Specialized SCADA protocols

A protocol controls the message format common to all devices on a network. Common protocols used in radio communications and telemetry systems include the HDLC, SCADA systems, software and protocols 73 MPT1317 and Modbus protocols.

High level data link control (HDLC) protocol

HDLC has been defined by the international standards organization for use on both multipoint and point-to-point links. Other variations of this protocol include SDLC (synchronous data link control used by IBM) and ADCCP (advanced data communication control procedure used by ANSI). HDLC is a bit-based protocol. Other protocols are based on characters (e.g. ASCII) and are generally slower. It is interesting to note that it is a predecessor to the LAN protocols.

The two most common modes of operation of HDLC are:
• Unbalanced normal response mode (NRM): This is used with only one primary (or master) station initiating all transactions.
• Asynchronous balanced mode (ABM): In this mode each node has equal status and can act as either a secondary or primary node.

The CSMA/CD protocol format

The HDLC protocol describes the complete communications process and provides a complete set of rules for controlling the flow of data around a network. The CSMA/CD protocol is not as comprehensive as HDLC and is concerned with the method used to get data on and off the physical medium. HDLC and CSMA/CD can be incorporated together for a more complete protocol.

Distributed network protocol

The distributed network protocol is a data acquisition protocol used mostly in the electrical and utility industries. It is designed as an open, interoperable and simple protocol specifically for SCADA controls systems. It uses the master/slave polling method to send and receive information, but also employs sub-masters within the same system. The physical layer is generally designed around RS-232 (V.24), but it also supports other physical standards such as RS-422, RS-485 and even fiber optic. There is large support within the SCADA industry to use DNP as the universal de facto standard for data acquisition and control.