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CH1786NH

CH1786 Family of Ultra Small 2400bps Modems

Cermetek Microelectronics 

Cermetek Microelectronics, Inc.

CH1786 Family of Ultra Small 2400bps Modems

DESCRIPTION OF FUNCTIONAL BLOCKS

AND DISCUSSION OF BASIC OPERATIONS

Figure 1 contains a functional block drawing of the

CH1786. The CH1786 is a highly integrated, full-

function modem, comprised of a modulator/

demodulator, controller, NVRAM and an FCC Part 68

approved and UL 1950/CSA C22.2 950 listed PSTN

line interface.

Modulation/Demodulation and Modem Controller.

These functions are provided by a monolithic modem

integrated circuit. This IC has built-in facilities to

accommodate integrated AT command control and

contains the necessary resident interfaces for general

communication and routing to the DAA.

Telephone Line Interface or DAA. The CH1786

family of modems is designed to meet 1500VAC RMS

isolation and provide 2122V peak surge protection.

Consequently, the CH1786 satisfies U.S. FCC Part

68 and DOT CSA CS-03 Part I Canadian

requirements, and will meet other international

approval agency requirements that specify these

levels of isolation.

With the exception of the U.S., most countries

(including Canada) require submission of the final

product or system containing the CH1786 to the

appropriate governing regulatory agency for approval.

Typically this is accomplished by submitting the final

end product to an independent test house or

consultant for evaluation. The test house or

consultant then forwards the test results and

applicable documents to the regulatory agency. Call

Cermetek for a list of suggested consultants.

Non-Volatile RAM (NVRAM). The NVRAM present

on the CH1786 is sufficient to store up to two user

customized modem configurations. The AT&Wn

command will store the active modem configuration in

the selected NVRAM locations by specifying an n of 0

or 1.

The AT&Yn command selects the modem

configurations to be automatically recalled and made

active upon a reset or power up. The ATZn

command immediately recalls and activates a stored

configuration. Refer to Tables 4, 5 and 6 for storable

S-Registers and available commands.

The NVRAM can save up to four telephone numbers

with up to 36 digits or modifiers in each telephone

number. The AT&Zn=s command will store s, the

telephone number dial string. The individual NVRAM

telephone number storage locations are selected by

specifying an n of 0,1,2,or 3. The ATDTS=n

command will cause the modem to dial the stored

telephone number in location n. NVRAM is not

available on the CH1786LC or CH1786NH.

SUPPORT FEATURES

AT Command Set. A 40-character command line is

supported. The command line starts with AT and

may contain standard or enhanced commands. The

commands are compatible with EIA Document

TR302.2/88-08006.

Serial Host Interface. The serial interface is a V.24

(EIA-232) compatible interface. Ten Bits total: 1 start

bit, 1 stop bit, and eight data bits which can be either

eight bits with no parity or seven odd or even with

parity. The start bit is LOW going. RXD and TXD

data is non-inverted. See pin description in Table 2.

Speaker Interface. The SPK output reflects the

receiver analog input and provides a signal that can

be used to monitor call progress. The SPK signal can

drive a 300 Ω load directly. Typically, the SPK signal

is input into an audio power amplifier and the

amplifier, in turn, drives the speaker coil. The

speaker is activated with the ATMn command. The

speaker volume is adjusted using the ATLn

command, where n is 0,1,2,or 3. Increasing numbers

correspond to higher volume. Figure 6 indicates one

method of driving an external 8 Ω speaker.

Phone Control. Cermetek ’s CH1786 contains a pin

called Voice or Data (V/D). The V/D pin toggles high

when the modem goes off hook. This pin can be

used to activate a relay which can switch a telephone

on or off the Tip and Ring incoming PSTN lines. This

feature allows any telephone associated with the

CH1786 to be disconnected when a data call is in

progress, thereby preventing the data from being

disturbed by an inadvertent telephone pick-up. See

Figure 2.

Speed and Parity Selection. Prior to call initiation,

the host controller trains the modem to the host

speed (2400,1200 or 300bps) and parity (odd, even,

mark, space, or none) via a host-initiated training

sequence. This training also sets the speed of the

data for originate calls. The CH1786 will

automatically adjust to the originator’s speed when

answering calls.

The CH1786 matches the host’s parity when it returns

status messages to the host. During a data

connection, however, the modem passes parity

through without interpretation or alteration.

Sleep Mode. To minimize power consumption, the

CH1786 includes a power down feature called Sleep

Mode. When activated, the CH1786 will

automatically enter Sleep Mode after 0 to 254

seconds of inactivity. The inactivity delay is selected

using the ATS24 command. The CH1786 is

delivered from the factory with a 5 second inactivity

delay enabled (i.e., ATS24=5). The CH1786 returns

to normal operation when a ring signal is received or

2003 Cermetek Microelectronics, Inc.

Page 2

Document No. 607-0004 Revision L1 (06/03)

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