CH1786 Family of Ultra Small 2400bps Modems
INTRODUCTION
FEATURES
The CH1786 family of modems are a full function,
Standards
ITU-U
V.22bis,V.22,Bell
FCC Part 68 approved 2400bps modem.
These
212,and Bell 103
modems provide a fast, easy and flexible way to
integrate a modem into any OEM product while
approvable
utilizing the minimum amount of PCB space (1.01 ”x
1.27 ”x 0.52 ”). The CH1786 family only requires two
external interfaces: a ITU-U 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,
peak surge protection minimum
hence,
is
compatible
with
available
industry
communication software.
standby mode
All CH1786 modems support asynchronous operation
at 2400bps, 1200bps, and 300bps to both Bell and
adaptive
and
fixed
compromise
ITU-U standards. The resident PSTN line interface,
equalization
or Data Access Arrangement (DAA), while being FCC
approved, is also Canadian DOT approvable and can
be approved in other countries that require 1500VAC
and telephone numbers
RMS isolation requirements per UL 1950 Edition 3.
The CH1786 family of modems operate off a single 5-
volt supply. The low power operation and automatic
CH1786 FAMILY
standby mode make these modems ideally suited for
CH1786
NVRAM, Voice/ Inject, Hermetic,
portable equipment. In addition, their small physical
Operating Temperature:
size affords maximum flexibility in equipment design.
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 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.
Figure 2. Voice/ Data Port Control
2004 Cermetek Microelectronics, Inc.
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Document No. 607-0004 Revision L4 (08/04)
Cermetek Microelectronics, Inc.
CH1786 Family of Ultra Small 2400bps Modems
DESCRIPTION OF FUNCTIONAL BLOCKS
AT Command Set. A 40-character command line is
AND DISCUSSION OF BASIC OPERATIONS
supported. The command line starts with AT and
Figure 1 contains a functional block drawing of the
may contain standard or enhanced commands. The
CH1786. The CH1786 is a highly integrated, full-
commands are compatible with EIA Document
function
modem,
comprised
of
a
modulator/
TR302.2/88-08006.
demodulator, controller, NVRAM and an FCC Part 68
approved and UL 1950/CSA C22.2 950 listed PSTN
Serial Host Interface. The serial interface is a V.24
line interface.
(EIA-232) compatible interface. Ten Bits total: 1 start
bit, 1 stop bit, and eight data bits which can be either
Modulation/Demodulation and Modem Controller.
eight bits with no parity or seven odd or even with
These functions are provided by a monolithic modem
parity. The start bit is LOW going. RXD and TXD
integrated circuit.
This IC has built-in facilities to
data is non-inverted. See pin description in Table 2.
accommodate integrated AT command control and
contains the necessary resident interfaces for general
Speaker Interface.
The SPK output reflects the
communication and routing to the DAA.
receiver analog input and provides a signal that can
be used to monitor call progress. The SPK signal can
Telephone Line Interface or DAA. The CH1786
drive a 300 Ω load directly. Typically, the SPK signal
family of modems is designed to meet 1500VAC RMS
is input into an audio power amplifier and the
isolation and provide 2122V peak surge protection.
amplifier, in turn, drives the speaker coil.
The
Consequently, the CH1786 satisfies U.S. FCC Part
speaker is activated with the ATMn command. The
68
and
DOT
CSA
CS-03
Part
I
Canadian
speaker volume is adjusted using the ATLn
requirements, and will meet other international
command, where n is 0, 1, 2, or 3.
Increasing
approval agency requirements that specify these
numbers correspond to higher volume.
Figure 6
levels of isolation.
indicates one method of driving an external 8 Ω
speaker.
With the exception of the U.S., most countries
(including Canada) require submission of the final
Phone Control. Cermetek’s CH1786 contains a pin
product or system containing the CH1786 to the
called Voice or Data (V/D). The V/D pin toggles high
appropriate governing regulatory agency for approval.
when the modem goes off hook. This pin can be
Typically this is accomplished by submitting the final
used to activate a relay which can switch a telephone
end product to an independent test house or
on or off the Tip and Ring incoming PSTN lines. This
consultant for evaluation.
