SCADA on the 7705 SAR

Overview

SCADA systems are used to monitor and maintain networks from remote monitoring locations. SCADA uses a controlling unit/RTU architecture with a single controlling unit that supports multiple RTUs.

The ISC supports software applications that specifically meet the requirements of TDM-based SCADA systems. The ISC is supported on the 7705 SAR-8 and the 7705 SAR-18.

The SCADA MDDB and PCM multidrop bridge applications feature similar architecture and functionality, with the main exception being that the MDDB application uses a serial RS-232 interface, while the PCM multidrop bridge application uses an E&M analog interface.

Multi-drop data bridge

The MDDB application provides a centralized digital bridging functionality that allows a SCADA bridge to be configured between a controlling unit and remote RTUs. The bridge allows a single data message stream to be broadcast from a controlling unit to multiple RTUs and allows a single RTU to communicate back to the controlling unit.

In a SCADA network, the 7705 SAR provides the communications infrastructure to connect the central controlling units to multiple RTUs at remote locations, where the controlling units and RTUs communicate over serial RS-232 links (synchronous or asynchronous). The 7705 SAR-8 or 7705 SAR-18 located at the controlling unit site contains the ISC, which provides the MDDB bridge functionality and acts as the MDDB controlling unit. Remote 7705 SAR nodes connected to RTUs are referred to as MDDB RTUs. See 7705 SAR for information about the ISC. See To configure SCADA on a 7705 SAR for more information.

The remote nodes are connected to the SCADA bridge over an IP/MPLS network. The ISC supports up to 16 SCADA bridges. Each bridge supports 32 branches. Two branches (branch 1 and branch 2) are dedicated connections to the SCADA controlling units; the other 30 branches connect to the RTUs.

Note: Larger bridges can be built by cascading individual bridges internally within a single ISC and using the controlling unit output from one bridge as the RTU input to another bridge. Larger bridges can be cascaded across multiple ISCs by using an RS-232 link.

PCM multi-drop data bridge

The PCM multidrop bridge application provides multidrop bridging for SCADA systems that use 4-wire analog modems to connect remote RTUs to a controlling unit. Incoming analog signals from the controlling unit are converted to PCM (Mu-Law or A-Law) for transport between a remote RTU and the controlling unit. The ISC broadcasts the controlling unit stream to all remote RTUs. Only the addressed remote unit will respond to the broadcast and the response must be transported through the bridge back to the controlling unit via an E&M interface. If the network RTUs support two SCADA systems over the same interface by separating them into high-frequency and low-frequency bands, the PCM multidrop bridge always selects the two loudest branches to be passed through the bridge for communication with the controlling unit.

Redundancy

Most SCADA systems have redundant controlling units where both monitor all traffic but only one controlling unit transmits at a time. The 7705 SAR SCADA system supports duplicated links - one to each controlling unit. Each bridge has two branches that connect to the controlling unit. The bridge supports two controlling unit links with only one being active. If redundancy is required, the second link can be manually configured and activated by forcing a switchover. Data from the bridge to the controlling unit is broadcast to both controlling units. The switchover allows only one controlling unit to access the bridge at a time. The 7705 SAR does not support automatic switchover capability.

Branch squelch

A condition may occur where a single RTU continues to send data to the controlling unit after the normal response period has expired. This condition locks up the bridge so that no other RTU can transmit data back to the controlling unit. To resolve this condition, the squelch command can be enabled on a bridge (it is disabled by default) by configuring a timeout period that, once expired, raises an alarm and triggers the squelching function. Squelching blocks the errant RTU so that other RTUs can continue to use the bridge. The squelch reset command is used to put the bridge back into a normal state. See To configure SCADA on a 7705 SAR for more information about configuring a squelch reset.