CH1724 Low-Cost Modem Module



The Cermetek CH1724 compact module provides a complete, low-speed modem. As a leadless surface mount module, the CH1724 requires less than half the board space of the leading competitor and is less than half as tall as well.

Despite being a low cost module, the CH1724 includes many advanced features such as; error correction, non-volatile memory for configuration storage and stored number dialing, and active line monitoring to permit the CH1724 to unobtrusively share the telephone line with standard telephones and other equipment.

Every CH1724 includes conveyed FCC Part 68 registration and Global Telecom compatibility. This eases the compliance burden on the designer of security, process control, medical monitoring, point-of-sale, or remote diagnostic systems.


CH1724 - 2400 bps; 0°C to 70°C

CH1724 ET - 2400 bps; -40°C to 85°C



  Small Size: 1.6” x 0.8” x 0.220”

•  Data transfer rates; 300 bps to 2400 bps

•  Control & configuration via AT commands.

•  3.3 and 5 volt compatible serial interface

•  Error control and correction

•  Line current and voltage monitoring to prevent modem operation from interfering with voice communications.

•  Complete integrated, Global DAA

•  Nonvolatile configuration storage

•  Contact ID and SIA Format Security protocols

•  User transferable FCC Part 68 registration

▪ Industry Canada CS-03 Registration

•  World-wide telephone network compliance.

•  UL60950 Compliance

•  Low Power operation; 3.3 Volts, 130 mW

•  RoHS compliant

Figure 1:  Functional Block Diagram of CH1724


CH1724 Design Notes

The following design notes are intended to assist the developer in achieving maximum utility from the CH1724 modem module.

Power Supply: The CH1724 decodes millivolt range analog signals from the telephone line.  Steps must be taken to ensure power supply noise on all supply lines, including ground, remains below 50 mV peak-to-peak. At any frequency between 20 kHz and 150 kHz must be less than 500 microvolts peak. Dedicated power and ground planes are recommended. Failure to provide clean power could cause the CH1724 to function erratically.

The CH1724 requires a single +3.3V ± 0.3 volt supply.  It is recommended that by-pass capacitors be placed on the power supply as close to the modem’s supply input as practical. Cermetek recommends a 10 mF Tantalum capacitor in parallel with a 0.01 µF ceramic capacitor.

Error Correction: The CH1724 employs error correction to preserve data integrity. MNP error correction monitors blocks of data for errors and executes a retransmission of any data blocks containing errors.  The AT\N command selects the use of error correction in the CH1724.  The modems negotiate error correction on completion of the modem handshake.  Both modems must have error correction available for error correction to be successfully negotiated.  The ATW command causes the modem to report the status of the error correction negotiations as part of the CONNECT response.

Flow Control: Flow control prevents the loss of data due to a buffer overrun when employing error correction or data buffering.  With flow control activated, AT&K command, the modem signals the host when its transmit data buffer nears capacity.  Likewise the host signals the modem if its input buffer fills. Two methods of signaling can be used:  hardware flow control uses the RTS and CTS control lines for signaling; software flow control employs XON and XOFF characters in the data stream to signal a buffer full condition.

Loop Monitoring: The CH1724 monitors loop current and voltage to prevent the modem from interfering with other devices connected to the same telephone line including interruption of an ongoing call or preventing an emergency call from being made. The ATLS command controls line monitoring.

Non-Volatile RAM (NVRAM): The CH1724 uses NVRAM to store a suer defined modem profile and phone numbers for automated dialing. The AT&W command stores the active modem configuration in the NVRAM.  The CH1724 is delivered with factory default setup parameter values pre-stored in NVRAM.

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 profile configuration.  Refer to Tables 4, 5 and 6 for storable S-Registers and available commands.

The CH1724 uses NVRAM to store two telephone numbers with up to 60 digits or modifiers each.  The AT&Zn=s command stores, the telephone number dial string "s" in location "n; n equals 0 or 1."  The ATDTS=n command will cause the modem to dial the stored telephone number in location n.

World-wide Telecom Compliance:  The CH1724 can be configured for telecom requirements from around the world.  The AT+GCI command includes settings for more than 75 countries.

Serial Interface:  The CH1724 connects to the system host through a 5 volt tolerant 3 volt serial interface.  At a minimum TXD, RXD and Ground must be used.  Use of he remaining signals, inputs /RTS, /DTR, and /Reset and outputs /CTS, /DCD, /DSR, and /RI is optional.  If you elect to not use the /RTS and /DTR inputs, install la 1K pull-down resistor to force those signals to the active state.  If not used the /Reset line should be left open.  Any of the outputs which are unused can be left open.

