If multiple devices send PLC commands at exactly the same time, the result will be a signal collision. In this case, both commands will fail. If multiple commands are issued by the same controller nearly consecutively, I would expect that the controller would send the first but either cache or not issue the second command. I have not performed much experimentation on this, but have seen this a few times when issuing commands from an XPT wall switch/transmitter.
The thing that was odd here was in regards to the two HG machines (I'll call them HG1 and HG2). HG1 sent A2 ON, A3 ON, A4 ON, A5 ON within a couple seconds (however long it took to press the buttons in the HG web interface. HG1 showed that the lights were switched on, which makes sense since it only knows it issued the command and did not monitor anything nor receive a status from the modules (not available for most X10 equipment). HG2 somehow knew that the command was issued and turned those modules to ON as well. But, in reality the lights did not turn on. Only the first module turned on and the others remained off. So, either HG sent the command and the lights somehow ignored them or the two machines interacted via the network to communicate the module status. I know HG can do this, but I didn't think it was enabled by default.
Either way, if 4-5 modules are toggled within 1-2 seconds, I would expect one of three things.
1) Signal collision
2) Cached commands
3) Missed commands
Some modules may work, but overlapping signals might occur depending on the controller. If the controller is not polite, it will kill all transmissions. If it is polite, it will wait for the line to clear before issuing a command. It may cache commands and send them or it may just ignore some and send the first and one of the others.
