diff --git a/Machines/Commodore/1540/Commodore1540.hpp b/Machines/Commodore/1540/Commodore1540.hpp
index 0d1e7d52a..378ecb35c 100644
--- a/Machines/Commodore/1540/Commodore1540.hpp
+++ b/Machines/Commodore/1540/Commodore1540.hpp
@@ -52,7 +52,7 @@ class SerialPortVIA: public MOS::MOS6522<SerialPortVIA>, public MOS::MOS6522IRQD
 				case ::Commodore::Serial::Line::Clock:		_portB = (_portB & ~0x04) | (value ? 0 : 0x04);		break;
 				case ::Commodore::Serial::Line::Attention:
 					_portB = (_portB & ~0x80) | (value ? 0 : 0x80);
-					set_control_line_input(Port::B, Line::Two, !!(value&0x80));
+					set_control_line_input(Port::B, Line::Two, value);
 				break;
 			}
 		}
diff --git a/Machines/Commodore/SerialBus.cpp b/Machines/Commodore/SerialBus.cpp
index 7d7ca7a09..c4e3a3f2c 100644
--- a/Machines/Commodore/SerialBus.cpp
+++ b/Machines/Commodore/SerialBus.cpp
@@ -13,10 +13,13 @@ using namespace Commodore::Serial;
 void Bus::add_port(std::shared_ptr<Port> port)
 {
 	_ports.push_back(port);
+	for(int line = (int)ServiceRequest; line <= (int)Reset; line++)
+		set_line_output_did_change((Line)line);
 }
 
 void Bus::set_line_output_did_change(Line line)
 {
+	// i.e. I believe these lines to be open collector, active low
 	bool new_line_value = false;
 	for(std::weak_ptr<Port> port : _ports)
 	{
@@ -27,6 +30,7 @@ void Bus::set_line_output_did_change(Line line)
 		}
 	}
 
+	// post an update only if one occurred
 	if(new_line_value != _line_values[line])
 	{
 		_line_values[line] = new_line_value;