CH1786A Family of Ultra Small 2400bps Modems
INTRODUCTION
The CH1786A represents the next generation of full
function, FCC Part 68 approved 2400bps modems in
the CH1786 Family. These modems provide a fast,
easy and flexible way to integrate a modem into any
OEM product while utilizing the minimum amount of
peak surge protection minimum.
PCB space (1.01”x 1.27”x 0.52”).
The CH1786A
family only requires two external interfaces: a ITU-T
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 CH1786A can be controlled with industry standard
standby mode.
AT commands and, hence, is compatible with
available industry communication software. DTE
speed conversion allows commands and data to be
adaptive
and
fixed
compromise
sent at rates that differ from the modem connection
equalization.
speed.
In most cases, the CH1786A can be used
interchangeably with the CH1786. See Attachment I
for equivalency comparisons and differences by
and telephone numbers.
product.
All
CH1786A
modems
support
asynchronous
operation at 2400bps, 1200bps, and 300bps to both
Bell and ITU-T standards.
The CH1786A also
supports
Fast
Connect
and
LAPM/MNP
Error
Correction.
The resident PSTN line interface, or Data Access
Arrangement (DAA), while being FCC approved, is
also Industry Canada CS-03 Part I Registered.
Further, the CH1786A can be approved in other
countries that require 1500VAC RMS isolation
requirements per UL 1950 Edition 3.
The CH1786A family of modems operate off a single
5-volt supply. The low power operation and automatic
standby mode make these modems ideally suited for
Figure 1. Functional Block Diagram of CH1786A.
portable equipment. In addition, their small physical
size affords maximum flexibility in equipment design.
DESCRIPTION OF FUNCTIONAL BLOCKS
CH1786A comes with FCC Part 68 approval and is
AND DISCUSSION OF BASIC OPERATIONS
shipped from the factory with an FCC Part 68 label
Figure 1 contains a functional block drawing of the
indicating
the
registration
number
and
ringer
CH1786A. The CH1786A is a highly integrated, full-
equivalent. This label should be prominently displayed
function
modem,
comprised
of
a
modulator/
on the end product.
demodulator, controller, NVRAM and an FCC Part 68
approved and UL 1950 Edition 3/CSA C22.2 950 listed
PSTN line interface.
FEATURES
Modulation/Demodulation and Modem Controller.
and Bell 103.
These functions are provided by a monolithic modem
integrated circuit.
This IC has built-in facilities to
US:B46MM07BCHXXXXAYY
accommodate integrated AT command control and
contains the necessary resident interfaces for general
IC:176A-CHXXXXAY
communication and routing to the DAA.
2006 Cermetek Microelectronics, Inc.
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Document No. 607-0019 Revision A8 (03/06)
Cermetek Microelectronics, Inc.
CH1786A Family of Ultra Small 2400bps Modems
Telephone Line Interface or DAA. The CH1786A
SUPPORT FEATURES
AT Command Set. A 50-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.
Figure 2. Voice/ Data Port Control
Speaker Interface.
The SPK output reflects the
receiver analog input and provides a signal that can be
family of modems is designed to meet 1500VAC RMS
used to monitor call progress. The SPK signal can
isolation and provide 2122V peak surge protection.
drive a 300 Ω load directly. Typically, the SPK signal
Consequently, the CH1786A satisfies U.S. FCC Part
is input into an audio power amplifier and the amplifier,
68 and Industry Canada CS-03 Part I requirements,
in turn, drives the speaker coil.
The speaker is
and will meet other international approval agency
activated with the ATMn command.
The speaker
requirements that specify these levels of isolation.
volume is adjusted using the ATLn command, where n
is 0, 1, 2, or 3. Increasing numbers correspond to
With the exception of the U.S., most countries
higher volume.
Figure 5 indicates one method of
(including Canada) require submission of the final
driving an external 8 Ω speaker.
product or system containing the CH1786A to the
appropriate governing regulatory agency for approval.
Phone Control. Cermetek’s CH1786A contains a pin
Typically this is accomplished by submitting the final
called Voice or Data (V/D). The V/D pin toggles high
end product to an independent test house or
when the modem goes off hook. This pin can be used
consultant for evaluation. The test house or consultant
to activate a relay which can switch a telephone on or
then
forwards
the
test
results and
applicable
off the Tip and Ring incoming PSTN lines.
This
documents to the regulatory agency. Call Cermetek
feature allows any telephone associated with the
for a list of suggested consultants.
