Hi Dan,
I could only conjecture that past problems may have been caused by modules "stepping" on each other. But I do know that there would be random days times when certain lights would not come on when called for by 5 modules, where I had several of each type of interface, serial and USB, all with the same schedule.
On thing that will certainly help is the fact that the USB and serial units have a different arrangement for calculating sunrise and sunset, in terms of the number of days that are specified for the changes to be made. But, with the serial and USB creating different sunrise/sunset times, the possibility still remains that there could be some stepping on each other.
I would have to assume that the actual RF carrier and modulation output is the same for both, since they are addressing the same modules.
But, to play it safe, I have created different files to use for different units.
The old serial units have 4 advantages: they have the front feedthrough receptacle - they have a 3-wire grounded power connection - they only need/use 2 AAA cells. But of course, they have a number of disadvantages. - the 8.3 filenames, smaller internal memory, and are lacking the RF transceiver section. But for $10 apiece, they certainly aren't expensive. And I've had far less "crashes" of the serial units. These crashes of unknown source, make a fantastic jamming device out of the interface modules (both serial and USB). When they go into that failure mode, they send out enough garbage to block all wired and wireless commands from going anywhere.
Ron Hirsch
[TTA Edit: Clarified the RF Transceiver statement.]