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TW(4)		     FreeBSD/i386 Kernel Interfaces Manual		 TW(4)

NAME
     tw	-- TW-523 X-10 device driver

DESCRIPTION
     Tw	is the driver for the TW-523 power line	interface, for use with	X-10
     home control products.  The X-10 protocol is compatible with a number of
     home control systems, including Radio Shack ``Plug	'n Power(tm)'' and
     Stanley ``Lightmaker(tm).''

     The driver	supports read()	write()	and select() system calls.  The	driver
     allows multiple processes to read and write simultaneously, but there is
     probably not much sense in	having more than one reader or more than one
     writer at a time, and in fact there may currently be a race condition in
     the driver	if two processes try to	transmit simultaneously	(due to	unsyn-
     chronized access to the sc_pkt structure in tw_sc).

     Transmission is done by calling write() to	send three byte	packets	of
     data.  The	first byte contains a four bit house code (0=A to 15=P).  The
     second byte contains a five bit unit/key code (0=unit 1 to	15=unit	16,
     16=All Units Off to 31 = Status Request).	The third byte specifies the
     number of times the packet	is to be transmitted without any gaps between
     successive	transmissions.	Normally this is 2, as per the X-10 documenta-
     tion, but sometimes (e.g. for bright and dim codes) it can	be another
     value.  Each call to write() can specify an arbitrary number of data
     bytes, but	at most	one packet will	actually be processed in any call.
     Any incomplete packet is buffered until a subsequent call to write() pro-
     vides data	to complete it.	 Successive calls to write() leave a three-
     cycle gap between transmissions, per the X-10 documentation.  The driver
     transmits each bit	only once per half cycle, not three times as the X-10
     documentation states, because the TW523 only provides sync	on each	power
     line zero crossing.  So, the driver will probably not work	properly if
     you have three-phase service.  Most residences use	a two-wire system, for
     which the driver does work.

     Reception is done using read() The	driver produces	a series of three
     character packets.	 In each packet, the first character consists of
     flags, the	second character is a four bit house code (0-15), and the
     third character is	a five bit key/function	code (0-31).  The flags	are
     the following:

     #define TW_RCV_LOCAL    1	/* The packet arrived during a local transmission */
     #define TW_RCV_ERROR    2	/* An invalid/corrupted	packet was received */

     The select() system call can be used in the usual way to determine	if
     there is data ready for reading.

SEE ALSO
     xten(1), xtend(8)

     TW-523 documentation from X-10 Inc.

FILES
     /dev/tw?
	      the TW523	special	file

AUTHORS
     Eugene W. Stark <stark@cs.sunysb.edu>

FreeBSD	9.2		       October 30, 1993			   FreeBSD 9.2

NAME | DESCRIPTION | SEE ALSO | FILES | AUTHORS

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