CRMET CH1786XNE, CH1786NH, CH1786NE, CH1786LCNE, CH1786LC Datasheet

CH1786 Family of Ultra Small 2400bps Modems

2003 Cermetek Microelectronics, Inc. Page 1 Document No. 607-0004 Revision L1 (06/03)

INTRODUCTION

The CH1786 family of modems are a full function,
FCC Part 68 approved 2400bps modem. These
modems provide a fast, easy and flexible way to
integrate a modem into any OEM product while
utilizing the minimum amount of PCB space (1.01 ”x

1.27 ”x 0.52 ”). The CH1786 family only requires two
external interfaces: a CCITT V.24 serial interface that
can be routed directly to a UART, and a Tip and Ring
interface which goes directly to an RJ-11 jack for the
PSTN line connection. The CH1786 can be
controlled with industry standard AT commands and,
hence, is compatible with available industry
communication software.

All CH1786 modems support asynchronous operation
at 2400bps, 1200bps, and 300bps to both Bell and
CCITT standards. The resident PSTN line interface,
or Data Access Arrangement (DAA), while being FCC
approved, is also Canadian DOT approvable and can
be approved in other countries that require 1500VAC
RMS isolation requirements per UL 1950 Edition 3.

The CH1786 family of modems operate off a single 5­volt supply. The low power operation and automatic
standby mode make these modems ideally suited for
portable equipment. In addition, their small physical
size affords maximum flexibility in equipment design.

Figure 1. Functional Block Diagram of CH1786.

The CH1786 comes with FCC Part 68 approval and is
shipped from the factory with an FCC Part 68 label
indicating the registration number and ringer
equivalent. This label should be prominently
displayed on the end product.

FEATURES

• Supports Standards CCITT V.22bis,V.22,Bell
212,and Bell 103

• FCC Part 68 approved and DOT CSA CS-03 Part I
approvable

• UL 1950 and CSA C22.2 950 Listed

• UL File Number: E104957

• AT Command structure with extensions

• 1500 VAC RMS isolation barrier minimum, 2122V

peak surge protection minimum

• Single 5 volt operation

• Low power operation with automatic reduced power

standby mode

• Automatic adaptive and fixed compromise
equalization

• Size: 1.01 ”x 1.27 ”x 0.52 ”(nominal)

• NVRAM allows storage of custom configurations

and telephone numbers

CH1786 FAMILY

CH1786 NVRAM, Voice/ Inject, Hermetic,

Operating Temperature:
0°C to +70°C

CH1786ET NVRAM, Voice/ Inject, Hermetic,

Operating Temperature:

-40°C to +85°C

CH1786NH Non-Hermetic, Operating

Temperature:
0°C to +70°C

Figure 2. Voice/ Data Port Control

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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, full­function 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

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upon an input low signal on the TXD pin.
ATS24=255 disables the Sleep Mode and is the
default if no value is set in register S24.

A SLEEP output signal is available to control power to
external devices. In Figure 6, a FET controlled by the
SLEEP signal turns of the external speaker amplifier
when the CH1786 enters Sleep Mode. In Sleep
Mode, power is reduced to approximately 50% of
normal operating power.

Guard Tone. A guard tone of 550 Hz or 1800 Hz can
be generated at 6 dB or 9 dB below the transmit level,
respectively, by using the &Gn command. Refer to
“Cermetek AT Commands and S-Registers reference
Guide” or the Cermetek web site at

http://www.cermetek.com

.

Answer Tone. A CCITT (2100 Hz) or Bell (2225Hz)
answer tone is generated depending on the selected
configuration. Refer to “Cermetek AT commands and
S-Registers Reference Guide” or the Cermetek web
site at http://www.cermetek.com

.

Data Encoding. The data encoding conforms to
CCITT recommendations V.22bis or V.22, or
Bell212A, or 103, depending on the selected
configuration. Refer to “Cermetek AT commands and
S-Register Reference Guide” or the Cermetek web
site at http://www.cermetek.com

.

Line Equalization. Transmitter and receiver digital
filters compensate for delay and amplitude distortion
during operation on nominal phone lines. In addition,
automatic adaptive equalization in the receiver
minimizes the effects of inter symbol interference.

