Lantronix ETS Reference Manual - 5.2.2 Modem Wiring
Ports
Modems
5-7
1
The modem and ETS wait for a connection. The ETS provides DTR to the modem so that the modem
will accept any incoming calls. The modem does not provide DSR to the ETS since it is not connected
via the phone line.
2
The modem receives a call, asserts the Ring signal, and then asserts Carrier Detect (CD) to signal it
accepted the connection.
3
The ETS sees its DSR input signal go high when the modem asserts DSR and/or CD.
4
The ETS logs the port in, after which the remote caller presses Return and gets a login prompt or au-
toconnect from the ETS.
5
The user works for a while, then decides to log out and exit.
6
If the user logs off the ETS locally, the ETS drops the DTR line long enough to signal the modem to
hang up the remote connection (roughly 3 seconds).
7
If instead the remote modem hangs up before the user logs out, CD (the ETS’s DSR) is de-asserted,
and the ETS closes the local session.
For outgoing calls, the sequence might be:
1
A local user connects to the modem service. The ETS keeps DTR high for the modem and pays no
attention to incoming DSR yet.
2
The user dials out and connects to a remote service.
3
The modem now raises CD (and therefore the DSR signal on the ETS) as a sign that it is connected.
The ETS now begins monitoring the DSR line while the session progresses.
4
If the user logs out of the remote connection, the remote modem hangs up and the local modem de-
asserts DSR. The ETS then logs the user off the service and makes the service available again.
5
If the local connection is closed (for example, by a Disconnect), the ETS drops DTR to signal the mo-
dem to hang up the remote end and closes the session.
5.2.2 Modem Wiring
There are two fundamental wiring issues to address: Data Terminal Ready (DTR), and Data Set Ready
(DSR). Both issues, along with Carrier Detect (CD), are discussed in this section.
The ETS normally asserts DTR. When Modem Control is enabled, the ETS will drop DTR for three seconds
each time the port is logged out and each time a network user disconnects from a modem service. The
modem must be configured to hang up and recycle when DTR is dropped. Security problems may result if
the modem is not configured in this way.
The ETS must know when a connection is lost so it can log out the port and close any sessions. Otherwise
security problems will result when the next user logs into the port. The ETS also needs to know when a valid
connection does not exist so it can ignore spurious characters from the modem. Otherwise the ETS may
interpret the characters from the modem as a user logging in, causing the port to be unavailable.