cpp
+++ b/src/com.cpp
@@ -21,7 +21,12 @@ void T_com::writeToSerial(const QByteArray &data, uint16_t sendLength)
     sendBuffer=data;
     sendLen=sendLength;
 
-    if (readCmds.size() == 0) { // no other read command active
+    // logic: exactly one command is sent to DC. no other command can be sent
+    // until the respond has been read from the serial line.
+
+    static int errorCnt = 0;
+
+    if (readCmds.size() == 0) { // no other read command active: ok
         if (CatSerial->isOpen())
         {
             if (CatSerial->error() != QSerialPort::NoError) {
@@ -53,14 +58,32 @@ void T_com::writeToSerial(const QByteArray &data, uint16_t sendLength)
                     return;
                 }
             }
-            readCmds.append(data.constData()[2]);
+
+            // save last command sent.
+            readCmds.append(data.constData()[2]); // 2: index of the last command
+
         } else {
             qCritical() << __func__ << ":" << __LINE__
                         << "ERROR sending" << data.toHex(':') << "port is not open";
         }
     } else {
-       qCritical() << __func__ << "" << __LINE__ << "ERROR detected active read cmd" << readCmds[0];
-       readCmds.clear();
+        qCritical() << __func__ << "" << __LINE__ << "ERROR detected active read cmd" << readCmds[0];
+        if (CatSerial->isOpen()) {
+            int availableBytes = CatSerial->bytesAvailable();
+            qCritical() << __func__ << "" << __LINE__ << "ERROR available bytes" << availableBytes;
+            if (availableBytes == 0) {
+                errorCnt += 1;
+                if (errorCnt > 100) {
+                    readCmds.clear();
+                    errorCnt = 0;
+                }
+            } else {
+                errorCnt = 0;
+            }
+        } else {
+            qCritical() << __func__ << ":" << __LINE__
+                        << "ERROR sending" << data.toHex(':') << "port is not open";
+        }
     }
 }