C
ermetek
microelectronics
CH1787 Small Functional Modem
INTRODUCTION
The CH1787 is a small full function modem that is FCC Part
68 approved. It has been designed to be used in
applications where there is little or no controller intelligence
to command the modem. This allows the user to operate
the modem via hardware resources only, not requiring AT
Command execution for basic operation. For those
applications where there is controller intelligence, the
CH1787 operates like a standard AT Command driver
2400bps, V.22bis asychronous modem.
The CH1787 is ideal as a remote modem such as found in
alarm products and in industrial controllers. Upon some
operational event, the modem will dial a prestored telephone
number under pin activation to make a modem connection
with a host or another site. The modem can also be set to
automatically answer incoming calls or manually answer a
call, using an answering pin. A call can be ended via a
hang-up pin.
FEATURES
GENERAL DESCRIPTION OF FUNCTIONAL
BLOCKS
Figure 1 is a functional block diagram of the CH1787. Each
block will be described. The CH1787 is a highly integrated,
full function modem, comprised of a modulator/demodula-
tor, controller, and an FCC Part 68 approved telephone in-
terface, also called a Data Access Arrangement (DAA) and
NVRAM.
Supports Standards CCITT V.22bis, V.22,
Bell 212, and Bell 103
FCC Part 68 approved and DOC approvable
Does not require a microprocessor to operate
Pin activated hang-up
Pin activated answer
Manual originate and answer pins
AT Command structure available
1000 VAC isolation barrier
Single 5 volt operation
Low power sleep mode
Automatic adaptive and fixed compromise
equalization
Test modes and diagnostics
Size: 2.0 x 1.25 x 0.53
NVRAM allows storage of custom configurations
and telephone numbers
Operating temperature 0
o
-70
o
C
Extended temperature -40
o
C to 85
o
C order
CH1787ET
Modulation/Demodulation and Control
This Functional Block is comprised of a monolithic modem
integrated circuit, with built-in facilities to accommodate in-
tegrated AT command control and resident interfaces for
general communication and routing to the DAA.
Controller
The controller is a programed microprocessor that provides
commands to the modem in response to external pin acti-
vation. The following pins are controlled by the micropro-
cessor and are described in detail in Table 1. These pins
are operational when the modem is in use at 2400bps only.
ANS - Manual Answer Pin Input - Answer mode-
Places modem in answer mode
ORG - Manual Originate Pin Input - Originate mode -
places modem in originate mode
ORA- Automatic Dial Pin Input - Dials one of two
prestored numbers based on TST
TST - Steering Pin Input used with ORA to select the user
stored number or Cermetek test number
HNG- Hang-up Pin Input used to terminate an active
modem connected
AAR- Automatic Answer Pin - used to place the modem
in Auto Answer Mode
Figure 1. CH1787 Functional Block Diagram
See pin description Table 1 for more details.
Cermetek Microelectronics, Inc.
Document No. 603-0182 Rev. D (11/98)
1
DAA
The CH1787 is designed to meet North American telephone stan-
dards as set by FCC Part 68 and DOC. The telephone line inter-
face is designed to meet 1000VAC and 1500 volt peak surge
isolation, among other parameters. As such, it will meet U.S. and
Canadian requirements and other international requirements that
specify that level of isolation. The CH1787 comes with FCC Part
68 approval, a label is provided with the registration number and
ringer equivalent. This label should be prominently displayed.
As with most countries, except the U.S., Canada requires sub-
mission of the product containing the CH1787 for DOC approval.
This can be done by submitting the design to a test house or
consultant. Call Cermetek for a consultant list.
NVRAM
NVRAM can save up to two user-customized modem configura-
tions. The AT&Wn command will store the active modem con-
figuration in one of two NVRAM locations as selected by an n of 0
or 1. The AT&Yn command selects one of the stored modem
configurations to be automatically recalled and active upon a re-
set or power up. The ATZn command immediately recalls and
activates a stored configuration. See Tables 2 and 3 for storable
S-Registers and Commands.
NVRAM can save up to four telephone numbers with up to 36
digits or modifiers in each telephone number. The AT&Zn=s com-
mand will store s, the telephone number dial string. The ATDTS=n
command will cause the modem to dial one of the four stored
telephone numbers. The NVRAM storage location for the four
telephone numbers is selected by an n of 0, 1, 2, or 3. Location
1
is used for the ORA stored number and Cermetek number.
Figure 2. Voice/Data Port Control
SLEEP MODE
Supported Features
AT Command Set
A 40-character command line is supported. The command line
starts with AT and may contain standard or enhanced commands.
