CM19a is an RF-Only Device - It Does Not "Talk" on the Powerline: That means that without some sort of RF to Powerline transceiver it can not control any lamp or appliance modules. The solution is to get a TM751 (or an RR501) Transceiver Module from X10 and plug it into an outlet within radio range of the CM19a. The combination of CM19a + TM751 + This Plugin has been tested extensively and works fine. Likewise for the RR501 Transceiver.
CM19a Also Does Not "Listen" on the Powerline: Even with a Transceiver, the CM19a is totally unaware of anything happening on the powerline that originates from other controllers (such as plug-on keypads or timers). If the CM19a is your only controller - no problem. But if you are using other controllers, this plugin will not be able to keep track of status properly when the other controllers turn things on and off. There is no such problem with the CM15a - this is a CM19a hardware limitation.
X10 Powerline Network is Not Like Ethernet: X10 is old technology and it is quite slow by modern standards. Commands will normally take at least 1 second apiece to complete, and until one command is completed the next one can not start. Multiple controllers can co-exist on the same powerline, but if more than one controller tries to talk at once there will be a collision; in most cases neither controller's command will succeed. There is no retry provision in the protocol, so when collisions occur things just fail to happen as expected. For this reason it is preferred (although not required) that the CM15a/CM19a connected to the computer running this plugin be the only controller in the house. Repeaters and other X10 "boosters" can also, at times, cause collisions, particularly when a long series of X10 commands is being sent as quickly as the CM15a/CM19a can process them.
Macros Can Sometimes Cause Problems: It is possible, by using the "ActiveHome Pro" software, to load "Stored Macros" and timers into the CM15a's memory. The macros run when a triggering event takes place (such as a time of day). Macros are useful and are not actually incompatible with the X10 Plugin software, but they can cause intermittent problems by "bogging down" the CM15a. This is not as common with CM15a as it was with CM11a, but it can be a bit difficult to diagnose if it does happen. Temporarily clearing all macros out of CM15a memory is a good troubleshooting tactic if you are having occasional failed commands. Note: this does not apply to CM19a adapters since they do not support macro storage.
Dimming is Not Intuitive: X10 Lamp and Wall Switch modules can not be ramped up gradually from off. The hardware insists upon coming on at full brightness when a turned-off module receives a Bright or a Dim. Bright behaves just like On: the light comes on at full power and stays there. Dim turns the light on at full power first and then ramps it down by the percentage you asked for. A light that is dimmed down to zero brighness is still considered to be on. Sending that light's module an On command does nothing; you must either turn it Off and then On, or use a Bright command to ramp it up to some brightness. Also, as mentioned earlier, dimming is relative (Dim-BY) not absolute (Dim-TO), so there is no way to directly set brigtness to a predetermined level.
All of the Above are restrictions imposed by the X10 hardware and protocols, not limitations in the X10 Plugin software. Experiment with your X10 gear using a hardware controller (e.g. a keypad) and see for yourself!
Have Fun with this Plugin and your X10 systems. But keep in mind that X10 is not 100% perfect. So please don't try using it to control your stove, your electric lawnmower, the launching of intercontinental ballistic missiles, or anything else that could start a fire or hurt somebody if it operated unexpectedly due to a glitch or the actions of a visitor on your Web site!