The test house or
feature allows any telephone associated with the
consultant then forwards the test results and
CH1786 to be disconnected when a data call is in
applicable documents to the regulatory agency. Call
progress, thereby preventing the data from being
Cermetek for a list of suggested consultants.
disturbed by an inadvertent telephone pick-up. See
Figure 2.
Non-Volatile RAM (NVRAM). The NVRAM present
on the CH1786 is sufficient to store up to two user
Speed and Parity Selection. Prior to call initiation,
customized modem configurations.
The AT&Wn
the host controller trains the modem to the host
command will store the active modem configuration in
speed (2400,1200 or 300bps) and parity (odd, even,
the selected NVRAM locations by specifying an n of 0
mark, space, or none) via a host-initiated training
or 1.
sequence. This training also sets the speed of the
data
for
originate
calls.
The
CH1786
will
The
AT&Yn
command
selects
the
modem
automatically adjust to the originator’s speed when
configurations to be automatically recalled and made
answering calls.
active upon a reset or power up.
The ATZn
command immediately recalls and activates a stored
The CH1786 matches the host’s parity when it returns
configuration. Refer to Tables 4, 5 and 6 for storable
status messages to the host.
During a data
S-Registers and available commands.
connection, however, the modem passes parity
through without interpretation or alteration.
The NVRAM can save up to four telephone numbers
with up to 36 digits or modifiers in each telephone
Sleep Mode. To minimize power consumption, the
number. The AT&Zn=s command will store s, the
CH1786 includes a power down feature called Sleep
telephone number dial string. The individual NVRAM
Mode.
When
activated,
the
CH1786
will
telephone number storage locations are selected by
automatically enter Sleep Mode after 0 to 254
specifying an n of 0, 1, 2, or 3.
The ATDTS=n
seconds of inactivity. The inactivity delay is selected
command will cause the modem to dial the stored
using the ATS24 command.
The CH1786 is
telephone number in location n. NVRAM is not
delivered from the factory with a 5 second inactivity
available on the CH1786LC or CH1786NH.
delay enabled (i.e., ATS24=5 ). The CH1786 returns
SUPPORT FEATURES
to normal operation when a ring signal is received or
2004 Cermetek Microelectronics, Inc.
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Document No. 607-0004 Revision L4 (08/04)
Cermetek Microelectronics, Inc.
CH1786 Family of Ultra Small 2400bps Modems
upon an input low signal on the TXD pin.
pin becomes active. The CH1786ET products can
ATS24=255 disables the Sleep Mode and is the
thus be powered down to zero.
default if no value is set in register S24.
Table 1. Connection Rates.
A SLEEP output signal is available to control power to
Answering
Connection Rate Resulting
external devices. In Figure 6, a FET controlled by the
Modem Initial
When Calling Modem Initial
SLEEP signal turns of the external speaker amplifier
Trained Rate
Rate Is:
when the CH1786 enters Sleep Mode.
In Sleep
300
1200
2400
Mode, power is reduced to approximately 50% of
300
300
1200
1200
normal operating power.
1200
300
1200
1200
2400
300
1200
2400
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 ITU-U (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 ITU-
U 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
Figure 3. Zero Power Operation.
during operation on nominal phone lines. In addition,
automatic adaptive equalization in the receiver
Referring to Figure 3, the controller activates the relay
minimizes the effects of inter symbol interference.
switch, supplying power (+5V) to the CH1786ET. In
this configuration, the ring signal is used to “wake up”
Transmission Speed.
In normal operation, the
the CH1786ET when the CH1786ET is in the power
originating modem initiates the call and attempts to
down state.
connect to the answering modem at a speed
The pins of the CH1786ET will be in an undefined
established by the originating modem’s controller
state when power is switched off. This must be taken
prior to call initiation. This is referred to as the Initial
into consideration to assure that no unwanted signals
Trained Rate. Upon receiving the call, the answering
are presented to the CH1786ET during power down.
modem will attempt to connect to the originating
modem at its Initial Trained Rate. If these two rates
Ring Indicator (RI). The RI pin follows the frequency
are identical, the connection is made. If the speeds
of the ring signal and toggles low when the CH1786
differ, the answering modem must adjust its rate or
detects an incoming call. The ring signal is typically
terminate the call. Table 1 indicates the connection
20 to 30 Hz and is on for 2 seconds and off for 4
rate that will result when the calling modem’s Initial
seconds. Although not TTL compliant, the RI pin can,
Trained Rate and the answering modem’s Initial
nonetheless, be utilized to activate external circuitry
Trained Rate are different.
including the external RST pin.
