Technical data

Managing Peripheral Devices
8.7 Managing Modems
Table 8–8 Types of Modem Control That Devices Support
Type of Modem
Control Description
No modem control The host and the modem cannot intercommunicate the status of
the host or the modem. It is possible to use a modem on this port;
however, this type of port is not recommended for a modem.
Without modem control, the modem cannot signal the host that
the telephone call has been disconnected and that the host must
take appropriate action: suspend or log out the associated user
process. (See Step 5 for the associated security implications.)
Furthermore, without modem control, you must set or wire the
modem so that it always answers incoming calls, because the
modem cannot know if the host is able to respond. (This too has
security and modem control implications.)
Limited modem
control
The host and the modem can intercommunicate and can take
actions based on the status of the other device. Limited modem
control is the best choice for most applications.
Full modem control The host and the modem can intercommunicate and can pass an
extensive amount of control and status information. Both the host
and the modem can take actions based on the status of the other
device.
Limited modem control, which has similar capabilities, has largely
superseded this configuration. Limited modem control also requires
fewer wires on the connection, making it the more economical
choice.
Refer to the device documentation to determine the type of modem control
signal that the device and modem support. This determines the number of
wires and the wiring connections needed for communications. The following
examples show types of modem control and the wires they require:
• DECconnect supports limited modem control, which requires two of the
six wires in the DECconnect cabling. The other four wires are used for
the following purposes:
Transmitting data
Receiving data
The transmit ground
The receive ground
• Full modem control requires more than two wires dedicated to the modem
control signaling.
• Devices that do not support modem control require no wires dedicated to
modem signaling.
With modem commands or custom-wired cabling, you can force a modem to
operate with a device that does not support modem control. However, this is
not recommended for general use on a host system, because this wiring can
potentially result in security problems.
Managing Peripheral Devices 8–31