diff --git a/arduino/arduino_terminal/arduino_terminal.ino b/arduino/arduino_terminal/arduino_terminal.ino
index b093462..97b4ab5 100644
--- a/arduino/arduino_terminal/arduino_terminal.ino
+++ b/arduino/arduino_terminal/arduino_terminal.ino
@@ -5,6 +5,12 @@
  * The Python Terminal Monitor or the Arduino IDE serial monitor is used to send 
  * and receive commands to/from the Z80.
  * 
+ * Seen from the PC, the terminal receives two bytes: a command byte and a value byte.
+ * Commands:
+ *  0x00 WRITE:     The next byte is sent as-is to Pat80. If the terminal interface buffer
+ *                  is not empty, the new byte will replace the older one.
+ *  0x01 BUFFER:    The terminal interface returns the number of bytes waiting to be sent.
+ * 
  * Seen from the Pat80, the terminal interface has two registers:
  * DATA Register at addr 0x00 (\RS) contains the last received byte from the pc
  * DATA_AVAILABLE Register at addr 0x01 (RS) contains the number of bytes in the buffer,
@@ -17,6 +23,9 @@
 // RS 12   // Input, low = DATA register, high = DATA_AVAILABLE register
 // DATA BUS (Input/Output, active high): 3, 4, 5, 6, 7, 8, 9, 10;
 
+const byte COMMAND_WRITE = 0x00;
+const byte COMMAND_BUFFER = 0x01;
+
 byte incomingBuffer = 0;  // Incoming from computer, to the Pat80
 byte outgoingBuffer = 0;  // Outgoing to computer, from the Pat80
 byte availableBytes = 0;  // Available bytes in the incoming buffer (for the DATA_AVAILABLE register)
@@ -33,9 +42,17 @@ void setup() {
 
 void loop() {
   if (Serial.available() > 0) {
-    incomingBuffer = Serial.read();
-    availableBytes = 1;   // TODO: Implement a 256 byte buffer and store the avail bytes number in this var
+    switch (Serial.read()) {
+      case COMMAND_WRITE:
+        incomingBuffer = Serial.read();
+        availableBytes = 1;   // TODO: Implement a 256 byte buffer and store the avail bytes number in this var
+        break;
+      case COMMAND_BUFFER:
+        Serial.write(availableBytes);
+        break;
+    }    
   }
+  
   if (outgoingBuffer != 0) {
     if ((outgoingBuffer >= 8 && outgoingBuffer <= 13) || (outgoingBuffer >= 32 && outgoingBuffer <= 127)) {
       // Printable character