NOTE
When using the RI pin, it is recommended that a
A 2400-baud connection rate will only result if both
Schmitt Trigger or the Isolated Envelope Detect
modems are initially set at 2400 baud.
Circuit in Figures 3 or 5 be placed between the RI pin
and the external load.
Zero Power Mode (CH1786ET only).
If an
application calls for zero power during standby
Power Supply. The CH1786 modem module is a
periods, the power may be switched off using external
complex set of sub-systems. During the course of
circuitry and then reapplied when the CH1786’s RI
normal operation the CH1786 decodes analog signals
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CH1786 Family of Ultra Small 2400bps Modems
from the telephone line that are in the millivolt range.
Enter: ATS7=255<CR>
Disables data-carrier time
Steps must be taken by the user to guarantee that
Result: OK
out
power supply noise on all supply lines, including
ground, does not exceed 50 mV peak to peak. Any
Enter: ATS10=255<CR> Disables lost-carrier time
frequency between 20 kHz and 150 kHz must be less
Result: OK
out
than 500 micro volts peak.
If necessary, use
dedicated power and ground planes.
Failure to
The following command sequences illustrate
provide such operating conditions could cause the
implementation of common Voice/Tone Port
CH1786 to malfunction or to function erratically.
applications.
The CH1786 requires a single +5V ±5% supply. It is
1. To answer a voice call.
recommended that by-pass capacitors be placed on
(a) Enter: ATH1<CR>
the power supply as close to the modem’s supply
Result: OK
input as practical.
It’s recommended a 10 µ F
Tantalum capacitor in parallel with a 0.01 µF ceramic
(b) Drive V.INJ. HIGH to activate relay.
capacitor be used.
(c) Begin voice conversation.
Voice/DTMF Tone Injection Port.
The CH1786
2. To switch to data mode.
provides two pins to allow the user to share the
(a) Drive V.INJ. LOW
telephone line interface associated with the modem
(b) At the originate modem:
for voice and DTMF applications. Figure 4 contains a
Enter: ATX1<CR>
schematic indicating one possible configuration for
Result: OK
voice/ tone utilization.
3. To disconnect (hang-up) a voice call or a data
WARNING
call.
The CH1786 has been FCC Part 68 approved as a
(a) Enter: ATH<CR>
data modem only. Utilization of the Voice/DTMF
Result: OK
Tone Port requires further registration. FCC will
require that the system, including the CH1786 and
4. To place a call to 1234567 using the CH1786’s
the handset or DTMF transceiver, adhere to Part 68
DTMF tone generator.
rules.
Voice/Tone Injection is not active on
(a) Enter: ATDT1234567;C0<CR>
CH1786NH.
Result: OK
(b) Drive V.INJ. HIGH
Typically, voice communication would precede data
communications.
For this case, the following
commands should be used to configure the CH1786
MODEM CONTROL
(Refer to Figure 4):
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
Figure 4. Voice/Tone Injection.
trained to the host’s speed (2400, 1200, 300bps)
and parity (odd, even, mark, space or none).
Enter: ATS0=<CR>
Disables auto-answer
Result: OK
These activities are briefly described below.
2004 Cermetek Microelectronics, Inc.
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Cermetek Microelectronics, Inc.
CH1786 Family of Ultra Small 2400bps Modems
Reset. Upon applying power to the CH1786, the
<CR> represents carriage return (ASCII 13
CH1786 automatically generates an internal reset
or HEX 0D).
pulse. The user may also reset the modem externally
by applying a high-going reset pulse to the RST pin
When entering commands to the modem, the
for at least 10ms after the +5V power supply has
backspace character-control-H (ASCII 8 or HEX 08)
stabilized. Delay sending commands to CH1786 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
CH1786 time to properly reconfigure.
command line. A command line may be as long as
40 characters, excluding the letters AT . By way of
Training the Modem. Each modem must be trained
example, the command below instructs the CH1786
to match its host’s speed and parity so that it is able
to configure itself to not echo characters when in
to recognize serial asynchronous commands sent to it
command mode E0 and then put itself in answer
by the host’s is UART. The host must retrain the
mode A .
modem each time a reset pulse is applied on RST or
after a RESET serial command has been issued.
