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V.23 Compatible Modem



  • 1200/75 bps Full Duplex V.23 Compatible Modem with: Optional 75bps Back Channel
    Optional 1200bps Data Retiming Facility
  • 3.58MHz Xtal/Clock Rate
  • Optional Line Equalisation
  • Conforms to Relevant Sections of V.23 and ETSI Specs


  • Automatic Meter Reading: BT SIN 236 and 237
  • Line Powered Telemetry
  • Unattended Security Systems
  • Telecommand Systems
  • Payphones; Feature Phones
  • Vending Machines
  • Marine and Amateur Radio Data

Supply Requirement:

  • 3.0 to 5.5 V power supply

The FX604 is a low power CMOS integrated circuit for the reception and transmission of asynchronous 1200bps data in accordance with CCITT V.23 and ETSI specifications. It is also capable of generating the 75bits/sec 'back channel'.

The device incorporates an optional Tx and Rx data retiming function that removes the need for a UART in the associated µC when operating at 1200bps.

The device can disable the back channel or be operated so only the mark or space tone is produced.
An optional line equaliser is incorporated into the receive path, this is controlled by an external logic level.

The FX604 may be used in a wide range of telephone telemetry systems. With a low voltage requirement of 3.0V it is suitable for both portable terminal and line powered applications.

A very low current 'sleep' mode (1mA typ.) and operating current of 1mA typ. mean that the device is ideal for line powered applications. A 3.58MHz standard Xtal/Clock rate is required.

Block Diagram

FX604: for the reception and transmission of asynchronous 1200bps data in accordance with CCITT V.23 and ETSI specifications

Design Support Information


FX/MX604 and FX/MX614 FAQ

Q. Can I use the FX614 in the same circuit as an MX614 or vice versa? The FX604 and MX604 V.23 modems are also pin and function compatable with each other.

A. Yes. The FX614 and the MX614 are completely pin and function compatible so code and hardware can be re-used in new designs. Similarly the FX604 and MX604 V.23 modems are also pin and function compatible with each other so are interchangeable.

Q. 604 - Why do the FX604 and FX614 apparently receive data when there is no input signal?

A. The FSK receiver of the FX604 and FX614 has a very high sensitivity and in the absence of a valid FSK signal will decode noise as data. This is not a fault but is due to the wideband properties of noise where sufficient energy may be present within the FSK decoder's bandwidth.

To prevent spurious reception a baseband carrier detect output (DET) is used to determine the validity of the received signal. The detector block is a simple energy detector that receives predominantly in-band energy from a band-pass filter in the device input. The DET should be monitored during reception and used to 'intelligently' decide whether the data is likely to valid, suspect or invalid. During periods when there is no data reception the DET will be predominantly low but is likely to false so the receiver should be powersaved where practical.

DET does not internally gate the FSK receiver because this would prevent data reception during short duration events such as fades when data recovery may still be possible.

Q. 604 - Are the FX614 and MX614 compatible with the HART specifications?

A. Yes they are, the FX614 and MX614 both appear to be suitable for encoding and decoding HART signals but have not been specifically designed to do so.

But just to add some further points:

  • The HART physical layer is not a telephone line. Instead it is a twisted pair 4-20mA DC current loop extended from a power source, located in a control room, to a field mounted transducer a.k.a. "a transmitter" in process controls industries.
    Accordingly, the interface to HART wiring is different than used for a telecom line interface and HART does not use phone dialling or related connection establishment signalling.
  • HART 5 specifies 5 to 20 character preamble, where each character includes 11 bits:
    so (5*11*1/1200=45m sec).
    DET high = Teon = 25ms min.
    Therefore the FX614 will react to preamble and catch the data.
  • HART 6 specifies 3 to 20? character preamble, where each character includes 11 bits:
    so (3*11*1/1200=27.5m sec).
    DET high = Teon = 25ms min.
    Therefore the FX614 will react to preamble and catch the data.
  • It is mandatory that preamble the parity bit is set such that a mark bit '1' is present.
    This is recommended as the device will not latch on to the correct timing bit if 2 consecutive low bits are input.
    i.e. the parity during preamble is ODD. So the string of the preamble data will consist of:
    1 Start bit = LOW
    8 High data bits = HIGH
    1 Parity bit (odd) = HIGH (for this particular ‘FF’ data character)
    1 Stop bit = HIGH
  • HART definitely uses Bell 202 FSK signalling so the FX614 FSK modulator and demodulator (1200/2200Hz) are compatible with HART signals.
  • HART is a multi-drop network so more than one field mounted transmitter may be connected in parallel.
  • HART is master/slave protocol in which all network transactions are initiated by a master device which addresses an individual slave in each transmission.
  • A HART network has only a single pair of wires which carry both power and signal to field devices. 99.99% of all HART field devices have only two electrical terminals. These two terminals simultaneously deliver power to the field device and also carry superimposed signals.
  • A HART master typically includes a field power supply that sources power, for use by field devices, to HART wiring.
  • When used in single drop mode (a single field mounted transmitter on the HART bus) then the bus will operate in a dual analogue/digital mode in which both a DC analogue current signal and a digital HART signals are present.  The minimum DC analogue current signal level is, by industry standard, ~3.84mA.  This means that all a field device's circuits, including uC, display, sensor, power supply, modem, and pushbuttons must operate on a total current budget of <3.84mA.
    The exception is a field device that uses an external power source and does not take its power from the HART bus. The FX614 is suitable for use in master devices where power is in ample supply because it is not taken from the HART bus.
    NOTE - Some system applications use the loop power therefore the current consumption will be:
    I = FX614 current + transducer current. The maximum current consumption of the FX614 @ Vdd=5V is 2.5mA. Therefore for when using a 4-20mA output transducer this makes the loop current range = 6.5 to 22.5mA.
  • This current constraint is the result of safety requirements that severely limit the maximum power delivered to field wiring so inadvertently miss-wired or damaged wire cannot start a fire or cause an explosion (A lot of flammable and explosive materials are present at industrial sites where they are produced e.g. oil, flour, pesticides, etc.)

Q. 604 – Does CML offer a suitable replacement device for the TCM3105 (Bell202) modem application?

A. CML has several devices that can comply to the Bell202 specification: FX614 / CMX624 / CMX850 / CMX866 / CMX867A / CMX868A an B / CMX869A and B / CMX878.

The FX614 is a very popular device which is used as a suitable replacement for the TCM3105.
The FX614 offers a cheap and simple solution to wire-line and radio modem applications.

Please refer to the following publication for a simple cheap amateur radio solution using the MX614_TCM3105_article.

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