CH1786A to be disconnected when a data call is in
progress, thereby preventing the data from being
Non-Volatile RAM (NVRAM). The NVRAM present
disturbed by an inadvertent telephone pick-up. See
on the CH1786A is sufficient to store up to two user
Figure 2.
customized modem profile configurations.
The
AT&Wn command will store the active modem
Speed and Parity Selection. Prior to call initiation,
configuration in the selected NVRAM profile by
the host controller trains the modem to the host speed
specifying either an n of 0 or 1. The CH1786A is
(2400, 1200 or 300bps) and parity (odd, even, mark,
delivered with Factory Default setup parameter values
space, or none) via a host-initiated training sequence.
pre-stored in NVRAM profile 0.
This training also sets the speed of the data for
originate calls. The CH1786A will automatically adjust
The
AT&Yn
command
selects
the
modem
to the originator’s speed when answering calls.
configurations to be automatically recalled and made
active upon a reset or power up. The ATZn command
The CH1786A matches the host’s parity when it
immediately recalls and activates a stored profile
returns status messages to the host. During a data
configuration. Refer to Tables 4, 5 and 6 for storable
connection, however, the modem passes parity
S-Registers and available commands.
through without interpretation or alteration.
The NVRAM can save up to four telephone numbers
Sleep Mode. To minimize power consumption, the
with up to 31 digits or modifiers in each telephone
CH1786A includes a power down feature called Sleep
number.
The AT&Zn=s command will store s, the
Mode.
When
activated,
the
CH1786A
will
telephone number dial string. The individual NVRAM
automatically enter Sleep Mode after 1 to 255 seconds
telephone number storage locations are selected by
of inactivity. The inactivity delay is selected using the
specifying an n of 0, 1, 2, or 3.
The ATDTS=n
ATS24 command and defaults to 0 which will disable
command will cause the modem to dial the stored
this feature.
The CH1786A returns to normal
telephone number in location n.
operation when a ring signal is received or upon an
input low signal on the TXD pin. A SLEEP output
signal is available to control power to external devices.
2006 Cermetek Microelectronics, Inc.
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Document No. 607-0019 Revision A8 (03/06)
Cermetek Microelectronics, Inc.
CH1786A Family of Ultra Small 2400bps Modems
In Figure 5, a FET controlled by the SLEEP signal
found
on
the
Cermetek
web
site
located
at
turns off the external speaker amplifier when the
http://www.cermetek.com.
CH1786A enters Sleep Mode. In Sleep Mode, power
is reduced to about 1/3 of normal operating power.
Answer Tone. A ITU-T (2100 Hz) or Bell (2225Hz)
answer tone is generated depending on the selected
Zero Power Mode. The “P” versions of the CH1786A
configuration.
Refer to “AT Commands and S-
Family supports Zero Power Mode operation. If an
Registers” for a more detailed discussion.
application calls for zero power during standby
periods, this feature allows the power to the CH1786A
Data Encoding. The data encoding conforms to ITU-
to be switched off using external circuitry and then
T recommendations V.22bis or V.22, or Bell212A, or
reapplied when the CH1786A’s RI pin becomes active.
103, depending on the selected configuration. Refer
The CH1786A can thus be powered down to zero.
to “AT Commands and S-Register” for a more detailed
discussion.
Referring to Figure 3, the controller activates the relay
switch, supplying power (+5V) to the CH1786A. In this
Line Equalization. Transmitter and receiver digital
configuration, the ring signal is used to “wake up” the
filters compensate for delay and amplitude distortion
CH1786A when the CH1786A is in the zero power
during operation on nominal phone lines. In addition,
state.
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
CH1786A-P
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 when the calling modem’s Initial Trained Rate and
answering modem’s Trained Rate are different.
Error Correction. The CH1786A supports LAPM and
MNP error correction. Error correction is enabled via
the \Nn command. Enabling error correction overrides
&Qn and sets S36 and S48 values.
Refer to
Cermetek, “AT Commands and S-Registers”, for a
Figure 3. Zero Power Operation.
more detailed discussion.
As indicated in Figure 2, the user connects an external
Table 1. Connection Rates.
pull-up resistor (Cermetek recommends 30k ohms) to
Connection Rate Resulting
an active 5V source thereby providing a small current
Answering Modem
When Calling Modem Initial
to the CH1786A through the RI pin when the CH1786A
Initial Trained Rate
Rate Is:
is powered down (i.e., no power applied to the VCC
300
1200
2400
pin).
