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V.22 bis Modem with AT Command Set



  • V.22bis, V.22 and Bell 212A QAM/DPSK
  • V.23, Bell 202, V.21 and Bell 103 FSK
  • Integral AT Command Set with 'Fast Connect'
  • V.23 and Bell 202 'Fast Turnaround' Support for Type 1 Caller Line Identification
  • DTMF/Programmable Tones: Tx and Rx
  • 'Zero-Power' and 'Powersave' Standby Modes
  • DE8661 DemoKit and Reference Diagram Available


  • Telephone Telemetry Systems
  • Remote Utility Meter Reading
  • Security Systems
  • Industrial Control Systems
  • Electrical Cash Terminals and ATMs
  • Pay-Phones
  • Cable TV Set-Top Boxes
  • EPOS Terminals

Supply Requirement:

  • 2.7 to 5.5 V power supply

The CMX866 is a multi-standard modem for use in telephone based information and telemetry systems.

Control of the device is via AT commands over a simple 9600bps serial interface, compatible with most types of host µController. An RS232 compatible interface can be created by the addition of a Level Converter. The data transmitted and received by the modem is also transferred over the same serial interface.

The on-chip µController interprets these AT commands and controls an internal DSP, which provides the modem and anciliary functions such as Ring Detection, Call Progress Detection, Hook Switch control and DTMF autodialling. User-specific DSP functions are also available via the AT command set.

Hardware support is provided for V.23 and Bell 202 Fast Turnaround and for rapid return to AT Command mode. A Fast Connect mode hasm been implemented to reduce modem connection time.

Flexible line driver and receive hybrid circuits are integrated on chip, requiring only passive external components to build a 2 or 4-wire line interface.

Complete examples of 2-wire line interfaces to an external host µC and to an RS232 interface, including the additional components required for Type 1 CLI, are provided.

The device features a Hook Switch relay drive output and a Ring Detector circuit that remain operational when the CMX866 is in ‘Zero-Power’ or Powersave mode, providing an interrupt which can be used to wake up an external host µC, as well as the CMX866, when line voltage reversal or ringing is detected.

Block Diagram

CMX866: a multi-standard modem for use in telephone based information and telemetry systems

Design Support Information



Q. I am interested in using the programmable tone detection feature of the chip. Do you have any tools to help me calculate the required coefficients?

A. The CMX868 filter coefficient generator spreadsheet can also be used for the CMX866, and this spreadsheet can be found at: WirelineFL_1.htm..

Q. How can I use the CMX866 in a leased line configuration?

A. If you need a permanent or part time connection between two modems, a leased line is a popular option. In most leased line applications both the answering and originating modems are left in the off-hook state indefinitely. This means that even when data is not being sent, there will still be activity in the form of a carrier signal. If the line is broken for some reason, the modems should detect this, go on-hook, and re-establish the link.

The interface from modem to leased line can be either 4 line of 2 line (usually 4), and the modulation methods and protocols are typically already known on both ends of the link.

To achieve a successful connection the modem must go off-hook and immediately start a handshake (V.23, V.22 or V.22bis are typical). No dial tone, ring back or busy tones will be present on the line and the 2100Hz answer tone may not be present. Additionally, as there is no direct way of initiating the sequence with the answering modem (no ringing signal is generated at the answering modem), the modem must test the line (typically every 30 seconds or so) whilst the line is idling (before data or carrier signal is present).

To establish a link with the CMX866 in originating mode:

A Tx=0 ;Blind dialing; the modem is told not to expect a dial tone when going off hook.

S6= 0; No wait for dial tone; we want the modem to try and connect immediately.

ATD1; Go off hook, dial DTMF digit and attempt to connect to the answering modem (detects 2100Hz or scrambled 1's).

S7 = >30 and <50sec ;wait 30 seconds, if modem not present wait a further 20 seconds and repeat.

To establish a link with the CMX866 in answering mode:

Establish a >30 second timer

Off-hook and Generate Answer Tone: ATA

If no connection is established, re-establish the 30 second timer.

All other settings such as V.22bis, gain etc should remain the same as normal.

Q. I am using the CLI code available in your Applications Note section on your website (; however I do not have access to the RESETN pin in my design and wish to re-enter AT command Mode following CLI decoding?

A. It is possible to re-enter AT command mode without issuing RESETN, following CLI routine execution as the interpreter is based on an 8051 core. It is achieved by inserting a long jump command (LJMP) to 1000h at the end of the exit routine in the CLI routine hex dump. This has not been tested by CML but has been shown to work.

Please note the CML disclaimer notice for applications and FAQ information.

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