Jack,
I think the seller provides a good description of the differences (for future readers):
RR501 2-Way Transceiver
Two-way module/receiver which will act on X10 line
signals as well as RF signals
The "appliance" module receptacle
can be set to either unit 1 or 9
Can be polled for "status" (I didn't know this for years - usually reserved for more expensive modules - not $8 ones.)
Has collision avoidance
TM751 Mini Transceiver
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Will only react to RF signals, not X10 line signals. it will however still put the 16 unit code signals for one house code on the line (when instructed to by X-10 RF)
Can only be
unit 1 (not changeable)
Cannot be polled for "status"
Does not have collision avoidance
X10 pushes the TM 751, it's smaller, but the RR501 is a better piece of equipment.
For your other problems - just ask - experts like Brian are here and able to help. Almost all X10 problems are solvable, some require an expenditure to fix - but compared to other technologies, that expenditure is low.
For now, glad it's working!!!
GLT ( oops...I mean George)