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7.5 Connecting a modem in direct mode
This mode uses a modem in a ‘dumb’ mode, and purely relies on the hardware control
lines. Telephone number selection, and all other configuration must be performed at the
modem.
The modem is controlled using DTR (Data Terminal Ready), and indicates it’s status
using DCD (Data Carrier Detect).
Note: This mode is not recommended, easier operation is achieved in ATMODE.
Call request
A call is requested by making DTR active (high). At this point the MODEM dials it’s
stored number, and attempts to establish the call with the remote end. When the call
connect has been completed the modem raises it’s DCD line, confirming it is connected
with the remote location.
A call already being active is indicated by the DCD line being active (high) from the
MODEM.
Call answer request
If a call is incoming, the modem answers the call, establishes the physical link, and then
raises DCD to inform the DVST unit that a call is present. The DVST unit responds to this
by taking DTR active (high).
Call disconnect.
This is requested by dropping the DTR signal. A successful disconnect is confirmed by
the DCD line being dropped by the MODEM. In this mode the DTR line remains low.
Configuration of modem in direct mode
No configuration can be performed by the system, however the following are required of
the modem :
Select synchronous operation.
Dial stored number on low to high transition of DTR.
Disconnect on a high to low transition of DTR.
DCD must follow remote carrier.
Ignore DTR to answer.
Auto-answer. ( If required ).
No flow control enabled.
Page 22 Dedicated Micros Ltd
Modems DVST Installation Guide