AT commands control the operation of several modem interface signals. The AT&C command can force /DCD to the active state; The AT&D command controls how the modem use of /DTR; the AT&S command can force /DSR to the active condition. AT&K3 controrls teh use of /CTS for low control. 

CH1724 AT commands

At commands control CH1724 operation. This section describes the AT command format and lists the commands, registers and result codes.

Command Mode: The CH1724 enters command mode on power-up, reset, a lost connection, or receipt of the escape code. In command mode the modem accepts commands from the host on /TXD. Appropriate result codes are returned on /RXD.

Note: For backward compatibility some functions are controlled by multiple commands.  In these instances the last command issued determines the function setting.

Command Line Format: AT commands follow a strict format.  Each command line, except A/, begins with the prefix AT. The "A" and "T" may be either both upper case or both lower case but cannot be of different cases. The modem determines data rate of the host equipment by measuring the width of the incoming bits of the “A” and “T.”

Multiple commands may be combined into a single command line of up to 60 characters. Commands are executed in the sequence they appear upon receipt of a carriage return. Spaces inserted into the command line are not placed in the buffer.

A backspace is used to edit the command line can before execution. The backspace erases the previous character in the command line. Register S5 allows the user to select a character other than backspace to edit the command line.

If the command buffer overflows, the modem issues an "ERROR" result code, and the command line is not executed. 

Re-Execute Last Command - The A/ command causes the CH1724 to re-execute the previous command line. This is the only command which does not use the "AT" prefix.

Omitted Parameters - Most commands include a parameter which determines how the functions will be set. When the command parameter is omitted from the command string, it is assumed to be a 0.

Escape Characters - A 3 character escape sequence may be entered to switch the modem into command mode while on line.  The escape character, set by Register S2, must be entered 3 times in succession to execute the escape. A command must then be entered within the period defined by S12 to enter command mode.  The default escape sequence is "+++."

Result Codes - The modem issues a result code after each action.  Result codes may be sent as full words, one or two digit numeric codes, or may be disabled all together.  Each result code ends with a carriage return when numeric result codes are chosen.  When full word result codes are chosen, a Line Feed and Carriage Return precede and follow each result code.

Security Standards

The CH1724 supports a pair of Security industry Association standard protocols; Contact ID (SIA DC-05-1999.09) and SIA Format (SIA DC-03-1990.01).  the CH1724 selects both of these protocols with variations on the standard modem AT commands. 

Contact ID: The command AT-A1<CR> selects Contact ID mode..  Contact ID uses a modified dialing command to transfer the Contact iD message, see command below.


In the above command "xxx" represents the number to be dialed.  The "1" identifies the digits that follow as Contact ID data.  The characters "yyyy" represent the Contact ID message including the CRC.  The Contact ID message may contain a maximum of 63 characters.  Additional data can be sent using the AT!1zzz or !zzz commands. 

Other commands and registers are also used in the Contact ID messaging.  The AT-B command causes the modem to resend the previous Contact ID message.  S10, S61, and S90 control the Kissoff tone timing at end the transaction. S42 an S43 control the timing of the handshake tones.

Contact ID mode also uses a unique set of responses as shown below.

b    Busy Tone Detected

c    Connect

K    Valid Kissoff Tone Detected

k    Kissoff Tone Exceeds the Time Limit Set by S10

N    No Carrier Detected

n    No Dial Tone Detected

O    OK, Valid Command Received

R    Ringing

r     Ringback Tone Detected

t     Dial Tone Detected

^     Kissoff Tone Detection Required

,     Dialing Complete     

SIA Format: The command AT-A2<CR> selects SIA Format..  SIA Format uses a modified dialing command to transfer the SIA formatted message, see below.

ATDxx!2YYYY<CR> or ATD!xxx!2yyyy<CR>

"xxx" is the number to be dialed.  The "2" identifies the following digits as SIA formatted data. The characters "YYYY" are the SIA fromatted message.  A maximum of 63 characters may be included in the Contact ID message.  Additional data can be sent using AT!2zzz. 

Other commands and registers are also used in the SIA messaging.  The AT-E and AT-Q commands determine handling of the Acknowledgement tone.  AT-R selects the SIA Format Column Parity calculation. S42, S61, S62, S7, S90, S92, S96, S99 and S102 control handshaking in SIA mode.

