Multiple X-10 RF Transceivers Sharing a Housecode

The details below apply to X-10 transceivers made in the past several years. Since RF travels at the speed of light, all transceivers will receive the RF signal at essentially the same time. Once they decode the RF, they transmit to the powerline starting with the next zero crossing (with the exceptions noted below) regardless of ZC polarity. Some earlier transceivers always waited for a rising ZC.

TM751

Two TM751 transceivers will coexist without problems except when the transceivers are on opposite phases. When on opposite phases, unit 1 & 9 codes will always collide on the powerline but all other codes will coincide. This has to do with the internal relay which is activated before sending the corresponding code to the powerline. The SCR can only be triggered with a positive signal so the TM751 waits for a positive ZC. Since ZC on the opposite phase is 180 out of phase, the two transceivers will transmit 1/2 cycle out of sync. Below, each digit represents a powerline 1/2 cycle with 1 indicating a 120kHz burst and 0 indicating a silence.
11100110100110101001011110011010011010100101   Phase A transmits
_11100110100110101001011110011010011010100101  Phase B transmits 1/2 cycle later
111101111101111111011111110111110111111101111  Receivers see this invalid sequence¹
   ^ ^ CM11A, CM15A, RR501 stop when they send 0 but detect 1
       CM15A, RR501 start anew after powerline is clear

¹Monterey Analyzer will erroneously report each 1110 sequence within the collision debris as a Bad Start Code. 
For other unit codes...

11100110100110101001011110011010011010100101  Phase A transmits
11100110100110101001011110011010011010100101  Phase B transmits 
11100110100110101001011110011010011010100101  Receiver sees a valid signal

RR501

Two RR501 transceivers on the same phase will coexist. If on opposite phases there will be collisions for units 1 & 9. The RR501 has collision detection and automatic retransmit. The first to detect a collision will stop sending to the powerline and then start anew after the powerline is clear. This will cause duplicate commands.

RR501 & TM751

This combination has collisions on units 1 & 9 even if on the same phase. Other units coexist as far as collisions but will see duplicates.

HCPRF

I only have one HCPRF so I can only test it in combinations. With an RR501 on the same phase, ALL codes are duplicated. With a TM751 on the same phase, ALL codes are duplicated. With a CM15A on the same phase, ALL codes are duplicated.

CM15A

I only have one CM15A so I can only test it in combinations. With an RR501 on the same phase, ALL codes are duplicated. With a TM751 on the same phase, ALL codes are duplicated. With a Leviton HCPRF on the same phase, ALL codes are duplicated.

FINAL NOTE

The reason that the HCPRF and CM15A always duplicate codes may be because they follow the X-10's recommendations and wait for 8, 9, or 10 half cycles of silence before trying to send. XTC798.DOC says...

When a transmitter has a message it wishes to transmit, it must wait for access to the power line for either 8, 9, or 10 half power line cycles - during which the line must have been continuously clear of data '1' bits. If a '1' bit is detected, it must restart its access timing and wait for another 8, 9, or 10 cycles. After line access has been achieved, the transmitter must check the line during the transmission of a '0' bit (no carrier) to see that no other transmitter is transmitting. If a collision occurs, the transmitter must abort its transmission immediately and again go though the line access procedure. The choice of 8, 9, or 10 half cycles is chosen randomly for each line access attempt.

That would always allow other, non-conforming devices (e.g. TM751) to transmit first.

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