Datasheet

Jon Waddington
23
3.4.2 Serial Interface
The CM12 X10 controller has a PC interface which uses RS232 serial communications.
The controller generates X10 signals and transmits them over the power line, performing
the corresponding function on the modules, depending on the user’s input on the
computer. The Information System uses an object to imitate the computer’s serial
interface and communicate with the controller.
The required voltage levels of RS232 signals are bi-polar. The voltage level of +3V to
+12V indicates a logic ‘0’ and a voltage level of -3V to -12V indicates a logic ‘1’ [30].The
Propeller uses lower voltage logic levels where a logic ‘1’ is 3.3V and logic ‘0’ is 0V,
therefore, a voltage level converter IC is required to invert the logic level and also to
convert the voltage to RS232 standards. The MAX3232 IC was chosen for this purpose .
The circuit diagram is shown in figure 3.4.2. Pin 0 of the Propeller is connected to the
input ‘T1IN’ of the MAX3232. This is the data to be transmitted. Pin 1 of the Propeller is
connected to the ‘R1OUT’ output of the MAX3232. This is the output from the X10
controller.
An RS232 message begins with a start bit of a logic ‘0’ which syncs the clock of the two
devices. A byte is then transmitted, starting with the least significant bit. Each bit period
is the same as the start bit period [31].
A stop bit of logic ‘1’ is sent to indicate that the message has ended. This can act as a
delay before the next message is transmitted as the line idles at logic ‘1’ until it is pulled
down by the start bit.
Figure 3.4.2. Circuit Diagram showing the Serial Interface