SIA mode uses a unique set of responses as shown below.

b          Busy Tone Detected

c          Connect

E          Negative Acknowledgement Detected

N          No Carrier Detected

n          No Dial Tone Detected

O          OK, Valid Command Received

P          Positive Acknowledgement Detected

R          Ringing

r           Ringback Tone Detected

t           Dial Tone Detected

T          Acknowledgement Time Out

,           Dialing Complete           


AT commands

An asterisk indicates the factory default

A - Answer Command - Causes the modem to immediately go off-hook and attempt link negotiations.

D - Dial Command - Causes the modem to go off-hook and dial. Below is a list of the characters accepted in the dialing string.

0-9, A-D, #, *    =    Dialing Digits

   L       =    Redial Last Number Dialed

   P       =    Pulse dial

   S=n   =    Dial number stored in location n 

   T       =    Tone dial

   W      =    Wait for dial tone

   ,        =    Pause for the duration of S8

   !        =    Switch hook flash

   ;        =    Return to the command state

   @      =    Wait for 5 seconds of silence

En - Command Echo - Determines if the modem returns the commands received from the host.

   n=0    Do not echo commands

   n=1    Enable command echo *

Hn - Switch Hook Control - Controls the connection to the telephone line.

   n=0    Switch hook relay opens; modem on-hook

   n=1    Switch hook relay closes; modem off-hook

In - Modem Identification - Provides product data..

   n=0    Modem identity

   n=3    Driver Version Number

   n=4    Data Pump Firmware Version

   n=9    Country ID

   n=11  Connection Data (see page ?)

Ln - Speaker Volume - Selects speaker output level.

   n=0    Low Volume

   n=1    Low Volume

   n=2    Moderate Volume *

n=3       High Volume

 LSn – Line Monitor Features – Controls modem operation on a shared telephone line.

   n=0    Line Monitor Disabled *

   n=1    Display Line Voltage Value

   n=2    Display Line Availability

   n=3    Disconnect on Extension Pickup

   n=4    Report Extension Pickup; remain off-hook

   n=5    Display Loop Current Value

   n=6    Display Line Sense Thresholds

n=7    Do not go off-hook; if line unavailable, disconnect on extension pickup

Mn - Speaker Activity - Selects speaker activity

   n=0    Speaker off

   n=1    Speaker on until carrier received *

   n=2    Speaker remains on with modem off-hook

n=3    Speaker on until carrier except during dialing

Nn - Link Negotiations - Selects how Register S37 will be used to set the link negotiations

n=0    Connect only using the speed selected by S37

n=1    Begin negotiations at the selected speed but allow fallback *

On - On Line - Puts the modem back into data mode from the on-line command mode.

   n=0    Return On Line with no retrain *

   n=1    Initiate retrain while returning On line.

n=3      Initiate rate renegotiation on return On line.

Qn - Responses - Determines if the modem will send responses to the host. 

   n=0    Send responses *

   n=1    No Responses

Sr? - Interrogate Register – Read register value.

Sr=n - Set Register Value - Set register value.

Vn - Result Codes - Determines what the type of result codes to be issued. 

   n=0    Numeric Result Codes

   n=1    English Word Result Codes *

Wn - Expanded Result Codes – Allows inclusion of protocol messages with the Connect responses.

   n=0    Report Host Receive Speed

   n=1    Report Host Receive Speed and protocol type

   n=2    Report Modem Receive Speed and protocol *

Xn - Result Code Set - Sets modem responses (see Table 3 for full response set)

n=0    Allows OK, RING, ERROR, CONNECT and NO CARRIER responses

n=1    Allows OK, RING, ERROR, CONNECT (with connect speed) and NO CARRIER responses

n=2    Allows OK, RING, ERROR, CONNECT (with connect speed), NO CARRIER and NO DIALTONE responses

n=3    Allows OK, RING, ERROR, CONNECT (with connect speed), BUSY, NO CARRIER, and BLACKLISTED responses

n=4    Allows OK, RING, ERROR, and NO CARRIER responses *

n=5    Allows OK, RING, ERROR, NO CARRIER and NO DIALTONE responses *

Z - Reset - Causes an immediate modem soft reset and reconfigures the modem to the stored values

&Cn - Data Carrier Detect Operation - Determines how the modem will present /DCD.

n=0   /DCD forced active at all times

n=1   /DCD indicates modem carrier signal state *

&Dn - Data Terminal Ready - Selects how the modem will react to the removal of  /DTR from the host.

n=0    Modem ignores status of /DTR

n=1    If /DTR is removed with the modem on line, the modem enters on-line command mode

n=2    /DTR must be active to maintain link *

n=3    Reset modem with on to off transition of /DTR.