Enter: ATE0A<CR>
Result: OK
Modem Training Command Sequence.
The
CH1786 is trained by sending it the following
AT Command Set. The available command set is
sequence:
divided into four types of commands: dial modifiers,
basic
commands,
ampersand
and
percent
Enter: AT<CR>
commands. Refer to the complete list in Table 5.
Result: OK
Where: A and T may be either upper or lower case
AT Command Data Rate. With the serial interface,
but must be the same case.
the rate is speed sensed for parity and format.
<CR> represents carriage return
The AT sequence is referred to as an attention
THE STATUS MESSAGES
sequence. The CH1786 will respond with one of the
The CH1786 responds with a status message after
following status messages, depending on whether it
each command is executed. This status message
is optioned for Terse (abbreviated) or Verbose
may either be a single digit followed by a carriage
(English) status messages.
return or a carriage return and line feed with a
message in English, followed by a carriage return and
Result: 0<CR> (Terse)
line feed.
or
The basic status code subsets are enabled with the
Result:
<CR><LF>OK<CR><LF> (Verbose)
Xn command.
Where n=0,1,2,3,4 establishes the
Where:
<CR> represents carriage return (ASCII 13
result codes allowed by the user.
or HEX 0D).
<LF> represents line feed (ASCII 10 or
X0:
Result Codes 0, 1, 2, 3, 4 allowed
HEX 0A).
X1:
Result Codes 0, 1, 2, 3, 4, 5, 10 allowed
X2:
Result Codes 0,1, 2, 3, 4, 5, 6, 10 allowed
After responding with an OK, the CH1786 is in idle
X3:
Result Codes 0, 1, 2, 3, 4, 5, 7, 10 allowed
mode and is ready to accept additional commands.
X4:
Result Codes 0, 1, 2, 3, 4, 5, 6, 7, 10 allowed.
An alternate attention sequence A/ may be sent
which behaves much like the AT sequence except
NOTE
that it causes the previously entered command
The CH1786 is factory set to X4, which allows all
specified with an AT prefix to be executed. When
result codes.
given, both the AT and A/ must be in upper case
ASCII.
No carriage return is needed for the A/
command.
MODEM STATES
The CH1786 can be in either command mode or data
THE COMMAND FORMAT
mode. When the modem is idle, it is in command
Typical commands consist of three elements: the
mode by default and will recognize commands.
attention sequence, the commands themselves, and
When data transmission is in progress, the CH1786 is
a terminating carriage return.
in the data mode state and will not recognize
commands.
To force the CH1786 to recognize
AT[commands]<CR>.
commands, the host must send an escape sequence
Where: AT represents attention sequence.
to the CH1786 forcing it out of data mode and into
[Commands] represents command strings.
command mode.
2004 Cermetek Microelectronics, Inc.
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Document No. 607-0004 Revision L4 (08/04)
Cermetek Microelectronics, Inc.
CH1786 Family of Ultra Small 2400bps Modems
Figure 5. CH1786 Application Diagram
2004 Cermetek Microelectronics, Inc.
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Document No. 607-0004 Revision L4 (08/04)
Cermetek Microelectronics, Inc.
CH1786 Family of Ultra Small 2400bps Modems
Table 2. CH1786 Pin Descriptions
PIN
NAME
TYPE
FUNCTION
1
RING
I/O
RING . Directly connects to the telephone line’s Ring lead through a user supplied
RJ-11C jack.
2
TIP
I/O
TIP . Directly connects to the telephone line’s Tip lead through a user supplied RJ-
11C jack.
3
RXA
O
ANALOG VOICE INJECTED . Receive Signal. Let float if not used. Not active in
CH1786NH.
4
TXA
I
ANALOG VOICE INJECTED . Transmit Signal. Let float if not used. Not active in
CH1786NH.
5
SPK
O
SPEAKER . Audio output for speaker. See speaker control schematic in