Cermetek Microelectronics, Inc.
CH1788 Surface Mount PLCC 2400bps Modem
2006 Cermetek Microelectronics, Inc.
Page 2
Document No. 607-0018 Revision A6 (01/06)
with all modems. The ability to execute AT
commands is consistent with industry standards.
Telephone Line Interface or DAA.
The CH1788 is
designed to meet 1500VAC RMS isolation and
provide 2122V peak surge protection. Consequently,
the CH1788 satisfies North American and some
international requirements. Note that supplemental
protection is required for FCC Part 68 conveyed
approval (see Figure 3 of this document). This
supplemental protection also improves field reliability
and facilitates FCC part 15 approval.
SUPPORTED FEATURES
AT Command Set.
Internal to the CH1788 is an
AT
command interpreter.
AT
commands and their
associated S-registers provide macro functionality.
These commands are summarized in this document
and are detailed in Cermetek publication CH1788 AT
Command and S-Register Reference Guide.
The CH1788 supports command concatenation. A
maximum of 63 characters (excluding
AT
) may be
included on each command line. S-registers provide
added programmability by allowing key parameters to
be configured for a specific application.
Serial Host Interface.
The serial interface is a
standard TTL V.24 5V DC interface. It is functionally
similar to an RS232 interface.
IMPORTANT NOTE
Do not use RS232 interface voltage levels on the
serial pins. Damage may result. Adhere to the TTL
V.24 5V DC interface standard. See Schematic 7
for a suggested circuit to interface RS-232 logic to
the CH1788 TTL V.24 logic.
Command Characters.
Each data byte or command
character consists of ten bits: 1 start bit, 1 stop bit,
and eight data bits. The data bits can be either eight
bits with no parity or seven odd or even with 1 parity
bit. The start bit is LOW going. RXD and TXD data is
non-inverted. See the pin descriptions in Table 2.
Data Encoding.
The data encoding is user
configurable. The
ATB
command sets the decoding.
The following ITU-T standards are supported:
V.22bis, V.22, Bell 212A, 103, Bell 202 or V.23.
Speed Buffered Mode.
Speed Buffering allows the
serial data interface or DTE speed (i.e., the speed
between the host microcontroller and the CH1788) to
be fixed regardless of the connect or DCE speed (i.e.,
the speed between the CH1788 and the remote
modem).
The option exists to disable Speed Buffering by using
the
ATY0
command. If Speed Buffering is disabled,
the data connection DCE speed will be equivalent to
the originating modems serial connection DTE speed.
In other words, disabling Speed Buffering forces the
answering modem to connect with a DCE speed
equivalent to the serial DTE speed of the originating
modem.
Data Transmission Speed.
The CH1788 allows the
DTE speed to be set to 300, 1200, 2400, or 9600bps.
The DCE speed can be set to 2400, 1200, or 300bps.
A connect message will appear on RXD indicating the
connected data rate.
General Operation.
By definition, the originating
modem initiates the PSTN connection and the
answering modem accepts the incoming call. The
connectivity speed (i.e., DCE speed) is determined by
the originating modem.
The originating modem’s DTE speed is established,
in turn, with an initializing
AT
command issued by the
application’s host controller. Speed Buffering allows
the CH1788 to communicate with the host controller
at a DTE speed that is greater than the connect DCE
speed.
Assuming that the default configuration of the
CH1788 has not been modified with the
ATB
or
ATY0
commands, the answering modem will attempt to
connect to the originating modem at the highest
possible data rate supported by the originating
modem. The connection rate can be less than the
maximum of 2400bps, for example, if the PSTN line is
impaired in some way.
By default upon power up, the CH1788 DTE speed is
set to 9600bps and Speed Buffering is enabled.
Typical Implementation.
In a typical
implementation, the CH1788 has a DTE speed of
9600bps and will connect to the remote modem at a
DCE speed of 2400bps. Speed Buffering allows the
CH1788 to negotiate the DCE speed independent of
the DTE speed. Data transfer to the host will be at
the DTE Speed Buffered rate of 9600bps. In this
case, flow control is not required.
It is possible for the DTE speed to be slower than the
DCE speed. For example, a serial interface DTE
speed of 1200 bps and a data rate DCE speed of
2400 bps. In this case, flow control must be
employed by the host to prevent buffer overflow and
lost data.
POWER DOWN
The CH1788 has two power down modes: idle power
standby and zero power standby.
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