The set of signals that are actually used depends upon which form of the
device was opened. If the ttttttttyyyydddd name was used, only TTTTDDDD, RRRRDDDD, and SSSSGGGG
signals are meaningful. These three signals are typically used with
"dumb" devices that either do not need any sort of data flow control or
use software flow control (see the description of the iiiixxxxoooonnnn, iiiixxxxaaaannnnyyyy, and
iiiixxxxooooffffffff options in _s_t_t_y(1) for more information on setting up software flow
control). If the ttttttttyyyymmmm device is used, the DDDDCCCCDDDD, and DDDDTTTTRRRR signals are also
used. These signals provide a two way handshake for establishing and
breaking a communication link with another device and are normally used
when connecting via a modem. When the port is initially opened, the host
asserts the DDDDTTTTRRRR line and waits for the DDDDCCCCDDDD line to become active. If the
port is opened with the OOOO____NNNNDDDDEEEELLLLAAAAYYYY flag, the open succeeds even if the DDDDCCCCDDDD
line is not active. A hangup condition occurs if the DDDDCCCCDDDD line
transitions from active to inactive. See _o_p_e_n(2), and _t_e_r_m_i_o(7) for more
information. If the ttttttttyyyyffff device is used, all of the signals are used.
The additional signals provide for full hardware flow control between the
host and the remote device. The RRRRTTTTSSSS line is asserted by the host
whenever it is capable of receiving more data. The CCCCTTTTSSSS line is sampled
before data is transmitted and if it is not active, the host suspends
output until it is.
The DIN-8 serial port connectors on the Indy, Indigo and MENET 4-Enet,
6-serial board (XT-FE-4TX-6A) can be used to communicate with serial
devices using RS-422 protocol. User can use the stream ioctl commands,
SSSSIIIIOOOOCCCC____EEEEXXXXTTTTCCCCLLLLKKKK and SSSSIIIIOOOOCCCC____RRRRSSSS444422222222, defined in /_u_s_r/_i_n_c_l_u_d_e/_s_y_s/_z_8_5_3_0._h to switch
between internal/external clock and RS-232/RS-422 protocols. Another
command that can be useful is SSSSIIIIOOOOCCCC____IIIITTTTIIIIMMMMEEEERRRR; it informs the driver how long
it should buffer up input data, in clock ticks, before sending them
upstream. Data can sometimes be sent upstream before, but never after,
this time limit. This feature reduces the cpu cost of receiving large
amounts of data by sending data upstream in large chunks. This duration
can also be configured into the kernel by tuning the duart_rsrv_duration
variable. On Origin, Onyx2, O2 and Octane systems, the serial hardware
sets an input timer based on the value passed in through SIOC_ITIMER. For
soft flow control, this input timer may result in excessively slow
response to an XOFF request since an XOFF char is not detected by the
