AT&T IPC-1600 User Manual page 78

Definition of IPC-1600 Device Node Names
Software Device Class
The /dev/ttyis xyy device type must be used for I/O devices (terminals,
printers, MODEMs, etc.) when software flow control is desired.
The ttyisxyy class of device node names does not support hardware flow
control via the standard Electronic Industries Association (EIA) RS-232C
hardware handshaking signals (CTS and RTS). Flow control is provided
under software control. Any change in the status of Data Carrier Detect
(DCD) causes the getty to respawn.
The /dev/ttyisxyy name directs the IPC-1600 to ignore all RS-232C
control signals, except Data Carrier Detect (DCD), and to use software flow
control over the Transmit Data (TD) and Receive Data (RD) lines. Characters
can be transmitted and received when DCD is not asserted. The connection is
dropped when DCD is asserted and then dropped.
5-4 IPC-1600 USER'S GUIDE