See
Cermetek
detailed
command
summary-
www.cermetek.com.
To minimize the modem power consumption, the CH1787 includes
a power down feature called the Sleep Mode. When activated the
CH1787 will automatically enter the Sleep Mode after 0 to 255
seconds of inactivity. The time of inactivity is selected by the ATS24
command and 5 seconds is the default time. The modem returns
to normal operation when a ring signal is received or is upon an
input low signal on TXD. ATS24 = 255 disables the Sleep Mode
and is the default.
_____
A SLEEP output signal is available to control power to external
devices. In Figure 5, a FET controlled by the SLEEP signal turns off
the external speaker amplifier when the modem enters the Sleep
Mode. In the sleep mode, power is reduced by 50% of normal
operating power.
Serial Host Interface
The serial interface is V.24 (EIA-232-D) compatible interface. See
pin description.
Transmission Speed
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. The SPK signal is usually input to
an audio power amplifier and the amplifier drives a speaker coil.
Figure 5 shows how to drive an 8
Ω
speaker.
The speaker can be turned on and off with the ATMn command.
The speaker volume can be adjusted by the ATLn command,
where n is 0, 1, 2, or 3.
The transmission rate of the host computer must be 300, 1200, or
2400 bps. The modem will connect at the selected speed or will
fall back to the speed set by the remote or answering modems
serial interface (the DTE transmission speed). That determines
the originate speed. The modem can be either originating (calling
modem) or answering (remote modem). The following table
shows the speeds:
Originate Mode
Answer Mode
300
1200
2400*
300
300
1200
1200
1200
300
1200
1200
2400
300
1200
2400
*
Pin activated operation at 2400bps only. Other speeds may be
used with AT Command operation.
Phone Control
The CH1787 contains a pin called Voice/Data (V/D). This pin
toggles high when the modem goes off hook and can be used to
activate a relay which can switch a telephone on or off the Tip and
Ring Telco lines. This allows the telephone to be disconnected
when a data call is in progress, preventing the data from being
disturbed by an inadvertent telephone pick-up. See Figure 2.
Speed and Parity Selection
Before a call, the modem adjusts to the host speed (2400, 1200, or
300 bps) and parity (odd, even, mark, space, or none) via a host-
initiated training sequence. This also selects the speed of the
data for originate calls. The modem automatically adapts to the
callers speed on answer calls.
The modem matches the hosts parity when it returns status mes-
sages to the host. During a data connection, however, the modem
passes parity through without interpretation or alteration.
Cermetek Microelectronics, Inc.
Document No. 603-0182 Rev. D (11/98)
2
POWER SUPPLY
The modem module is a complex sub-system that may be treated
as any other component. Special attention should be paid to the
power supply connections. The modem decodes analog signals
from the telephone line that are in the millivolt range. Al though the
modem is designed to withstand significantly induced power sup-
ply noise, there is a limit. Steps must be taken 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 50 mV peak. If necessary, use dedi-
cated power and ground planes. Failure to provide such operating
conditions could cause the modem to malfunction.
The CH1787requires a single +5V ±5% supply. It is recommended
that by-pass capacitors be placed on the power supply as close to
the modems supply input as practical. It is recommended that a
10
µF
Electrolytic capacitor in parallel with a 0.01µF ceramic
capacitor be used.
WARNING: The CH1787 has been FCC Part 68 approved as a
data modem. Utilization of the Voice/Tone Port requires further
registration. FCC will require that the system, including the CH1787
and the handset or DTMF transceiver, adhere to Part 68 rules.
ENABLE RXA
4.7K
MECHANICAL RELAY ONLY.
A ZERO IMPEDANCE SWITCH MAY REPLACE
THE RELAY AND TRANSISTOR CIRCUIT.
+5V
2N2222
1N914
1 RING
2 TIP
+15V
20K
3
20K
+
2
-
TL064
0.1uF
82K
0.1uF
20K
63.4K
VOICE OR DTMF
SIGNAL
A1
54K
20K
0.1uF
2.2uF
nonpolar
4
1
CH1787
A
499
B
7 RXA
8 DTR
9 ANS
10 V/D
11 RST
12 RI
13 CTS
14 DSR
15 DRA
16 TXD
17 DIR
18 IRQ
19 SLEEP
AAR 34
GND 33
VCC 32
RXD
HNG
CON
TST
SPKR
NC
NC
NC
DAG
31
30
29
28
27
26
25
24
23
HS 22
DCD 21
GND
20
6
7
0.1uF
11
-
5
+
TL064
28K
A2
0.1uF
ADJUST A2 FOR THE DESIRED LEVEL OF AXT VOICE TO BE ECHOED TO AXR.
