HP-UX Reference (11i v2 07/12) - 7 Device (Special) Files, 9 General Information, Index (vol 10)

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modem(7) modem(7)

Terminal Port Access Types

There are three types of modem access: call-in connections, call-out connections, and direct (no modem con-

trol) connections. A given port may be accessed through all three types of connection by accessing different

ﬁles. The modem access type of a terminal ﬁle is determined by the ﬁle’s major and/or minor device

numbers.

The call-in type of access is used when the connection is expected to be established by an incoming call.

This is the type that would be used by getty (1M) to accept logins over a modem. When an

open() is

issued to such a ﬁle, the driver may wait for an incoming call and will then raise the CONTROL based on

the current mode (see below) of the port. When the port is closed, the driver may or may not lower the

CONTROL depending on the

HUPCL bit.

The call-out type of access is used when the connection is expected to be established by an outgoing call.

This would be used by programs such as uucp(1). When an

open() is issued to such a ﬁle, the driver will

immediately raise the CONTROL and wait for a connection based on the mode currently in effect. When

the port is closed, the driver may or may not lower the CONTROL depending on the

HUPCL bit.

The direct type of access is used when no driver modem control is desired. This could then be used for

directly connected terminals that use a three-wire connection, or to talk to a modem before a connection

has been established. The second case allows a program to give dialing instructions to the modem. Neither

the

CLOCAL nor the HUPCL bits have any effect on a port accessed through a direct ﬁle. (However, both

bits may be inherited by other types of ﬁles; see the Terminal Port Access Interlock section below.) An

open() to a direct ﬁle does not affect the CONTROL and does not depend on any particular state of the

STATUS to succeed. When the ﬁle is closed, the driver will not affect the state of the CONTROL. If a

modem connection has been established, it will continue to exist. Setting the speed of a direct ﬁle to B0

(see termio(7) ) will be considered an impossible speed change and will be ignored. It will not affect the

CONTROL.

Modem Line Control Modes

There are two modes of modem line control: CCITT mode and simple mode. A given port may have only

one of these two modes in effect at any given point in time. An attempt to open a port with a mode other

than the one in effect (from a PENDING or SUCCESSFUL open() on a different ﬁle) will cause the

open() to be returned with an [ENXIO] error. The modem access type of a terminal ﬁle is determined by

the ﬁle’s major and/or minor device numbers.

CCITT mode is used for connections to switched line modems. The CONTROL for CCITT mode are Data

Terminal Ready (DTR) and Request to Send (RTS). The STATUS are Data Set Ready (DSR), Data Carrier

Detect (DCD), and Clear to Send (CTS). Additionally, the Ring Indicator (RI) signal indicates the presence

of an incoming call. When a connection is begun (an incoming call for a call-in ﬁle or an

open() issued to

a call-out ﬁle), the CONTROL are raised and a connection timer (see the Modem Timers section below) is

started. If the STATUS become raised before the time period has elapsed, a connection is established and

the

open() request is returned successfully. If the time period expires, the CONTROL are lowered and

the connection is aborted. For a call-in ﬁle, the driver will wait for another incoming call; for a call-out ﬁle,

the open() will be returned with an [EIO] error. Once a connection is established, loss of either DSR or

CTS will cause the CONTROL to be lowered and, if the device is a controlling terminal, a hangup signal

will be sent to the controlling process.

If DCD is lost, a timer is started. If DCD resumes before the time period has expired, the connection will

be maintained. However, no data transfer will occur during this time. The driver will stop transmitting

characters, and any characters received by the driver will be discarded. (However, on some implementa-

tions data transmission cannot be stopped. See the DEPENDENCIES section.) If DCD is not restored

within the allotted time, the connection will be broken as described above for DSR and CTS.

If the modem connection is to be broken when the

close() system call is issued (i.e. HUPCL is set), then

the CONTROL will be lowered and the close() will be returned as successful. However, no further

open()s will be allowed until after both DSR and CTS have been lowered by the modem, and the hangup

timer (see the Modem Timers section below) has expired. The action taken in response to an open() dur-

ing this time will be the same as if the port were still open. (See the Terminal Port Access Interlock section

below.)

When a port is in CCITT mode, the driver has complete control of the modem lines and the user is not

allowed to change the setting of the CONTROL or affect which STATUS are actively monitored by the

driver (see the Modem Ioctls section below). This is to provide strict adherence with the CCITT recommen-

dations.

HP-UX 11i Version 2: December 2007 Update − 2 − Hewlett-Packard Company 87