&F - Restore Factory Configuration - Returns modem configuration to its factory settings.

&Kn - Local Flow Control - Determines the operation of flow control between the modem and local host.

   n=0    No Flow Control

   n=3    RTS/CTS (hardware) Flow Control *

   n=4    XON/XOFF (software) Flow Control

&Sn - Data Set Ready - Determines how the modem will present /DSR to the host.

n=0    Force /DSR active *

n=1    /DSR Indicates the modem’s readiness to communicate

&Tn - Modem Test Modes - Control diagnostics

   n=0    Terminate Test in Progress

n=1   Local Analog Loopback test

&V - View Active Configuration - Presents current modem configuration information

&W - Store Active Configuration - Places the current configuration into the selected memory location where it can be recalled on a soft reset.

&Zx=n - Store Number n in Location  “x” - Permits placement of a telephone number in the selected memory location.  Dialing the stored number is accomplished with the s=n dial modifier where n is the memory location of the stored number.

   x=0    Location 0

x=1   Location 1

\Fn – Fast Connect – Controls the V.22 Fast connect and V.23 turn around functions.

n=0    V.22 Fast Connect and V.23 Turnaround modes disabled.

n=1    V.22 Fast Connect Enabled

n=5    V.23 turnaround using RTS

n=6    V.23 Turnaround using DTR

\Jn – Data Buffer – Determines if data buffers will be used on non-error controlled connections.

   n=0    Data Buffer Enabled *

n=1    Modem will not connect at a rate faster than the serial interface speed.

\Kn - Response to receipt of Break Signal (Data Mode) Determines how the modem will handle a received break signal

n=0    Enter on-line command mode, do not transmit break

n=1    Clear data buffers, transmit break

n=2    same as n=0

n=3    Immediately transmit break

n=4    same as n=0

n=5    Transmit nondestructive break in sequence with data

n=6    Ignore the break signal

\Nn - Error Control Mode - Selects which protocols will be used during data connections.

n=0,1  Buffer Data, No Error Control

n=2    Error Correction required

n=3    Auto Error Correction *

n=6    SDLC Formatted Data

 -An – Security Mode – Selects the Security mode used by the modem

   n=0    Normal Operation

   n=1    Contact ID Format

   n=2    SIA DC-03-1990.01 Format

-B – Retransmit Contact ID message – Causes the modem to immediately resend the most recent Contact ID message

 -En – Detect SIA Format Acknowledgement Tone – Determines if the modem will detect the SIA Format Acknowledgement Tone.

   n=0    Do not detect Acknowledgement Tone

   n=1    Detect Acknowledgement Tone

-Kn – Response to Invalid SDLC Frame format – Determines how the modem will respond in SDLC mode if the data frame from the host to not conform to the SDLC standard.

   n=0    send “bad crc” message

   n=1    send abort character

-Qn – Hang-up on Receipt of Acknowledgment Tone – This command selects how the modem will disconnect in response to receipt of the Acknowledgement Tone.

   n=0    Do not Disconnect

   n=1    Disconnect upon Timeout

   n=2    Disconnect upon negative acknowledgement

   n=3    Disconnect on positive acknowledgement

-Rn - SIA Format Column Parity Calculation – This command selects whether or not the modem will calculate the Column parity. 

   n=0    Do not Calculate Column Parity

   n=1    Calculate column parity

+ER=n - Error Control Reporting - Determines if the Error Control status of the connection will be reported. 

      0 = Error Control Report enabled *

      1 = Error Control Report Disabled

+GCI = a Country Selection (Hex code) - Chooses the modem’s country configuration. The country configuration is always stored in nonvolatile memory.  