DECREASE RECEIVE GAIN SIGNAL. AXR. BY DECREASING VALUE OF A1.
Figure 3. Voice/Tone Injection
Power-up Reset
After applying power to the modem, an internally generated reset
pulse is created. The user can also reset the modem externally by
applying the high-going reset pulse to RST for at least 10ms after
the +5V power supply has stabilized. Delay sending commands
to CH1787 for 100-200ms.
MODEM CONTROL
The CH1787 modem may be controlled by sending serial ASCII
command sequences. The commands are sent to the modem
serially on TXD. After execution of the command, the modem re-
turns a serial status message on RXD, to indicate the completion
status of the command.
Training the Modem
The modem must be trained to match the hosts speed and parity
so that it is able to recognize serial asynchronous commands
sent to it by the host UART. The host must retrain the modem each
time a reset signal is applied on RST or after a RESET serial
command. The modem is trained by sending it the following three-
character sequence.
Initializing the Modem
Before commands may be sent to the modem, the modem must
be initialized. This consists of two events: 1) after power-up, a
hardware reset pulse must be applied to the modem, and 2) the
modem must be trained to the hosts speed (2400, 1200, 300bps)
and parity (odd, even, mark, space or none).
+Vcc
3
+Vcc
+Vcc
RI
FROM 1787
PIN 12
+
8
1
22K
4.7K
5
+
4.7uF
+
LM393
6
ISOLATED
ENVELOPE
RING DETECT
7
LM393
2
4
10K
.1uF
10K
Figure 4. CH1787 Application Diagram of Test Circuit
Cermetek Microelectronics, Inc.
Document No. 603-0182 Rev. D (11/98)
3
Table 1. CH1787 Pin Descriptions
PIN
1
NAME
RING
I/O
I/O
FUNCTION
Di rectly connects to the telephone li ne's Ri ng lead through a user suppli ed RJ-11C jack.
2
TIP
I/O
Di rectly connects to the telephone li ne's Ti p lead through a user suppli ed RJ-11C jack.
7
RXA
____
DTR
O
ANALOG VOICE INJECTED. Transi t and recei ve si gnal. Let float i f not used.
DATA TERMINAL READY i nput. Acti veLOW. Swi tchi ng off DTR can ei ther return modem to command
state, di sconnect phone call, or reset modem. DTR should be set LOW when not used.
Answer i nput has two modes of operati on dependi ng on i ts state duri ng reset. 1. When ANS i s low
duri ng reset the CH1787 wi ll go off hook i n the answer mode, sendi ng answer tone conti nuously
wai ti ng for an ori gi nati ng tone. Thi s mode of operati on i s used on a dedi cated non di al-up telephone
li ne (leased li ne). 2. When ANS i s hi gh duri ng reset, the modem wi ll i ni ti ate an answer tone whenever
ANS Pi n goes low duri ng normal operati ons. The modem wi ll send the answer tone for 30 seconds
and then stop. If ANS i s sti ll low at the completi on of the 30 seconds, the modem wi ll attempt another
answer tone sequence as long as ANS i s low. Used to manually answer an i ncomi ng call.
VOICE/DATA output i s used to swi tch between telephone and modem li ne use. When low the modem
i s i n the control mode and a voi ce ci rcui t can be swi tched out, RXA, TXA when hi gh the modem i s i n
the data mode and the i nput should be RXD/TXD.
RESET i nput (acti ve hi gh). Thi s i nput must be asserted HIGH for at least 10 ms to reset the modem.
RESET i s then returned LOW for normal operati on. If no system reset i s avai lable, let thi s pi n float to
enable i nternal reset.
RING INDICATION: Thi s si gnal follows the frequency of the ri ngi ng si gnal normally about 20 to 40 Hz
for 2 seconds off for for 4 seconds.
No Connecti on.
DATA SET READY output. LOW i ndi cates handshaki ng wi th a remote modem i s i n progress, and/ or
the data carri er of a remote modem i s detected.