Transmission Speed. In normal operation, the
originating modem initiates the call and attempts to
connect to the answering modem at a speed
established by the originating modem’s controller
prior to call initiation. This is referred to as the Initial
Trained Rate. Upon receiving the call, the answering
modem will attempt to connect to the originating
modem at its Initial Trained Rate. If these two rates
are identical, the connection is made. If the speeds
differ, the answering modem must adjust its rate or
terminate the call. Table 1 indicates the connection
rate that will result when the calling modem’s Initial
Trained Rate and the answering modem’s Initial
Trained Rate are different.

NOTE

A 2400-baud connection rate will only result if both
modems are initially set at 2400 baud.

Zero Power Mode (CH1786ET only). If an
application calls for zero power during standby
periods, the power may be switched off using external
circuitry and then reapplied when the CH1786 ’s RI

pin becomes active. The CH1786ET products can
thus be powered down to zero.

Table 1. Connection Rates.

Answering

Modem Initial

Trained Rate

Connection Rate Resulting

When Calling Modem Initial

Rate Is:

300 1200 2400

300 300 1200 1200
1200 300 1200 1200
2400 300 1200 2400

Figure 3. Zero Power Operation.

Referring to Figure 3, the controller activates the relay
switch, supplying power (+5V) to the CH1786ET. In
this configuration, the ring signal is used to “wake up”
the CH1786ET when the CH1786ET is in the power
down state.
The pins of the CH1786ET will be in an undefined
state when power is switched off. This must be taken
into consideration to assure that no unwanted signals
are presented to the CH1786ET during power down.

Ring Indicator (RI). The RI pin follows the frequency
of the ring signal and toggles low when the CH1786
detects an incoming call. The ring signal is typically
20 to 30 Hz and is on for 2 seconds and off for 4
seconds. Although not TTL compliant, the RI pin can,
nonetheless, be utilized to activate external circuitry
including the external RST pin.

When using the RI pin, it is recommended that a
Schmitt Trigger or the Isolated Envelope Detect
Circuit in Figures 3 or 5 be placed between the RI pin
and the external load.

Power Supply. The CH1786 modem module is a
complex set of sub-systems. During the course of

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normal operation the CH1786 decodes analog signals
from the telephone line that are in the millivolt range.
Steps must be taken by the user to guarantee that
power supply noise on all supply lines, including
ground, does not exceed 50 mV peak to peak. Any
frequency between 20 kHz and 150 kHz must be less
than 500 micro volts peak. If necessary, use
dedicated power and ground planes. Failure to
provide such operating conditions could cause the
CH1786 to malfunction or to function erratically.

The CH1786 requires a single +5V ±5% supply. It is
recommended that by-pass capacitors be placed on
the power supply as close to the modem’s supply
input as practical. It’s recommended a 10 µF
Tantalum capacitor in parallel with a 0.01 µF ceramic
capacitor be used.

Voice/DTMF Tone Injection Port. The CH1786
provides two pins to allow the user to share the
telephone line interface associated with the modem
for voice and DTMF applications. Figure 4 contains a
schematic indicating one possible configuration for
voice/ tone utilization.

Typically, voice communication would precede data
communications. For this case, the following
commands should be used to configure the CH1786
(Refer to Figure 4):

Figure 4. Voice/Tone Injection.

Enter: ATS0=<CR> Disables auto-answer
Result: OK

Enter: ATS7=255<CR> Disables data-carrier time
Result: OK out

Enter: ATS10=255<CR> Disables lost-carrier time
Result: OK out

The following command sequences illustrate
implementation of common Voice/Tone Port
applications.

1. To answer a voice call.
(a) Enter: ATH1<CR>
Result: OK

(b) Drive V.INJ. HIGH to activate relay.
(c) Begin voice conversation.

2. To switch to data mode.
(a) Drive V.INJ. LOW
(b) At the originate modem:
Enter: ATX1<CR>

Result: OK

3. To disconnect (hang-up) a voice call or a data
call.
(a) Enter: ATH<CR>

Result: OK

4. To place a call to 1234567 using the CH1786’s
DTMF tone generator.
(a) Enter: ATDT1234567;C0<CR>

Result: OK

(b) Drive V.INJ. HIGH

MODEM CONTROL

The CH1786 modem may be controlled by sending
serial ASCII command sequences. The commands
are sent to the modem serially on the TXD pin. After
execution of a received command, the modem
returns a serial status message that can be read on
pin RXD. This message indicates command
completion status. Refer to Table 3 for a complete
list of status messages.

