The pulses (?) at ZC + 2.5mS are not part of the Insteon PLC signal but from an unknown source.
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Thanks to Martin Custer at Automated Outlet, who loaned me an Insteon Starter Kit, I was able to conduct a hands on test of the Insteon technology. I was impressed by the concept and by the robust performance although my tests were relatively brief and informal as I had no way to program any signal schedules.
Insteon uses two RF<>PLC transceivers to bridge between the two electrical phases of the typical North American residence. I found this to be the weakest link, requiring the RF modules to be fairly close together (10-12 feet) for reliable communications. My residence is not very RF-friendly so others may find they get better RF range. In any event, it should not be much of a problem to locate the RF units where they can communicate reliably. (Other RF access points are planned.)
Other RF devices using the 900MHz region may also interfere with communication between the RF links. The Insteon frequency is fixed but you may be able to select another channel for the device(s) causing interference. This may be difficult if the offending device belongs to a neighbor.
Installing the starter kit was simple and straightforward. I merely followed the step-by-step instructions in the quick start guide that came with the kit.
Insteon uses a two-way communications protocol. Each message contains a checksum which allows receivers to validate the message and send an ACK or NAK. When signals are missed or corrupted, they are repeated automatically with up to five tries with an LED on the controller flashing rapidly to indicate the failure. All nodes on the network repeat messages as PLC so signal strength is good in the vicinity of all nodes. The PLC carrier is 131.65kHz which is close enough to X-10's 120kHz that the ELK ESM1 X-10 Signal Strength Meter can be used to check for the presence of Insteon PLC signals.
Insteon modules and switches can be programmed to respond to both Insteon and X-10 PLC and the tabletop controller can send both types of signals but only the Insteon signals are repeated and the RF units do not act as an X-10 phase coupler. Also each Insteon unit is two-way which loads the network and attenuates X-10 signals. While the interoperability allows you to mix and match the two systems, the more Insteon devices used, the more trouble you will likely see with X-10 while the Insteon part of the network will grow more reliable.
Insteon PLC uses BPSK (Binary Phase Shift Keying) which uses 2 phases of the carrier (180° apart) to signify binary 1 and 0. It is possible to recover the clock from the signal as well as measure any phase shift caused by the channel properties by use of a fixed header. BPSK has good noise immunity and is used for IEEE 802.3 and low bitrate IEEE 802.11.
The screenshots below were taken with a Protek 220 DSO via an ACT Scope-Test2. They show the Insteon PLC signal in increasing detail from A to D. (Each screenshot increases the horizontal resolution by a factor of 10 over the previous screenshot so the resolution of D is 1000 times A.) The 60Hz power signal is included for reference.
The carrier bursts are of about 2mS duration and span ZC, starting about 0.5mS before ZC. They are in groups of five. The protocol is two-way with ACK/NAK and each Insteon PLC module also repeats the signal. From studying the specifications, it appears they use 10 cycles of carrier (see screenshot E) to represent each bit (13,165 bits per second) which means it takes about 76µS per bit or a little over 600µS per byte. They use 5 bursts to send a single message. The powerline half-cycle that follows the five bursts is used to relay the signal to the opposite phase via RF.
The pulses (?) at ZC + 2.5mS are not part of the Insteon PLC signal but from an unknown source.
The 131.65kHz carrier frequency means that the Insteon signal will be affected by the same signal sinks (usually line filter capacitors) that affect X-10. The fact that each Insteon module is also a signal repeater may help counter signal sinks but I suspect some filters will still be needed. The techniques used to detect/demodulate the carrier probably means that Insteon modules have a narrow bandwidth and are more immune to noise than X-10 modules.
The RF units send periodic PLC signals the purpose of which is not readily apparent.
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