   a - Country Code

      00   =    Japan

      04   =    Germany

      07   =    Argentina

      09   =    Australia

      0A  =    Austria

      0D  =    Bangladesh

      0E  =    Barbados

      0F  =    Belgium

      14   =    Bolivia

      16   =    Brazil

      19   =    Bulgaria

      1E  =    Belarus

      20   =    Canada

      25   =    Chile

      26   =    People’s Republic of China

      27   =    Columbia

      2B  =    Costa Rica

      2D  =    Cyprus

      2E  =    Czech Republic

      31   =    Denmark

      35   =    Ecuador

      36   =    Egypt

      3C  =    Finland

      3D  =    France

      46   =    Greece

      49   =    Guatemala

      50   =      Hong Kong

      51   =      Hungary

      52   =      Iceland

      53   =      India

      54   =      Indonesia

      57   =      Ireland

      58   =      Israel

      59   =      Italy

      5E  =      Jordon

      61   =      Korea

      62   =      Kuwait

      64   =      Lebanon

      69   =      Luxembourg

      6C  =      Malaysia

      70   =      Malta

      73   =      Mexico

      77   =      Morocco

      7B  =      Netherlands

      7E  =    New Zealand

      7F  =    Nicaragua

      82   =    Norway

      83   =    Oman

      84   =    Pakistan

      85   =    Panama

      87   =    Paraguay

      88   =    Peru

      89   =    Philippines

      8A  =    Poland

      8B  =    Portugal

      8C  =    Puerto Rico

      8E  =    Romania

      98   =    Saudi Arabia

      9C  =    Singapore

      9F  =    South Africa

      A0  =    Spain

      A1  =    Sri Lanka    

      A5  =    Sweden

      A6  =    Switzerland

      A9  =    Thailand

      AE  =    Turkey

      B2  =    Ukraine

      B3  =    United Arab Emirates

      B4  =    United Kingdom

      B5  =    United States

      B7  =    Uruguay

      BB  =    Venezuela

      BC  =    Vietnam

      C1  =    Serbia and Montenegro

      F8  =    Latvia

      F9  =    Lithuania

      FC  =    Slovakia

      FD  =    Slovenia

      FE  =    Taiwan

+IFC = a,b  Local Flow Control  - Selects the flow control settings for the modem.

      a - Host control of data from the modem

            0 =    No Flow Control

            1 =    XON/XOFF (Software Flow Control)

            2 =    RTS/CTS (Hardware Flow Control) *

      b - Modem control of data from host

            0 =    No Flow Control

            1 =    XON/XOFF (Software Flow Control)

            2 =    RTS/CTS (Hardware Flow Control) *

+ILR=a - Report Serial Data Rate - Determines if the local serial data rate will be reported as part of the Connect message.

  0    =   Do not transmit local serial data rate with the Connect response

  1    =    Transmit the local serial data rate as part of the Connect message.

 +IPR=a - Fix Serial Data Rate - Sets the speed of the serial interface to a predetermined rate.

      a - Data Rate Setting

              0       =   Automatic rate detection

             110     =   110 bps

             300     =   300 bps

             600     =   600 bps

            1200    =   1200 bps

            2400    =   2400 bps

            4800    =   4800 bps

            9600    =   9600 bps

           14400   =   14,400 bps

           19200   =   19,200 bps

           38400   =   38,400 bps

           57600   =   57,600 bps

          115200  =   115,200 bps

+MS=a,b,c,d,e,f - Select Modulation - Selects the starting modulation for link negotiations. 

      a - Carrier

               V22B = V.22bis connection

               V22   = V.22 connection

               Bell212A = Bell 212A connection

               V23C = V.23 connection

               V21   = V.21 connection

               Bell103 = Bell 103 connection

      b - Automatic Modulation negotiations

               0 - Disabled

               1 - Enabled *

      c - Minimum receive data rate (300 to 2400 bps)

      d - Maximum receive data rate (300 to 2400 bps)

      e - Minimum transmit data rate (300 to 2400 bps)

      f - Maximum transmit data rate (300 to 2400 bps)

+VCID=n – Caller ID Selection – Controls the use of Caller ID in the CH1724 

    n=0    Caller ID Disabled

  n=1    Caller ID message presented as formatted data.

  n=2    Caller ID message presented as raw data.

CH1724 Register Settings

S0     Answer on nth Ring:  S0 sets the modem to automatically answer on the nth ring.  Setting S0 to 0 disables automatic answer. 

         Range:     0 to 255

         Units        Rings

         Default     0

S1     Ring Count: S1 is a read-only register showing the number of rings detected.  If no ring is detected within 8 seconds, S1 is reset.

         Range:  0 to 255

         Units     Rings

         Default  0

S2     Escape Character: Register S2 defines the ASCII escape character. Values 0-127 select valid characters; values 128 to 255 disable the escape sequence.

         Range:  0 to 255

         Units     ASCII Character

         Default  43 (+)

S3     Line Termination Character:  REgister S3 determines the ASCII character used to terminate commands and modem responses.   

         Range:  0 to 127

         Units     ASCII Character

         Default  13 (Carriage Return)

S4     Line Feed Character: Register S4 sets the ASCII character used as a line feed in modem responses. 

         Range:  0 to 127

         Units     ASCII Character

          Default  10 (Line Feed)

S5     Backspace Character: Register S5 defines the ASCII character used to edit the command line. 

