Applies To:
  • CitectSCADA 3.xx, 4.xx, 5.xx

I am not sure how to set up a modem for real time communications to a remote PLC, what baud rate to use or what could be causing the modem to drop off line or not establish a link properly. 

For the modem at the PLC end, set the modem according to the following guide. The suggestions below should work in most situations however when in doubt use them as a starting point and work from there;
  • Baud Rate:
    With regard to fax/data modems, baud rate refers to the symbol rate and is not the same as the bit rate. You need to choose a modem connect speed (modulation standard) that will work with both Citect and the PLC bit rate.
    For example: to get Citect to communicate with an AB SLC 5/03 with a 14.4Kbps/V.32bis modem, the following selection would be the only solution;
Citect Baud Rate (ports form) Modem Speed PLC Baud Rate
9600 V.32 9600bps 9600

The reason for using a slow rate is that the only other option in the SLC5/03 is 19,200 bps. You would need a better modem(28.8Kbps/V.34) to connect at 19,200 bps.

The idea is to set the modem to a constant speed with any fall-back option disabled. The method to achieve this varies from one brand of modem to the next, you will need to consult the modem manual and ensure that the modem negotiates at a constant speed with any auto rate change option turned off.
(For the example above check which of the following AT commands are supported by your modem, see ATB13 or AT&N6).

  • Error Correction:
    It is best to turn this option off in the modem, see AT%E0 or AT&M0.
  • Compression:
    This is best turned off and may prove to provide better long term reliability, see AT%C0 or AT&K0.
  • Local Handshake/Flow Control:
    This has been known to affect communications on some modems. It is best to turn this option off, see AT&K0 or AT&H0.
  • Result Codes (response codes) and Echo:
    If the modem sends result codes, example "OK", "CONNECT", "ERROR", etc, to the PLC it may not respond to a genuine request. This should be turned off, ATQ1E0.
    (Note: Once you do this the modem will no longer echo commands or results, but AT&V will still echo).
  • DTR control:
    The modem can bet set to respond to the DTR signal (enter command state, hang up, etc.), in the initial instance it is best to force DTR on, until the operation can be verified later, use AT&D0.
  • Modem Profile:
    The modem configuration can be stored in NVRAM, that is its stored profile. Do this once you have proven the modem operation. Use AT&W to save the configuration profile.

    To read the configuration at any time type AT&V or ATI4 to view the current active profile.

  • Auto Answer:
    Set the modem to Auto Answer mode, use ATS0=1 (answer incoming call after 1 ring).
  • Leased Line:

    This function is supported by some modems.  You will have to verify that a particular make and model is capable of this type of operation.  See AT&L.   (This may also required dip switch or other settings on the unit).


  1. The extended modem command sets vary between manufacturers, some use "&" others use "%",  "\" or "*" prefixes. Consult your modem manual.
  2. Current modems use a superior modulation method, achieve higher speeds and lower bit error rates than earlier modem standards, use a modem capable of at least V.32bis (ITU-T standards) to achieve satisfactory results. Ensure you enable Trellis Coding (if there is an option to disable it, see AT&U0 or AT*T1).
  3. Industrial modems may not necessarily give better results than commercial units, each make and model should be tested on its own merits.