SCADA modem important features for telemetry application

• Automatic smart features
Most asynchronous modems are compatible with the Hayes AT command set, which automates most modem features.

• Data rate
This is normally the first feature examined. Ensure to distinguish this from the baud rate and the difference between the data rate before compression (nominal) and the effective data rate when compression has been performed.

• Asynchronous/synchronous modes
The ability to switch between both modes allows more flexibility for future applications. This is sometimes provided in a dip switch form.

• Transmission modes
The most efficient method of data transfer is full-duplex. This is a preferable method of operation to half-duplex where the line turnaround time introduces a considerable amount of inefficiency into the data transfer.

• Modulation techniques
The two most popular modulation techniques are V.22 bis which supports 1200 and 2400 bps transmission and V.32 bis the other almost universal transmission capability (which has V.22 bis as a subset).

• Data compression techniques
There are four main compression standards used today (mainly for telecom switched lines). It is important that the modem has compatibility with these:
• MNP class 5 (the most popular)
• MNP class 7 (compatible with MNP 5)
• ACT
• CCITT V.42bis (arguably becoming the most popular

• Error correction/detection
The most popular error detection and correction mechanism is MNP-4. The CCITT have incorporated this standard into the V.42 standard, which allows for MNP-4 and link access procedure for modems (LAP-M).

• Flow control
This is useful in controlling the flow of data from an attached terminal so that it does not overload the modem. Ensure that the existing terminals and hardware support the necessary flow control protocols such as ENQ/ACK, RTS/CTS or XON/XOFF.

• Optimal blocking of data (or protocol spoofing)
Two modems would negotiate with each other for the specific file transfer protocol that should be used before transfer of data occurs. This avoids unnecessary acknowledgments from the connected terminal device to the modem. Hence if two modems can transfer 500 character blocks between them, but the terminal to modem only supports 100 character blocks, the modem would accumulate 5 sets of 100 character blocks and transfer this in one hit to the receiving modem. The receiving modem would then transfer 5 sets of 100 character blocks to the receiving terminal, which would then acknowledge each 100-character block in turn.

• Rack mounted/internal/stand-alone modems
Selection should be made on the basis of the application. A lot of industrial systems use rack mounted modems for saving on space and ease in providing the appropriate power supplies.

• Power supply
Typically most manufacturers of the latest modems have a separate power supply or derive power from the telephone lines.

• Self testing features
Ensure that the modem can perform a self-test and also the standard local andremote loop back tests.