TW(4) BSD/i386 Kernel Interfaces Manual TW(4)NAMEtw — 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.
FILES
/dev/tw?
the TW523 special file
SEE ALSOxten(1), xtend(8)
TW-523 documentation from X-10 Inc.
AUTHORS
Eugene W. Stark ⟨stark@cs.sunysb.edu⟩
BSD October 30, 1993 BSD
