Swiss Army Knife of SCADA Series: Part 2
Mar 24, 2022
Written by: Jon Tandy, Product Application Engineer
This article is Part 2 in the “Swiss Army Knife” series, describing the wide versatility of product features and application use cases for the Elecsys Industrial Data Gateway (IDG) products. The Director and RediGate edge gateways provide many capabilities that enable SCADA (Supervisory Control and Data Acquisition) and IIoT (Industrial Internet of Things) customers to add value to their control and monitoring systems.
In SCADA systems, a commonly used technique is “poll-response,” in which the SCADA master requests data from each field device and waits for the response data before sending another poll. In a small system, or when using a Local Area Network with abundant communication bandwidth, this method may work reasonably well. But when the system consists of many thousands of devices and a communication network with more latency (such as satellite or cellular), the total amount of time required to poll data sequentially from each field device can be excessive. This can result in potential safety or environmental hazards, or a control scheme that is not adequately responsive to physical conditions. This is especially true when communication may be intermittent, resulting in poll retries and timeouts. Another problem is that poll-response systems require the SCADA system to consume more resources processing all the data, all the time, even when most of the data may not have changed.
Several of Elecsys’ SCADA customers operate interstate oil or gas pipelines spanning thousands of miles, consisting of thousands of measurement devices and relatively slow communication networks. Even though network bandwidth and speed continue to improve, there is correspondingly an increase in the number of installed devices and the quantity of data to be monitored. The problem of poll-response latency can even affect customers with smaller networks, such as a manufacturing plant with high-speed Ethernet, where the volume of data or rate of data acquisition can overrun the available communication bandwidth or SCADA system data processing resources.
The Elecsys Director and RediGate edge gateways were designed to solve the problem of data acquisition latencies and processing overhead. Using Report-by-Exception (RBE) techniques, customers have reduced their overall system latency from 12-15 minutes down to 6 seconds! The gateway installed at each remote site polls devices in their native protocols, locally examines the data for changed values, and instantaneously publishes only the changed data for the SCADA system to consume. Because each site can report data simultaneously, rather than relying on a sequential polling cycle, the system becomes orders of magnitude more responsive.
Below are some of the features provided by Elecsys gateways that address important considerations when moving from a poll-response to a Report-by-Exception topology:
- Choice of RBE protocol – Elecsys typically uses MQTT for reporting real-time data, including a compact binary format, Sparkplug-B, JSON, and Google Protocol Buffers (Protobuf). We also provide Report-by-Exception to our HCP (Host Communication Processor) middleware for customers whose SCADA systems still depend on Modbus polling.
- Data Integrity – An RBE system must provide high assurance that the communication network is constantly connected and no data is lost. Elecsys gateways send a full data refresh upon initial connection, upon request from SCADA, or in the event of a lost packet, and then changed data in between. Regular heartbeat messages are utilized when data isn’t changing to ensure connection reliability.
- Deadband – Analog values usually change frequently with small variations, but reporting every change could overwhelm the system with unimportant data. Elecsys gateways include a Deadband feature so that each data point can be configured with a sensitivity threshold that limits reporting of small changes. The Deadband setting, along with the speed of local data acquisition, allows the user to balance data sensitivity versus communication bandwidth usage.
- Redundant Connections – Robust networking allows the use of multiple communication paths for failover in the event that one server or communication channel becomes temporarily unavailable.
- Store and Forward – Some of the RBE protocol options include a Store and Forward feature so that if a connection is lost to an MQTT broker, the changed data is stored locally on the gateway with a timestamp, to be reported later and archived in a time-series database.
- Periodic Reporting – Sometimes it is desired to periodically update data on a timed interval in addition to (or instead of) the RBE data. The Elecsys gateways also support sending data on demand, such as an alarm or batch data.
We invite you to “stay tuned” to future installments of this series of articles on important uses of the Elecsys IDG products. Please consider how Elecsys can help you improve your operation’s efficiency, productivity, and add value to your bottom line.
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