Cermetek Microelectronics, Inc.
CH1788 Surface Mount PLCC 2400bps Modem
2006 Cermetek Microelectronics, Inc.
Page 7
Document No. 607-0018 Revision A6 (01/06)
IMPORTANT NOTE
The 911 PPU feature requires the CH1788 must be
the first device on the installation’s PSTN line. All
other phone or higher priority instruments to be
down the line from the CH1788.
CALL PROGRESS MONITORING
Call Progress Monitoring allows a call to be monitored
during the dialing sequence. This feature becomes
active when the CH1788 goes Off-Hook and
automatically disables when DCD is detected. It is
also inactive if a connection attempt should fail and
the CH1788 goes back On-Hook. To enable this
feature, the DCD line must be activated with the
ATC1
command.
Setup.
This feature requires additional external
circuitry. See Schematic 2. Although the circuit in
Schematic 2 uses a variable resistor to adjust the
speaker volume, a fixed value resistor may be
substituted.
Alternate Setup.
Control of the speaker can
alternately be performed under command control. To
implement this alternate control method, modify the
circuit in Schematic 2 by connecting resistor R5 to A1
instead of DCD. This allows register S101 to be used
to turn the speaker on or off. Set S101 to 1 to turn
the circuit on, set S101 to 0 to turn the circuit off.
FAST CONNECT HANDSHAKING
The CH1788 supports a fast handshaking (often
called “quick connect”) feature.
Typically, the initial handshaking sequence for a V.90
modem (56Kbp) takes 20-30 seconds to complete. A
standard 2400bps (V.22bis) modem connects in 4-8
seconds. With the Fast Connect feature enabled, the
CH1788 will connect at 1200bps (Bell 212) in as little
as 0.5 seconds. This feature will work with any
compatible modem.
Setup: Originate Mode.
The Fast Connect feature,
when the CH1788 is in originate mode, is activated by
using the following command setup string:
Enter:
ATFY0X4K0S30=$10S89=$40<CR>
Result:
OK
Setup: Answer Mode.
The Fast Connect feature,
when the CH1788 is in answer mode, is activated by
using the following command setup string:
Enter:
ATL3<CR>
Result:
OK
IMPORTANT NOTE
The Fast Connect feature is operational only for DTE
speeds of 1200bps (Bell 212).
LONG SPACE DISCONNECT
The long space disconnect signal is used to force the
CH1788 to release the PSTN line and go back On-
Hook.
Setup.
To enable this feature, bit 7 of register S110
most be set to 1. See the details of register S110.
VOICE INJECTION
It is possible to share the CH1788’s internal DAA with
external applications. This allows one PSTN line to
be shared between an external voice application and
the CH1788 modem. This feature is controlled via
register S101 when using the configuration in
Schematic 3.
Setup.
This feature requires additional external
circuitry. See Schematic 3. This application
implements Voice Injection via the S1 and S2 pins
and requires a two to four wire converter and a relay
circuit. Set S101 to 1 to activate Voice Inject and set
S101 to 0 to deactivate Voice Inject.
Operation.
The two to four wire converter circuit
provides for simultaneous voice in and voice out (with
respect to ground). This circuit must be capacitor
coupled (i.e., C7) to the CH1788 and isolated from
ground (i.e., C6). Voice/data switching is controlled
by setting register S101 appropriately.
During Voice Injection, the voice signal must not
exceed 2.187 V
pp
(i.e., zero dB) per FCC maximum
permissible limits. When sending data, the voice
circuit must be fully disconnected from the line (i.e.,
Voice Inject deactivated) or data corruption will occur.
IMPORTANT NOTE
When utilizing the Voice Injection circuit, it is
necessary to submit the end product containing the
CH1788 Voice Injection feature to an FCC approved
testing facility for re-approval of the end product per
FCC Part 68 requirements.
MODEM CONTROL
The CH1788 may be controlled with ASCII AT
commands sequences. The commands are sent to
the CH1788 serially via the TXD pin. After execution
of a received command string, the CH1788 returns a
serial status message on RXD.
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