For a modem to respond to instructions, it needs to be switched into 'Command' mode. ClearSCADA uses 2 methods to instruct a modem to switch into command mode:
- ClearSCADA can send an escape sequence
- ClearSCADA 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.
NOTE: You can disable the sending of BREAK signals by setting the Duration to 0. This means that ClearSCADA will only use the escape sequence to instruct modems to switch to Command mode.
To change the duration of the BREAK signal:
- Access the ClearSCADA Server Configuration Tool (see Accessing the ClearSCADA Server Configuration Tool).
- Expand the Global Parameters branch of the tree structure.
- Select Channels to access the Channels section.
- 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.
- 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.
- Right-click on the system icon in the tree-structure, and select the Apply Changes option to apply the changes.
When you have finished defining the duration settings, you can either configure other Channels settings:
- Define the Settings for Channel Scanning Statistics Trends.
- Define how a PSTN Channel is Selected for Communications.
- Define the Settings for COM Ports and TCP/IP Connections.
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.
Further Information
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.