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CH1788

Surface Mount PLCC 2400bps Modem

Cermetek Microelectronics 

Cermetek Microelectronics, Inc.

CH1788 Surface Mount PLCC 2400bps Modem

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.

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.

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.

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.

Data Encoding. The data encoding is user

configurable. The ATB command sets the decoding.

The following CCITT 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).

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.

The option exists to disable Speed Buffering by using

the ATY0 command. If Speed Buffering is disabled,

2003 Cermetek Microelectronics, Inc.

Page 2

Document No. 607-0018 Revision A2 (07/03)

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