Using CitectSCADA > Communicating with I/O Devices > Communicating with Remote Devices via Modems > I/O device constraints for multi-dropping

I/O Device constraints for multi-dropping

If you are multi-dropping off a single modem, use your I/O Devices to issue the caller ID, not the modem. This is because using the modem to issue the ID will send the same ID no matter which I/O Device the call is relevant to. This makes it difficult to identify the I/O Device that triggered the call.

By using the I/O Device to issue the ID, the I/O Server will receive a unique caller ID for each I/O Device. However, not every I/O Device is capable of issuing caller IDs. If multi-dropping, use I/O Devices that can issue caller IDs.

To configure dial-up remote I/O Devices for communication with CitectSCADA:

  1. Run the Express Communications Wizard.
  2. Complete the wizard, selecting the relevant I/O Server, then the I/O Device, creating each new instance when necessary.
  3. On the Scheduling page of the wizard, select the Connect I/O Device to PSTN check box.
  4. Select an appropriate schedule for CitectSCADA to communicate with the remote I/O Device. (For a persistent connection - whenever CitectSCADA is running - select On Startup.) For example (all based on a Synchronize at time of 10:00:00):
  5. Select On Startup for a persistent connection. To disconnect a persistent connection, call the IODeviceControl() function with type 8.
  6. Type in the phone number necessary for CitectSCADA to dial the remote modem attached to the remote I/O Device. Include any pre-dial numbers necessary to obtain a connection to an outside PSTN line (dial tone) before dialing (for example, 0 (zero)) - if appropriate.
  7. On the next wizard page, if the device is configured to dial-in to CitectSCADA, create a unique identifying caller name for the remote I/O Device so that it can be identified by CitectSCADA.
  8. Follow the instructions on the next page of the wizard and click Finish.

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