         Range:  0 to 127

         Units     ASCII Character

         Default  8 (Back Space)

S6     Dial Tone Wait Time: Register S6 determines how long the modem waits for dial tone before dialing.   The Dial Tone Wait Time cannot be set to less than two seconds.    

         Range:  0 to 255

         Units     Seconds

         Default  3

 S7    Wait for Carrier after Dialing: Register S7 controls how long the modem waits for a valid carrier signal after dialing.  

         Range:  0 to 255

         Units     Seconds

         Default  50

S8     Comma Pause Time: Register S8 defines the duration of the pause caused by a comma in the dialing string.

          Range:  0 to 255

          Units     Seconds

          Default  2

S10   Kissoff Tone Window: Register S10 governs how long the modem will look for the Kissoff Tone during Contact ID operations.

         Range:     0 to 255

         Units     16 ms

         Default  78 (1.248 second window)

S11   DTMF Dialing Speed: Register S11 controls the DTMF tone duration and spacing.

         Range:  50 to 150

         Units:    milliseconds

         Default  55

S12   Escape Code Guard Timer: Register S12 sets the modem escape code guard timer. Any data received within the guard timer causes the modem to abort the escape from data mode.

        Range:  10 to 255

        Units     0.02 Seconds

        Default  50

S29   Hook Flash Duration: Register S29 controls the duration of the hook flash generated by the “!” dial modifier in the dialing string.

         Range:     0 to 255

         Units     0.01 Seconds

         Default  70

S30   Disconnect Inactivity Timer:  Register S30 determines how long the modem remains on line with no data flowing. A zero prevents the modem from disconnecting due to inactivity.

         Range:   0-255

         Units:    10 Minutes

         RDefault:    0

S36   Response to LAPM Negotiation failure:  Register S36 deeines the action taken by the modem if the selected error correction negotiations are unsuccessful.

        0    =    Disconnect

        1    =    Maintain link with no error correction

        3    =    Maintain link with no error correction

        4    =    A MNP negotiation is attempted, disconnect if unsuccessful

        5    =    MNP negotiation attempted, establish normal connection if unsuccessful

        7    =    MNP negotiation attempted, establish normal connection if unsuccessful *

S37   Line Data Rate:  Register S37 determines the maximum line data rate that can be negotiated

        0    =    Automatic rate negotiation *

        2    =    1200/75 bps (V.23)

        3    =    300 bps

        5    =    1200 bps

        7    =    2400 bps

S42   Data Delay Time: Register S42 sets the time during which the received data will be ignored after handshaking is initiated.

         Range:     0 to 255

         Units     2 milliseconds

          Default  125

S43   Contact ID Handshake Tone Validation Time: Register S43 governs the time the CH1724 allows to complete handshaking in the Contact ID mode.

         Range:     0 to 255

         Units     1 milliseconds

          Default  50

S61   Acknowledgement Tone Validation Time: Register S61 sets the time for the CH1724 to detect the acknowledgement or Kissoff tones in Security modes.

         Range:     0 to 255

         Units     8 milliseconds

         Default  35 (280 milliseconds)

S62   SIA Handshake Tone Validation Time: Register S62 defines the time allotted for the CH1724 to detect the handshake tones in SIA mode.

         Range:     0 to 255

         Units     8 milliseconds

         Default  113 (904 milliseconds)

S71   Delay Before Looking for Acknowledgement: In SIA mode Register S71 determines how long after transmitting a message before the modem looks for the Acknowledgement Tone.

         Range:     0 to 255

         Units     8 milliseconds

         Default  130 (1.04 seconds)

S90   Acknowledgement Timeout: Register S90 sets the time allotted for the CH1724 to receive an acknowledgement or Kissoff tone in Security modes before disconnecting.

         Range:     0 to 255

         Units     16 milliseconds

         Default  156 (2.496 seconds)

S92   Sync Delay: Register S92 controls the time between the Handshake Tone and the Synchronization signal in SIA mode.

         Range:     0 to 255

         Units     4 milliseconds

         Default  120 (480 milliseconds)

S96   Mute Delay: Register S96 determines how long the modem will transmit null characters after the message is sent in SIA mode.

         Range:     0 to 255

         Units     1 millisecond

         Default  54

S102  Speed Synchronizatoin Signals:  Register S102 selects how many Speed Synchronization Signals will be sent in SIA Format. Each sync signal is 26.67 milliseconds.

