Define the Settings for Modems

You need to define 2 durations for modems.

For a modem to respond to instructions, it needs to be switched into 'Command' mode. Geo SCADA Expert uses 2 methods to instruct a modem to switch into command mode:

  • Geo SCADA Expert can send an escape sequence
  • Geo SCADA Expert can send a BREAK signal

A BREAK signal is usually regarded as the more reliable method, and is sent by the relevant drivers to the modems.

The BREAK signals need to be of sufficient length. If the length (duration) is too short, the modem will not be able to detect the BREAK signal and so will not switch to command mode. If the duration is too long, there will be a unnecessary delay.

The Duration setting that you define applies to every PSTN channel on your system. The minimum duration is determined by the speed of the slowest PSTN channel.

You can disable the sending of BREAK signals by setting the Duration to 0. This means that Geo SCADA Expert will only use the escape sequence to instruct modems to switch to Command mode.

To change the duration of the BREAK signal:

  1. Access the Geo SCADA Expert Server Configuration Tool (see Accessing the Geo SCADA Expert Server Configuration Tool).
  2. Expand the Global Parameters branch of the tree structure.
  3. Select Channels to access the Channels section.

  4. Use the upper Duration field in the PSTN box to define the length of the BREAK signal (in milliseconds).

    The required duration of the BREAK signal can vary according to the models of modem being used. Typically, the default setting of 100 milliseconds is suitable, but you may need to increase this setting to nearer 200 milliseconds depending on the models of modem being used.

  5. If your modem cable is fully wired, you will also need to define the Duration of Post Hang-up Delay.

    If your modem cable is not fully wired, you can apply the changes.

  6. Right-click on the system icon in the tree-structure, and select the Apply Changes option to apply the changes.

For more information, see the topics that are listed in the gray footer section at the bottom of this topic. Select the relevant entry to display the topic that you require.

Or:

Continue with the server configuration. If you are unfamiliar with the server configuration process, we recommend that you proceed to learn about Driver Settings.

You can find more information about the CCITT recommendations for BREAK signal length at the Microsoft support website (www.support.microsoft.com). The BREAK signal information is included in Microsoft Knowledge Base Article - 119506.

You can use the Channels section of the Geo SCADA Expert Server Configuration Tool to define the duration of the delay between consecutive calls on a PSTN channel. A delay may be required when using network connected serial ports, such as a Comtrol DeviceMaster. With such setups, delays may be needed due to latency when reporting the reply from a modem to a hang-up command.

When a call has completed, the driver sends a hang-up command to the modem. The completion of the hang-up can be signalled in two ways - the reply from the modem and (with a fully wired cable) the carrier detect signal changing to Low. With a network connected serial port, there can be delay between the driver receiving the reply from the modem and the carrier detect signal change. This can cause a problem if there is another call pending when the first call completes, as the driver can start the new call and then receive the reply from the modem to the hang-up command. The reply from the modem is then mistakenly handled as being related to the new call, which causes the call attempt to fail with a 'No carrier' error.

To avoid this problem, you can define a delay that is applied after hanging up the modem (and so allow time for the modem reply to be received). The duration of this delay will vary depending on the amount of latency on your network, but can be between 200ms and 5000ms.

To define the duration of the delay:

  1. Access the Geo SCADA Expert Server Configuration Tool (see Accessing the Geo SCADA Expert Server Configuration Tool).
  2. Expand the Global Parameters branch of the tree structure.
  3. Select Channels to access the Channels section.
  4. Use the lower Duration field in the PSTN box to define the duration of the post hang-up delay.
  6. Close the Geo SCADA Expert Server Configuration Tool to apply the changes (or select another branch if further server configuration is required).

For more information, see the topics that are listed in the gray footer section at the bottom of this topic. Select the relevant entry to display the topic that you require.

Or:

Continue with the server configuration. If you are unfamiliar with the server configuration process, we recommend that you proceed to learn about Driver Settings.