An internal blocking diode provides supply
isolation.
300
300
1200
1200
1200
300
1200
1200
The pins of the CH1786A will be in an undefined state
when powered is switched off. This must be taken into
2400
300
1200
2400
consideration to assure that no unwanted signals are
presented to the CH1786A during power down.
NOTE
A 2400-baud connection rate will only result if both
Guard Tone. A guard tone of 550 Hz or 1800 Hz can
modems are initially set at 2400 baud.
be generated at 6 dB or 9 dB below the transmit level,
respectively, by using the &Gn command. For a more
detailed discussion, refer to Cermetek Document 613-
Ring Indicator (RI). The RI pin follows the frequency
0001, “AT Commands and S-Registers”, which can be
of the ring signal and toggles low when the CH1786A
detects an incoming call. The ring signal is typically 20
to 30 Hz and is on for 2 seconds and off for 4 seconds.
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Document No. 607-0019 Revision A8 (03/06)
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CH1786A Family of Ultra Small 2400bps Modems
Although not TTL compliant, the RI pin can,
schematic indicating one possible configuration for
nonetheless, be utilized to activate external circuitry
voice/ tone utilization.
including the external RST pin.
Typically, voice communication would precede data
+MS Command.
The user may override the
communications.
For this case, the following
automatic data rate function of the HS modems. This
commands should be used to configure the CH1786A
is accomplished by executing the +MS command.
(Refer to Figure 4):
This command allows the user to specify the desired
data modulation protocol. Further, the +MS command
Enter: ATS0=0<CR>
Disables auto-answer
may be used to force a desired data rate and/or
Result: OK
restrict the allowed data rate.
The supported
modulation schemes are listed above.
Enter: ATS7=255<CR>
Disables data-carrier time
Result: OK
out
The
following
examples
establish
the
V.22bis
modulation scheme:
Enter: ATS10=255<CR> Disables lost-carrier time
Result: OK
out
Enter:
AT+MS=V22B
Result:
OK
WARNING
This is identical to the following explicit +MS
The CH1786A has been FCC Part 68 and Industry
command:
Canada CS-03 Part I approved as a data modem
only.
Utilization of the Voice/DTMF Tone Port
Enter:
AT+MS=V22B,0,300,1200,300,1200
requires further registration.
FCC and Industry
Result:
OK
Canada will require that the system, including the
CH1786A and the handset or DTMF transceiver,
adhere to Part 68 and CS-03 Part I rules.
NOTE
The minimum transmission rate is not stored and will
default to 300bps upon execution of reset or upon
The
following
command
sequences
illustrate
power up.
implementation
of
common
Voice/Tone
Port
applications.
Refer to, “AT Commands and S-Registers”, for a more
1. To answer a voice call.
detailed discussion of the +MS command.
(a) Enter: ATH1<CR>
Result: OK
Power Supply. The CH1786A modem module is a
(b) Drive V.INJ. HIGH to activate relay.
complex set of sub-systems.
During the course of
(c) Begin voice conversation.
normal operation the CH1786A decodes analog
signals from the telephone line that are in the millivolt
2. To switch to data mode.
range. Steps must be taken by the user to guarantee
(a) Drive V.INJ. LOW
that power supply noise on all supply lines, including
(b) At the originate modem:
ground, does not exceed 50 mV peak to peak. Any
Enter: ATX1<CR>
frequency between 20 kHz and 150 kHz must be less
Result: OK
than 500 micro volts peak.
If necessary, use
dedicated power and ground planes.
Failure to
3. To disconnect (hang-up) a voice call or a data call.
provide such operating conditions could cause the
(a) Enter: ATH<CR>
CH1786A to malfunction or to function erratically.
Result: OK
The CH1786A requires a single +5V ±5% supply. It is
4. Place a call to 1234567 using the CH1786A’s
recommended that by-pass capacitors be placed on
DTMF tone generator, then use VOICE mode for
the power supply as close to the modem’s supply input
voice communications.
Note that the CO
as practical.
It’s recommended a 10 µ F Tantalum
command disables carrier generation.
capacitor in parallel with a 0.01 µF ceramic capacitor
(a) Enter: ATDT1234567;C0<CR>
be used.
Result: OK
(b) Drive V.INJ. HIGH
Voice/DTMF Tone Injection Port.