         Range:     0 to 255

         Units     Sync signals

          Default  10

Table 1: CH1724 Modem Responses

Verbose Result Code

Numeric Result Code




Command line executed successfully



Modem link established



An incoming Ring has been detected



Modem lost carrier, does not detect answer tone, or does not detect a carrier signal



Invalid command received



1200 bps connection established. When extended result codes are enabled “MNP4” is added for an error controlled connection and “NoEC” for a normal connection



No Dialtone was detected



A Busy Tone was detected



Remote modem did not answer



2400 bps connection established. When extended result codes are enabled “MNP4” is added for an error controlled connection and “NoEC” for a normal connection



A connection was established at the DTE rate of 38,400 bps. When extended result codes are enabled “MNP4” is added for an error controlled connection and “NoEC” for a normal connection



A connection was established at the DTE rate of 57,600 bps. When extended result codes are enabled “MNP4” is added for an error controlled connection and “NoEC” for a normal connection



A connection was established at the DTE rate of 115,200 bps. When extended result codes are enabled “MNP4” is added for an error controlled connection and “NoEC” for a normal connection

Figure 2: CH1724 Mechanical Specifications


 Table 2: CH1724 Mechanical Specifications










Module Length in inches





Module Width in inches





Module Height in inches





Solder Pad Diameter in inches





Solder Pad Castellation Diameter in inches





Center to center space between Pads in inches





Pin row Length in inches

Figure 3: CH1724 Recommended Landing Pattern


Pad Dimensions: Oval 0.130 inches by 0.070 inches

Pad Extension beyond CH1724 PCB: 0.040 inches

Row Center to Row Center: 0.74 inches

 Warning: No Vias, Traces or other Copper should be placed under the CH1724 module


CH1724 Surface Mount Soldering instructions

The CH1724 is subject to damage if over-exposed to heat during solder reflow operations.  Following the soldering instructions below will ensure that the process of soldering the module to the board does not damage the modem.

                                    Absolute Maximum Temperature:      250° C

                                               Maximum Time at 235° C:      15 Seconds

                            Maximum Time in reflow zone (217° C):      90 Seconds

                                        Maximum Preheat Dwell Time:      180 Seconds


Figure 4:  Maximum Recommended Solder Temperature Profile



Figure 5: CH1724 Solder Pad Configuration

Pin configuration

CH1724 pAD Descriptions






Ring and Tip connect the modem to the telephone line. FCC Part 68 Rules require a 1500 volt isolation barrier between the telephone line and all other circuits.  This isolation must be preserved throughout the system.

The telephone company places a DC "Battery" voltage across Tip and Ring on publich switched telephone lines. The CH1724 operates regardless of the polarity of this voltage. The "Battery" voltage drives up to 100 miliamps of loop current.

UL60950 requires minimum creepage and clearnce dinstances between conductive circuits; creepage is the distance between conductive points along a surface. 



Ring and Tip connect the modem to the telephone line. FCC Part 68 Rules require a 1500 volt isolation barrier between the telephone line and all other circuits.  This isolation must be preserved throughout the system.



This pin is reserved for future use and should remain unconnected.



/DSR is an active low output from the modem. The /DSR signal typically indicates that the modem has established a communications link.  The AT&S command defines the operation of the /DSR signal.



/DCD is an active low output from the modem. /DCD normally indicates that the modem has detected a valid carrier signal from the remote modem.  The AT&C command controls when the CH1724 asserts /DCD.



/DTR is an active low input to the modem. An active /DTR signals traditionally indicates that the host is available for communications. The AT&D command selects how the modem interprets activity on /DTR.



The /RI signal reports on the presence of an incoming ring signal When a ring occurs across Tip and Ring, the /RI output goes low and toggles with the cadence of the ring signal.



SPKR provides a Pulse Width Modulated version of the signal on Tip and Ring. This allows use of a speaker with the modem to monitor modem operations. The signal on SPKR is controlled by the ATL and ATM commands. 



Ground provides the reference voltage for all host interface signals



TXD provides the path for transmitted data and modem commands to be passed from the host controller to the modem. Data starts with a high to low transition. Mark or binary 1 condition is indicated by HIGH



RXD provides the path for received data and modem responses to be sent from the modem to the host controller. Data starts with a high to low transition. Mark or binary 1 condition is indicated by HIGH



/RESET is an active low input which initiates a modem hardware reset. /RESET must be active for a minimum of 100 milliseconds for a proper modem reset sequence. No external reset is required; if none is used the RESET signal should be left open



/CTS is an active low output from the modem. When hardware flow control is set, the modem asserts /CTS to indicate that it can accept data from the terminal equipment on /TXD.



No Connection; this pad should remain unconnected.



VCC provides 3.3 volt power to the modem. 



