Okay, so it seems that sync also works the other way around.

Machines/Commodore/1540/Commodore1540.cpp

@@ -30,7 +30,7 @@ void Machine::set_serial_bus(std::shared_ptr<::Commodore::Serial::Bus> serial_bu
 
 unsigned int Machine::perform_bus_operation(CPU6502::BusOperation operation, uint16_t address, uint8_t *value)
 {
-//	if(operation == CPU6502::BusOperation::ReadOpcode && (address >= 0xE9C9 && address <= 0xEA2D)) printf("%04x\n", address);
+	if(operation == CPU6502::BusOperation::ReadOpcode && (address >= 0xF556 && address <= 0xF56D)) printf("%04x\n", address);
 //	if(operation == CPU6502::BusOperation::ReadOpcode && (address == 0xE887)) printf("A: %02x\n", get_value_of_register(CPU6502::Register::A));
 
 /*	static bool log = false;
@@ -56,10 +56,6 @@ unsigned int Machine::perform_bus_operation(CPU6502::BusOperation operation, uin
 	}
 	else if(address >= 0x1800 && address <= 0x180f)
 	{
-		if(address == 0x1805)
-		{
-			printf("Timer\n");
-		}
 		if(isReadOperation(operation))
 			*value = _serialPortVIA->get_register(address);
 		else

Machines/Commodore/1540/Commodore1540.hpp

@@ -91,10 +91,23 @@ class DriveVIA: public MOS::MOS6522<DriveVIA>, public MOS::MOS6522IRQDelegate {
 		uint8_t get_port_input(Port port) {
 			if(port)
 			{
-				return 0x7f;	// imply not sync, write protect tab uncovered
+				return 0xff;	// imply not sync, write protect tab uncovered
 			}
 			return 0xff;
 		}
+
+		void set_port_output(Port port, uint8_t value, uint8_t direction_mask) {
+			if(port)
+			{
+				if(value&4)
+				{
+					printf("Head step: %d\n", value&3);
+					printf("Motor: %s\n", value&4 ? "On" : "Off");
+					printf("LED: %s\n", value&8 ? "On" : "Off");
+					printf("Density: %d\n", (value >> 5)&3);
+				}
+			}
+		}
 };
 
 class SerialPort : public ::Commodore::Serial::Port {

Machines/Commodore/SerialBus.cpp

@@ -45,14 +45,12 @@ void Bus::set_line_output_did_change(Line line)
 		if(locked_port)
 		{
 			new_line_level = (LineLevel)((bool)new_line_level & (bool)locked_port->get_output(line));
-//			printf("[%s] %s is now %s\n", typeid(locked_port).name(), (bool)locked_port->get_output(line) ? "high" : "low");
 		}
 	}
 
 	// post an update only if one occurred
 	if(new_line_level != _line_levels[line])
 	{
-		printf("[Bus] %s is %s\n", StringForLine(line), new_line_level ? "high" : "low");
 		_line_levels[line] = new_line_level;
 
 		for(std::weak_ptr<Port> port : _ports)
@@ -65,3 +63,25 @@ void Bus::set_line_output_did_change(Line line)
 		}
 	}
 }
+
+#pragma mark - The debug port
+
+void DebugPort::set_input(Line line, LineLevel value)
+{
+	_input_levels[line] = value;
+
+	printf("[Bus] %s is %s\n", StringForLine(line), value ? "high" : "low");
+	if(!_incoming_count)
+	{
+		_incoming_count = (!_input_levels[Line::Clock] && !_input_levels[Line::Data]) ? 8 : 0;
+	}
+	else
+	{
+		if(line == Line::Clock && value)
+		{
+			_incoming_byte = (_incoming_byte >> 1) | (_input_levels[Line::Data] ? 0x80 : 0x00);
+		}
+		_incoming_count--;
+		if(_incoming_count == 0) printf("[Bus] Observed %02x\n", _incoming_byte);
+	}
+}

Machines/Commodore/SerialBus.hpp

@@ -71,6 +71,18 @@ namespace Serial {
 			LineLevel _line_levels[5];
 	};
 
+	class DebugPort: public Port {
+		public:
+			void set_input(Line line, LineLevel value);
+
+			DebugPort() : _incoming_count(0) {}
+
+		private:
+			uint8_t _incoming_byte;
+			int _incoming_count;
+			LineLevel _input_levels[5];
+	};
+
 }
 }
 

Machines/Commodore/Vic-20/Vic20.cpp

@@ -57,6 +57,10 @@ Machine::Machine() :
 
 	// TEMPORARY: attach a [diskless] 1540
 	set_disc();
+
+	_debugPort.reset(new ::Commodore::Serial::DebugPort);
+	_debugPort->set_serial_bus(_serialBus);
+	_serialBus->add_port(_debugPort);
 }
 
 void Machine::write_to_map(uint8_t **map, uint8_t *area, uint16_t address, uint16_t length)

Machines/Commodore/Vic-20/Vic20.hpp

@@ -295,6 +295,7 @@ class Machine:
 		std::shared_ptr<KeyboardVIA> _keyboardVIA;
 		std::shared_ptr<SerialPort> _serialPort;
 		std::shared_ptr<::Commodore::Serial::Bus> _serialBus;
+		std::shared_ptr<::Commodore::Serial::DebugPort> _debugPort;
 
 		// Tape
 		Tape _tape;