x10 Firecracker message format and a C++ class to use it

• Download source and demo project - 44.5 Kb
• Download latest from my website

Introduction

The Firecracker(C) is a product by X10 industries. It is a matchbox sized unit that plugs into a serial port and transmits commands wirelessly one-way to a receiving unit that's plugged into an AC outlet, which then sends the same signal through the home's AC wiring to all other x10 control modules. There are several types of modules, the two main ones being a light controller and an appliance controller. This system can accommodate 16 house codes, with 16 controllers in each house code, for a total of 256 devices. The individual controllers can be set to a particular house and unit code by rotary switches. There are several different commands that can be sent out; On, Off, Dim, Brighten, All lights on, All lights off and All controllers off. The On and Off signals are sent to a house and unit code. This causes that controller to go into a listen mode after which it will respond to dim and brighten commands which aren't provided with a target. Each dim or brighten command causes about a 5% difference. The All lights on and All lights off are targeted to only a house code which would cause all light controllers (up to the max of 16) with that house code to respond appropriately. Finally, the All units off will turn off all light and appliance modules on that house code.

Background

The Firecracker does not rely on normal serial communications, instead it is driven by the RTS & DTR signal lines which also provide its power. The command frame is pretty simple, consisting of only 5 bytes. Refer to table 4 for the message layout. A couple of tricky areas are initialization of the device and clocking out the signals to the RTS/DTR lines with appropriate delays between bits. Initializing the device is achieved by setting the RTS/DTR lines low for a short time and then bringing them both high with another delay before commencing a message. Sending out bits consists starting out with both lines high and setting DTR low for an on bit and setting RTS low for an off bit. In both cases, there must be a delay of at least 500 microseconds before setting that same line high again. This process continues until the entire 5 bytes are sent out. Table 1 lists the house codes. Notice that the house code is a bit different depending on which bank the target controller is on. Table 2 lists the byte to send to turn a controller either on or off. Note that controllers 9-16 have the same code as 1-8. Bit 2 of the house code is what ultimately decides which bank the command is for. Table 3 lists the all-on-off commands. These are sent in place of a code from table 2. Finally, table 4 shows the format of each message. The 2 header bytes and 1 footer byte are always the same.

Table 1 - House codes

Note that the house code has bit 2 turned on when addressing units 9 - 16.

Table 2 - Unit codes and function

Note that the code for units 8 - 16 are the same. As explained in table 1, the house code indicates if we're addressing units 9 - 16.

Table 3 - Commands

Note that commands have bit 7 turned on.

Table 4 - Message format

Using the code

Just a handful of functions are needed to surface the functionality of the FireCracker. Open the COM port by calling Open() with desired COM port as a string as in "COM1". Demo projects for both VC6 and VS.NET are included.

• void Open(LPCSTR pszComPort); // "COM1", "COM2", etc...
• void Close(); // Releases the COM port
• void TurnOn(char cHouseCode, int nUnitCode); // Turns on a unit. House code is the letter from A thru P
• void TurnOff(char cHouseCode, int nUnitCode); // Turns of a unit
• void Dim(); // Dims the last unit turned on by about 5%
• void Brighten(); // Same as Dim() but brightens instead
• void AllLightsOn(char cHouseCode); // Turns all lights on in a house code. Up to 16 controllers
• void AllLightsOff(char cHouseCode); // Turns all lights off
• void AllUnitsOff(char cHouseCode); // Turns all light and appliance controllers off

History

• 5/7/2004 - Initial release.