X10 TM751 and "runaway" dims???

I've forgotten who it was who first posted that he could cure his TM751 by aligning the antenna at a right angle to the 120V wiring in the wall behind the outlet.

Take a powerstrip with a few things plugged into it including a lamp module and TM751.

Align the antenna with the power cords.

Send a Dim.

Play with the antenna orientation and length.

I found that I could induce and stop the endless dims at will with any of about 6 TM751s I tried.

If RF is present at certa>For some installations, sending any X10 RF dim signal to a

4-5 years ago I conducted extensive tests using a Ming 310MHz RF transmitter driven from the LPT port of a machine running W95 so I could control exactly what RF was being transmitted.

I sent a single RF code and using a CM11A determined that the TM751 and RR501 sent 3 contiguous dims to the powerline resulting in a 12% step. Three contiguous dims (12%) take 550ms (66/120) to transmit as PLC. Each additional dim takes 183.33ms (22/120). (If a PLC signal follows a previous dim/bright by no more than 5 idle half cycles it will res.ult in an additional 6% change.)

Starting at the end of the 105ms RF code I sent a continuous 1kHz modulated

310MHz RF signal for ~1000ms, varying the duty cycle until I found a burst width that would cause more dims to be sent to the powerline. For the TM751 this was 60% (~600µs on, 400s off). I then reduced the duration of the 1kHz signal until I found the point where one extra dim was triggered. This was at ~535ms after the start of the RF code or about 430ms after the 40ms gap. Another dim was triggered at ~735ms and ~935ms after the start of the RF code. These timings were very repeatable.

With the RR501 a 40% duty cycle (400µs on, 600µs off) present ~200-300ms after the start of the RF code would trigger an additional dim. The trigger point was nowhere near as repeatable as with the TM751. What is notable is that any device sending 5 copies of the RF code (e.g. HR12A) would trigger an extra dim.

I did not check to see just when the PLC code started in relation to the start of the RF code. The CM15A waits until near the end of the 40ms gap before it starts sending PLC but I do not know about the TM751 or RR501. Also, I incremented the duty cycle of the 1kHz modulation in 100µs steps. The TM751 did not respond to a 50% duty cycle but did to 60%. I did not try to refine it beyond that.

What this means is that if the data out line of the RF receiver section of the TM751 or RR501 is high for sufficient time at certain points after an initial RF dim signal an extra dim will be sent.

The RF section uses a data slicer circuit. The demodulated RF data envelope is applied to one input of a comparator with the other input having a capacitor that charges to 1/2 the average amplitude of the signal. When the signal is greater than the charge on the capacitor the comparator output is high, otherwise it is low. In the absence of an RF signal the TM751 RF receiver section outputs continuous noise pulses. Most are of short duration with an occasional longer one. It may be that inductive coupling of the PLC signal can result in longer noise pulses. I did not try putting a scope on the data out line to see what was present during an endless dim episode.

Many thanks Dave. Your informative and detailed responses are much appreciated. Am I correct in assuming that the encoding of the 310 MHz RF pulses is the same as the encoding of the corresponding PLC? I.e., if the pulse train for the PLC is "1110011010...", do the 310 MHz RF pulses follow the same pattern?

Regards, Charles Sullivan

No. The RF code includes address and function in one code. For example...

A1 ON 01100000 00000000

The RF encoding is described (partially) in the CM17A documentation. The payload bytes are the same as the 3rd & 4th bytes sent to the CM17A with the RF protocol being identical to the NEC IR protocol. The old rectangular keychain remote used a µPD6121 NEC chip to generate the codes.

Each of the two payload bytes are followed by their bitwise complements (B1 'B1 B2 'B2) and the bits are sent >Many thanks Dave. Your informative and detailed responses

Here's an example. I used "B2 ON" because A is 0110 which is symmetrical and doesn't make the bit order clear. In the bitstreams below spaces are meaningless in the transmission scheme - they are just there to delineate sections of the codes. The ------ represents 6 idle PLC half-cycles.

B2 On (RF) -> B2, B On (PLC)

RF: Lead-In 01110000 10001111 00010000 11101111 (40ms silence)

PLC: 1110 10101001 1010100101 1110 10101001 1010100101 ------ 1110 10101001

0101100110 1110 10101001 0101100110

RF codes are usually repeated 5 times. PLC is always sent twice.

Dave, Ouch! I knew that, but I guess my thoughts were fixated on the dim command. Thanks for snapping me back to reality, and for the chip reference.

Regards, Charles Sullivan