Modems at the I/O Server

Topology > I/O Devices > Communicating with Remote Devices via Modems > Modems at the I/O Server

Modems at the I/O Server

To decide how many modems to use at the I/O server end, decide what function each modem will perform. A single modem can do any one of the following functions:

Dial-out	Makes calls to remote I/O devices in response to a Citect SCADA request; for example, scheduled, event-based, operator request, and so on. Also returns calls from remote I/O devices.
Dial-in	Only receives calls from remote I/O devices, identifies the caller, then hangs up immediately so it can receive other calls. Citect SCADA then returns the call using a dial-back or dial-out modem.
Dial-back	Only returns calls from remote I/O devices.
Dial-in and dial-out	Receives calls from remote I/O devices and makes scheduled calls to remote I/O devices.
Dial-in and dial-back	Receives and returns calls from remote I/O devices.

Your modem setup depends on your system requirements. When making your decision, consider the following guidelines:

If you need to communicate with multiple remote I/O devices at once, you will need a separate modem for each I/O device. Otherwise you'll have to wait as I/O devices are contacted one after the other, and incoming calls may be held up.
If you have scheduled requests to I/O devices and you also need to urgently return calls from remote I/O devices that dial in, you will need at least one modem for each of these functions. For example, if you have a large number of remote I/O devices and you require fast responses by Citect SCADA, provide an additional modem at the I/O server. This would reduce the chances of it being engaged when an I/O device dials in (say, with an urgent alarm).
In a big system with many remote I/O devices or a system where calls from remote I/O devices can be time sensitive, it's a good idea to dedicate at least one modem to dial-back. This will give you quick responses to dial-In calls (from remote I/O devices). It also means your dial-out schedules won't be disrupted (if you use the same modem for returning calls and scheduled calls, the scheduled calls are forced to wait until the dial-back call is complete).