UM-0085-B09
DT80 Range User Manual
Page 200
RG
External Modem
Not applicable to DT8xM models
Modem (Remote) RS 232 Connection
Another common way of communicating with the DT80 is to connect its Host RS 232 port to a wired or wireless modem,
which communicates with another modem connected to the host computer at the other end of the comms link. This way,
the DT80 can be across town or across the world from the host computer, and the link can use PSTN (landline), radio,
GSM (cellular) or satellite communication. This is known as a modem connection to a remote DT80.
Figure 81: Modem connection
The "phone network" cloud in the diagram could be:
•
a dedicated wired or wireless link, e.g. a leased line or a dedicated radio link
•
the public switched telephone network (PSTN)
•
a cellular or satellite mobile network
The DT80 supervises the modem using standard Hayes-compatible "AT" commands. Certain command strings are
configurable, to allow the widest possible range of modems to be supported.
The DT80 can also control the modem’s power supply. If this facility is used, the DT80 can automatically reset the
modem if it determines that this is necessary. See
Powering the DT80’s Modem (P203)
.
Automatic Modem Detection
By default, the DT80 uses the state of its Host RS 232 port’s
DSR
terminal to determine whether a modem is connected
to the port. (
DSR
= "Data Set [i.e. modem] Ready")
If the
DSR
terminal is not held active by the connected device, the DT80 assumes that it’s connected directly to the host
computer and operates accordingly.
If the DT80’s
DSR
terminal is held active by the connected device, the DT80 assumes that it’s connected to a modem
and operates accordingly, initialising the modem, monitoring other Host RS 232 lines to determine when a modem
connection to the host computer has been established, and so on.
If required, you can override the automatic modem detection feature by setting the following profile:
PROFILE HOST_MODEM DETECTION=ALWAYS
(assume modem is always present, regardless of
DSR
state), or
PROFILE HOST_MODEM DETECTION=NEVER
(assume modem is not present)
The default setting for this profile is
PROFILE HOST_MODEM DETECTION=DSR
.
DT80-to-Modem Cable
The cable used to connect the DT80 host port to the modem should normally be a straight-through comms cable, as
shown in
. The DT80 uses the following RS232 control signals:
•
DSR
(Data Set Ready), for modem detection, as described above
•
DTR
(Data Terminal Ready), to tell the modem that a data terminal (i.e. the DT80) is connected to it, and also to
hang up a call
•
RTS
(Request To Send) and
CTS
(Clear To Send), to allow hardware flow control (see
)
•
DCD
(Data Carrier Detect), to allow the DT80 to detect when the modem is on-line (see
Communications Operation (P203)
).
•
RI
(Ring Indicator), to allow the DT80 to wake up when an incoming call is detected.