Modems at the I/O Device

You can have multiple I/O Devices connected to a single modem if they have the same communication requirements (phone number, baud rateThe number of times per second a signal changes in a communication channel. While the baud rate directly affects the speed of data transmission, the term is often erroneously used to describe the data transfer rate. The correct measure for the data rate is bits per second (bps)., data bitsGroup of binary digits (bits) used to represent a single character of data in asynchronous transmission., stop bitsThe number of bits that signals the end of a character in asynchronous transmission. The number is usually 1 or 2. Stop bits are required in asynchronous transmissions because the irregular time gaps between transmitted characters makes it impossible for the server or I/O device to determine when the next character should arrive., and parityA communications error-checking procedure. The number of 1's must be the same (even or odd) for each group of bits transmitted without error.). A separate port and modem needs to be used for each remote I/O Device with different communication requirements.

When deciding how many modems to use, consider the following:

• Once an I/O Device has been contacted, the connection can be retained for other I/O Devices in the group. If the I/O Server needs to request data from another I/O Device with the same communication details, it will wait until the current request has been completed, then it will use the same connection to make the second request.
• You can configure your modem to initiate telephone calls (call CitectSCADA), and/or receive telephone calls. This is done independently of CitectSCADA.

• Note: Wherever your modem is, you need to verify that its Data bits, Parity, Stop bits, and Serial Port Speed settings are compatible with the remote I/O Device as defined by the device's manufacturer or your communications link will not work.

See Also