XTB-II X10 Transmit Booster
by JV Digital Engineering

Updated 05/14/08 - Comments and questions are welcome

The original X10 Transmit Booster (XTB) was developed to improve the reliability of X10 automation systems.  That unit simply plugs between any X10 control module and the wall socket, and will amplify the 120KHz X10 signals generated by that module.  With its 6-watt transformer power supply, the XTB generates a strong signal on its own phase, but a good passive coupler is still required at the electrical distribution panel to propagate its output to the other phase.  The XTB-II combines the high-power XTB transmitter with two coupling networks to drive both phases directly, eliminating the need for a separate passive coupler.

The XTB-II is designed for a split-phase 240V system, and its PIC microcontroller normally blanks the superfluous X10 bursts used in 3-phase systems.  That allows the XTB-II to concentrate all its energy into the zero-crossing X10 burst, and it can generate a slightly stronger signal than the XTB.  Depending on line characteristics, the XTB-II can output over 20Vpp at 120KHz onto the AC line.

The XTB-II has 2-prong X10 input receptacle that functions similar to the one on the XTB.  A control module such as the CM15A can be plugged directly into that receptacle, and the XTB-II will drive its boosted output onto both phases.  A 3-prong grounded receptacle may be available on special request for installations that still use the CM11A.  To prevent excessive loading of the return signal, it is recommended that no more than three X10 transmitters be plugged into that receptacle.  The larger size of the XTB-II allowed room for larger inductors, and the X10 receptacle on the XTB-II can deliver up to 50 watts.  An internal fuse will blow if that rating is exceeded by a significant amount.

The XTB-II includes built-in TW523 emulation.  A digital I/O line can be run from an automation controller directly to the XTB-II.  The opto-isolated digital interface on the XTB-II is functionally identical to that of the TW523.  The XTB-II TW523 emulation differs from the actual TW523 in several regards.  The XTB-II error checks all incoming data, and does not produce any output when a collision is detected.  The XTB-II does not need a gap to separate X10 messages, and it will recognize each pair of bright/dim commands.  When enabled, the XTB-II also includes the ability to accept extended messages.

Like the XTB, the XTB-II includes an amplifier to boost low-level X10 return signals.  The XTB-II adds a bandpass filter in that path to attenuate to out-of-band noise, but line transients can still make it through the filter, and are amplified.  Devices that sensitive to noise, such as the PowerLinc 1132, may not work well with XTB-II.

The XTB-II now includes programmable mode options to further tailor its operation for best performance in each installation.  This includes a variable decode to try to recover X10 signals that are not much higher than the background noise level.  It also includes a basic repeater mode to boost line signals received from remote transmitters.  However, the XTB-IIR enhanced repeater version is a better choice for installations where the repeater capability is a primary factor.



The XTB-II does not simply plug into a standard receptacle like the XTB.  It should be installed adjacent to the electrical distribution panel where it can drive both phases directly.  The XTB-II has two internal terminal strips for whatever line connections are appropriate for a particular installation.  The XTB-II is now supplied with a 2-prong polarized X10 Input receptacle on the cover to simplify connections to the distribution panel.  It should be connected to neutral and both legs of a split-phase 120/240VAC distribution panel through a receptacle fed by a double-pole 15A or 20A circuit breaker.  A solid connection to neutral is essential to prevent damage to the XTB-II.

If the XTB-II contains a grounded receptacle on the cover, the ground must also be connected.  While a standard 240V 3-prong receptacle is adequate for the standard unit, I recommend 4-conductor power cord wired through a 14-20P/14-20R plug/receptacle for the grounded unit.  The locking strain relief on the XTB-II will accept up to .4-inch diameter wire.  It may be easier to wire the terminal strip with the board removed from the case.  #18 gauge wire is sufficient because the XTB-II is internally fused at 2 amps maximum.  Check the electrical connections carefully before applying power.  It is recommended that the terminal strip screws be re-torqued (with power switched off) to insure solid connections.


A single-output 240V 50Hz version of the XTB-II is available for international installations.  That unit does not include the 120V X10 Input receptacle on the cover.  Connections are through the two terminal strips inside.


The XTB-II can either function in the default X10 boost mode or the TW523 emulation mode.  It cannot do both at the same time.  The XTB-II powers up in the default X10 boost mode, and it cannot monitor powerline signals while in that mode.  While in the default X10 boost mode, any message received from the automation controller will cause the XTB-II to switch to TW523 emulation.  In that mode X10 inputs received through the X10 input receptacle will not be transmitted to the powerline unless the XTB-II repeater is enabled.  However, they will be decoded, and valid X10 messages will be sent to the automation controller.  Interrupting power for several seconds will return the XTB-II to the default X10 boost mode unless Mode 15 is ON.  Refer to the XTB-II Mode Options document for more information.

In the TW523 emulation mode, the XTB-II checks all incoming messages, and it only accepts those without apparent collisions.  Message length is determined by the next gap or start pattern, and no gap is needed to separate messages.  This allows the XTB-II to recognize each pair of bright/dim commands.  While all outgoing extended messages are transmitted, the XTB-II can be set to also decode incoming extended messages.  This mode has not been tested other than with a logic analyzer because my automation controller does not recognize extended messages.  Due to their extra length, extended messages are not repeated to reduce the possibility of collisions.  However, two-way extended communication with the automation controller is fully supported.

The X10 transmitter in the XTB-II auto tunes itself to 120KHz using the powerline as a reference.  By default, it only transmits the X10 burst following a zero crossing.  Within that window, the duration of the burst is controlled by the digital input.  If three-phase transmission is enabled, the transmission window is open throughout each powerline cycle.  European 50Hz versions have 3-phase transmission enabled by default.

Please refer to the Mode Options Document for the additional features available in the latest firmware.



XTB-II with optional grounded receptacle 16-4 Power Cable Installed



The XTB-II is available in both kit and custom assembled versions.  A single-output 240V 50Hz version for Europe is available in kit form.  An assembled 240V 50Hz version is also available on request.

Please contact me if you have any questions at: jeff@jvde.us
If you don't receive a response within 24 hours, try: xtbjeff@gmail.com