The CH1786A
provides two pins to allow the user to share the
telephone line interface associated with the modem for
MODEM CONTROL
voice and DTMF applications.
Figure 4 contains a
The CH1786A modem may be controlled by sending
serial ASCII command sequences. The commands
2006 Cermetek Microelectronics, Inc.
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Document No. 607-0019 Revision A8 (03/06)
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CH1786A Family of Ultra Small 2400bps Modems
are sent to the modem serially on the TXD pin. After
Training the Modem. Each modem must be trained
execution of a received command, the modem returns
to match its host’s speed and parity so that it is able to
a serial status message that can be read on pin RXD.
recognize serial asynchronous commands sent to it by
This message indicates command completion status.
the host’s is UART. The host must retrain the modem
Refer to Table 3 for a complete list of status
each time a reset pulse is applied on RST or after a
messages.
RESET serial command has been issued.
Modem Training Command Sequence.
The
CH1786A 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 CH1786A will respond with one of the
following status messages, depending on whether it is
optioned for Terse (abbreviated) or Verbose (English)
status messages.
Figure 4. Voice/Tone Injection.
Result: 0<CR> (Terse)
or
Result:
<CR><LF>OK<CR><LF> (Verbose)
IMPORTANT NOTE
Where:
<CR> represents carriage return (ASCII 13
Although every effort has been made to design the
or HEX 0D).
CH1786A products to be fully and completely
<LF> represents line feed (ASCII 10 or HEX
compatible with the existing CH1786 family, there are
0A).
some minor differences.
Refer to Table 14 for
equivalency information and refer to Attachment I for
After responding with an OK, the CH1786A is in idle
a summary of product differences.
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
INITIALIZING THE MODEM
causes the previously entered command specified with
Before commands may be sent to the CH1786A, the
an AT prefix to be executed. When given, both the AT
CH1786A must be initialized. Initialization is a two
and A/ must be in upper case ASCII. No carriage
step activity consisting of:
return is needed for the A/ command.
1. Hard Reset. This is accomplished by applying a
hardware reset pulse to pin RST or by switching
THE COMMAND FORMAT
the power off and then back on.
Typical commands consist of three elements: the
attention sequence, the commands themselves, and a
2. Initial modem training.
The CH1786A must be
terminating carriage return.
trained to the host’s speed and parity (odd, even,
mark, space or none).
AT[commands]<CR>.
These activities are briefly described below.
Where: AT represents attention sequence.
[Commands] represents command strings.
Reset. Upon applying power to the CH1786A, the
<CR> represents carriage return (ASCII 13 or
CH1786A automatically generates an internal reset
HEX 0D).
pulse. The user may also reset the modem externally
by applying a high-going reset pulse to the RST pin for
When entering commands to the modem, the
at least 10ms after the +5V power supply has
backspace character-control-H (ASCII 8 or HEX 08)
stabilized. Delay sending commands to CH1786A for
may be used to edit mistakes. AT and A/ may not be
200ms after reset has been initiated to allow the
edited.
Multiple commands may be placed in the
CH1786A time to properly reconfigure.
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 CH1786A
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Cermetek Microelectronics, Inc.
CH1786A Family of Ultra Small 2400bps Modems
to configure itself to not echo characters when in
AT COMMAND APPLICATIONS
command mode E0 and then put itself in answer mode
Dialing A Phone Number. The Dial command takes
A .
the form Dn , where n is a string of characters. In the
Enter: ATE0A<CR>
simplest form, n will be only the digits of the phone
Result: OK
number to be dialed.
AT Command Set. The available command set is
Example: Dial number.
divided into four types of commands: dial modifiers,
Enter:
ATD1234567<CR>
basic commands, ampersand and percent commands.
Result:
OK
Refer to the complete list in Table 5.
In response to this command, the CH1786A dials the
AT Command Data Rate. With the serial interface,
telephone number “123-4567” and then waits for a
the rate is speed sensed for parity and format.
carrier from the distant modem.
If no carrier is
detected within a given time (the default time is 30
seconds), the CH1786A automatically releases the line
THE STATUS MESSAGES
and sends a NO CARRIER result code. If a carrier is
The CH1786A responds with a status message after
detected, the CH1786A gives a CONNECT result code
each command is executed.
This status message
and goes on-line, permitting communication with the
may either be a numeric code (Terse Result Code)
distant modem.
followed by a carriage return; or a carriage return and
line feed with a message in English (Verbose Result
The Dial Command may also be issued without a