INITIALIZING THE MODEM

Before commands may be sent to the CH1786, the
CH1786 must be initialized. Initialization is a two step
activity consisting of:

1. Hard Reset. This is accomplished by applying a

hardware reset pulse to pin RST or by switching
the power off and then back on.

2. Initial modem training. The CH1786 must be

trained to the host ’s speed (2400,1200,300bps)
and parity (odd, even, mark, space or none).

These activities are briefly described below.

WARNING: The CH1786 has been FCC Part 68
approved as a data modem only. Utilization of
the Voice/DTMF Tone Port requires further
registration. FCC will require that the system,
including the CH1786 and the handset or DTMF
transceiver, adhere to Part 68 rules. Voice/Tone
Injection is not active on CH1786NH.

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Reset. Upon applying power to the CH1786, the
CH1786 automatically generates an internal reset
pulse. The user may also reset the modem externally
by applying a high-going reset pulse to the RST pin
for at least 10ms after the +5V power supply has
stabilized. Delay sending commands to CH1786 for
200ms after reset has been initiated to allow the
CH1786 time to properly reconfigure.

Training the Modem. Each modem must be trained
to match its host’s speed and parity so that it is able
to recognize serial asynchronous commands sent to it
by the host’s is UART. The host must retrain the
modem each time a reset pulse is applied on RST or
after a RESET serial command has been issued.

Modem Training Command Sequence. The
CH1786 is trained by sending it the following
sequence:

Enter: AT<CR>
Result: OK
Where: A and T may be either upper or lower case

but must be the same case.
<CR> represents carriage return

The AT sequence is referred to as an attention
sequence. The CH1786 will respond with one of the
following status messages, depending on whether it
is optioned for Terse (abbreviated) or Verbose
(English) status messages.

Result: 0<CR> (Terse)
or
Result: <CR><LF>OK<CR><LF> (Verbose)
Where: <CR> represents carriage return (ASCII 13

or HEX 0D).
<LF> represents line feed (ASCII 10 or
HEX 0A).

After responding with an OK, the CH1786 is in idle
mode and is ready to accept additional commands.
An alternate attention sequence A/ may be sent
which behaves much like the AT sequence except
that it causes the previously entered command
specified with an AT prefix to be executed. When
given, both the AT and A/ must be in upper case
ASCII. No carriage return is needed for the A/
command.

THE COMMAND FORMAT

Typical commands consist of three elements: the
attention sequence, the commands themselves, and
a terminating carriage return.

AT[commands]<CR>.
Where: AT represents attention sequence.

[Commands] represents command strings.

<CR> represents carriage return (ASCII 13

or HEX 0D).

When entering commands to the modem, the
backspace character-control-H (ASCII 8 or HEX 08)
may be used to edit mistakes. AT and A/ may not be
edited. Multiple commands may be placed in the
command line. A command line may be as long as
40 characters, excluding the letters AT. By way of
example, the command below instructs the CH1786
to configure itself to not echo characters when in
command mode E0 and then put itself in answer
mode A.

Enter: ATE0A<CR>
Result: OK

AT Command Set. The available command set is
divided into four types of commands: dial modifiers,
basic commands, ampersand and percent
commands. Refer to the complete list in Table 5.

AT Command Data Rate. With the serial interface,
the rate is speed sensed for parity and format.

THE STATUS MESSAGES

The CH1786 responds with a status message after
each command is executed. This status message
may either be a single digit followed by a carriage
return or a carriage return and line feed with a
message in English, followed by a carriage return and
line feed.
The basic status code subsets are enabled with the
Xn command. Where n=0,1,2,3,4 establishes the
result codes allowed by the user.

X0: Result Codes 0, 1, 2, 3, 4 allowed
X1: Result Codes 0, 1, 2, 3, 4, 5, 10 allowed
X2: Result Codes 0,1, 2, 3, 4, 5, 6, 10 allowed
X3: Result Codes 0, 1, 2, 3, 4, 5, 7, 10 allowed
X4: Result Codes 0, 1, 2, 3, 4, 5, 6, 7, 10 allowed.

NOTE

The CH1786 is factory set to X4, which allows all
result codes.

MODEM STATES

The CH1786 can be in either command mode or data
mode. When the modem is idle, it is in command
mode by default and will recognize commands.
When data transmission is in progress, the CH1786 is
in the data mode state and will not recognize
commands. To force the CH1786 to recognize
commands, the host must send an escape sequence
to the CH1786 forcing it out of data mode and into
command mode.

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Figure 5. CH1786 Application Diagram