ORA i s an i nput that i s acti ve low. When LOW i t wi ll di al one of two stored telephone numbers
dependi ng on the state of the TST pi n. The numbers are stored i n NVRAM. One i s a user defi ned
number that i s loaded i n by the user one ti me wi th the AT&ZO=(phone number) command. The other
number i s a prestored number that i s located at Cermetek(408____________) and i s i n a permanent
loop back answeri ng on the second ri ng. CH1787 wi ll attempt to connect wi th the stored number up to
15 tri es at 60 second i ntervals to obtai n a successful connecti on as long as ORA i s low. The state of
DCD i ndi cati ng carri er desi gnates a successful connecti on. If ORA goes hi gh, further attempts to
connect wi ll be termi nated. If after di sconnecti ng from a vali d connecti on, ORA sti ll low the modem wi ll
be unresponsi ve unti l ORA i s placed hi gh then low. The FCC requi reds that automati c di ali ng attempts
do not exceed 15 to the same number. TST low di al the user stored number.
TRANSMIT DATA. Seri al recei ve data i nput. Marki ng, or a bi nary 1 condi ti on i s transmi tted when a
HIGH i s asserted.
DIR i s an output that i ndi cates when TXD data may be sent to the modem and when RXD data i s
vali d from the modem. The user should moni tor thi s pi n or gate i t as shown i n Fi gure 4 to prevent data
from bei ng erronously i nterpreted by the user's host processer. DIR tells when the TXD and RXD li nes
are used for i nternal CH1787 connecti on. When DIR i s hi gh, vali d data i s on RXD and TXD. When
DIR i s low the user may i gnore RXD data and should not place any data on TXD si nce i t wi ll be
i gnored by the CH1787.
4.7KW resi stor to 5V
SLEEP output. A LOW i ndi cates modem i s i n low power i dle mode. Used to control power to other
devi ces. See Fi gure 5. Regi ster S24 i s the sleep ti mer regi ster. When the modem i s i nacti ve for a
peri od of ti me speci fi ed by S24, the CH1787 wi ll peri odi cally power down by about 50% of i ts normal
operati ng power. I/O li nes wi ll become undefi ned. The default for the CH1787 i s that sleep mode i s
i nhi bi ted. Thi s i s done by loadi ng 255 i nto S24.
8
I
9
___
ANS
I
10
__
V/D
O
11
RST
1
__
RI
N/C
___
DSR
I
12
O
13
-
14
O
15
____
ORA
I
16
____
TXD
I
17
DIR
O
18
IRQ
-
19
_____
SLEEP
O
Note: (1) If VCC has a slow power up ramp time, the internal reset may be ineffective.
Cermetek Microelectronics, Inc.
Document No. 603-0182 Rev. D (11/98)
4
P IN
N AM E
I/O
F U N C T ION
20
GND
I
GR OU N D .
21
____
DCD
__
HS
O
D ATA C AR R IE R D E T E C T
output.
L OW
i ndi cates a data carri er from a remote modem i s detected.
DCD follows carri er i s the default.
S P E E D IN D IC AT ION
. Hi gh speed select output. A LOW on thi s pi n i ndi cates the modem i s
operati ng at 2400bps.
Ori gi nate i nput (acti ve LOW) places to modem off hook i n the ori gi nati ng mode wi thout di ali ng. Used
to make a connecti on on dedi cated leased li nes between two poi nts. The remote modem must be i n
answeri ng mode.
22
O
23
____
ORG
I
24
NC
-
No Connecti on
25
26
27
28
NC
NC
SPK
___
TST
____
CON
____
HNG
____
RXD
VCC
GND
____
AAR
-
-
O
I
No connecti on.
No connecti on.
SPEAKER - Audi o Output. See Fi gure 5.
Test pi n i nput used i n conjuncti on wi th ORA to steer the di ali ng between a user stored number and a
pre-stored Cermetek test number. See ORA descri pti on.
Connect output i ndi cates a vali d 2400bps connecti on. Can be used i n place of DCD and HS to start
the communi cati ons. Can be used to i ndi cate the start of an exchange of data.
The hang-up i nput pi n (acti ve LOW), forces the modem to di sconnect. The HNG pi n i s only functi onal
when DCD i s acti ve (low).
R E C E IV E D ATA
. Seri al recei ve data output. Recei ved MARKING or a bi nary 1 condi ti on i s i ndi cated
by a HIGH.
5 Volts + 5%
GR OU N D
.
AAR pi n i s an i nput (acti ve LOW). When asserted low then hi gh, CH1787 wi ll auto answer duri ng a
Ri ng cycle. AAR can be ti ed low pri mari ly to enable Auto Answer on the fi rst RING. AAR wi ll not
overri de an AT auto answer condi ti on e.g., S0 > 1.
Note: Noi se should be less than 50MV.
29
30
31
32
33
O
I
O
I
I
34
I
Spare pins are available for custom functions. Contact Cermetek with your requirements.
Cermetek Microelectronics, Inc.
Document No. 603-0182 Rev. D (11/98)
5
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