/RTS is an active low input to the modem.  When hardware flow control is set, an active /RTS indicates to the modem that the host has data to send.



This pad is provided only for mechanical stability. No electrical connection should be made.



This pad is provided only for mechanical stability. No electrical connection should be made.

Table 3: CH1724 Electrical Specifications


















During Data Transfer

Ring Voltage Detected





Type B Ringer

Ring Frequencies Detected





Type B Ringer

Telephone Line Loop Current






Line Impedance






Transmit Level




Data Transfer




DTMF Signaling





VCC = 3.3 Volts; IOH = -1.8 mA





IOL = 1.8 mA






VCC = 3.3 Volts






VCC = 3.3 Volts






± 50 PPM

Table 4: CH1724 Absolute Maximum Ratings







Storage Temperature















5 Volt tolerant Interface

Operating Temperature









CH1724 ET

Model Numbers

Cermetek offers the CH1724 in teh standard commercial operating temperature range of 0°C to 70°C and in extended range of -40°C to 85°C.   Table 5 below lists the available CH1724 models.

Table 5: CH1724 Model Numbers


Summary of Features


2400 bps modem; Error Correction; NVRAM; 0°C to 70°C

CH1724 ET

2400 bps modem; Error Correction; NVRAM; -40°C to 85°C

Figure 6: CH1724 Typical connection diagram


Parts List for CH1724 Typical Connection Diagram

Reference Designation



C1, C2


Cap 680 pFd, Y2



PTC, TR600-150


1.   Capacitors C1 and C2 may be required for EMI filtering in your system.  Without these components you may experience unintended radiation when the telephone cable is attached.  C1 and C2 are high-voltage capacitors.  This 680 pFd, 5000 volt capacitor will direct high frequency harmonics to the system ground.

2.   F1 is a positive thermal coefficient (PTC) device which protects the modem from excessive current flow. This device is required for your system to pass UL60950.  Fuses may be used in place of the PTC. 


ACTA Product Registration Number: US: B46MM01BCH1724

This equipment complies with TIA/EIA/IS-968 rules. Located on the equipment is a label that displays the FCC registration number.  The FCC registration number includes information on the manufacturer, model number and ringer equivalence of the connection to the telephone network.  If requested this information must be provided to the telephone company. 

The ringer equivalence is used to determine the quantity of devices which may be connected to a telephone line.  An excessive number of devices on the telephone line may result in the devices not ringing in response to an incoming call. In most, but not all areas, the sum of the Ringer Equivalence Numbers should not exceed five (5.0).  To be certain of the number of devices that may be connected to the line, as determined by the total Ringer Equivalence Numbers, contact the local telephone company for the maximum Ringer Equivalence for the calling area.

This equipment cannot be used on the telephone company-provided coin service.  Connection to party Line Service is subject to State Tariffs.

If this equipment causes harm to the telephone network, the telephone company will notify you in advance that temporary discontinuance of service may be required.  If advance notice isn’t practical, the telephone company will notify the customer as soon as possible. Also, you will be advised of your right to file a complaint with the FCC if you believe this is necessary.

The telephone company may make changes in its facilities, equipment, operations, or procedures that could affect the operation of the equipment. If this happens, the telephone company will provide advanced notice in order for you to make the necessary modifications in order to maintain uninterrupted service.

If trouble is experienced with this equipment, please contact;

Company Name:      Cermetek Microelectronics, Inc.

Address:                 374 Turquoise Street, Milpitas, CA 95035

Telephone:              408-942-2200

Fax:                        408-942-1346

If the trouble is causing harm to the telephone network, the telephone company may request you to remove the equipment from the network until the problem is resolved.

This equipment uses the following USOC jacks: RJ11C

It is recommended that the customer install an AC surge arrestor in the AC outlet to which this device is connected. This is to avoid damaging the equipment caused by local lightening strikes and other electrical surges.

CS-03 Registration # 176A-CH1724

Cermetek reserves the right to make changes in specifications at any time and without notice.  The information furnished by Cermetek in this publication is believed to be accurate and reliable.  However, Cermetek assumes no responsibility for its use, or for any infringements of patents or other rights of third parties resulting from its use.  No license is granted under any patents or patent rights of Cermetek.

Cermetek provides the user with a FCC (USA) Approved module.  However, Cermetek does not have influence over nor knowledge of the specific user application environment.  Therefore, for FCC Approved or Approvable devices, the user assumes all risk for maintaining compliance to registration.

Printed in U.S.A


374 Turquoise Street | Milpitas, CA 95035 | LOCAL:  408-942-2200 | FAX:  408-942-1346