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:

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 Plant 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).

See Also

Modems at the I/O Device

Example Configurations for Modems at the I/O Server