Re: for Anand Dhuru

[nobody@xxxxxxxxxxxx (Dave Houston)]

Anand,

In some X-10 designs (e.g. TW523, CM15A) they use one of the windings of a
transformer as the inductor in an oscillator tank circuit and modulate that
from the PIC (or other controller). In others (e.g. RR501) they generate the
modulated 120kHz with the PIC. In both, they then use a tuned transformer to
couple the signal to the powerline. I would generate the 120kHz with the PIC
(easier than designing a stable oscillator and easier to tune). Even the
8-pin 12F683 has hardware PWM you can use for this. One of the app notes on
Bruce Reynolds site has PWM code (for another PIC) which you can adapt.

I think the difficulty in the AN236 type coupling is that it cannot supply
enough current. The X-10 transformer has a 5:1 turns ratio which steps down
the voltage but steps up the current. (For receiving it steps up the
voltage.)

The ST7537 app note has an excellent section on the powerline as a
communications network beginning on p4.

The "IT800 Line Driver Performance" app note from...

     http://www.yitran.com/download1.htm

has tables showing current & Vpp vs. network impedance at frequencies near
the 120kHz used by X-10.

Another advantage of using the line driver shown in the ST7537 app note is
that it is only activated during transmissions reducing signal attenuation
during reception.

The PIC16F88 is similar to the PIC16F628 but the 16F88 can use a bootloader.
There's a free bootloader available from www.sparkfun.com.

The PIC itself does not provide isolation so you need a transformer power
supply and line coupler if you want to make a connection from a PC.

If you don't want to use a transformer for isolation, you can optoisolate
the RS232 connection similar to the way the TW523 isolates its TTL
connection. There's a schematic for an optoisolated full duplex RS232 link
here...

     http://chaokhun.kmitl.ac.th/~kswichit/Rs232_web/Rs232.html

"Anand" <ardhuru@xxxxxxxx> wrote:

>Thanks for the valuable pointers again, Dave.
>
>I now have a collection of mains front end circuits, and am tring to
>decide on one (or a combination with ideas borrowed from some).
>
>I have one query, Dave; correct me if I am wrong.
>
>Assume that only an injector is required (reception not required).
>
>You have X10 data coming out of the PIC. *The transformer generates the
>120 KHz carrier*, and PIC data modulates this so one ends up with 120
>KHz modulated X10 data coupled by the transformer into the mains.
>
>Am I right so far? or is it that the data coming out of the PIC is
>already pre-modulated by the PIC at 120KHz (regardless of whether a
>direct or a transformer coupling is used)? I know the latter happens
>with the AN236 design.
>
>If my earlier assumption is correct, then in transformerless designs
>one would have to also generate the 120 KHz within the PIC (as in
>AN236), or externally using an oscillator to modulate the X10 data.
>
>You mentioned you did not get a decent transfer using the circuit in
>app. note AN236. This could perhaps be on account of the front end
>using 5 volts, as against the 18 - 30 volts hat X10 uses?
>
>I use a 16F877 when I connect using a PL513; I prefer to use a
>bootloader as far as possible. But when I'm experimenting with my own
>interface, I prefer to use a 16F628, as I wouldnt like to share any
>line with the PC with either a bootloader or an ICSP cable.
>
>Regards,
>
>Anand