Introduction
Direct wiring methods are commonly used for simple on-off device control, where a circuit is either open or closed. A typical example would be the wiring used for a light switch. Likewise, digital sensors are usually wired directly to monitoring panels in the world of industrial automation. A sensor responds to a certain environmental condition by closing the circuit for an indicator on the display panel. No programming is required, and the main considerations are simply the voltage, current, and distance involved.

As the distance increases, it becomes exponentially more difficult to wire these connections. The growing presence of Ethernet in many industrial sites has given system integrators other options for connecting digital sensors to remote display panels. We can learn about the real-world advantages and disadvantages of these different options from the experiences of a fictional frozen food company, XYZ Inc.

Isn't direct wiring easier?

XYZ Inc initially stored its inventory in large freezers that each required round-the-clock monitoring. For each freezer, a display panel right outside the entrance indicated the current status of items such as temperature, door, compressor, oil pressure, and power. Accordingly, the display panel was wired directly to the various sensors, with wiring embedded within the freezer walls.

As XYZ Inc expanded, they wanted to establish a control center to monitor each freezer along with other food processing equipment. They also wished to install additional sensors to monitor other aspects of their freezer operations. However, with sensor wiring already embedded in the freezer walls, XYZ Inc faced the following challenges:

• It would be expensive and difficult to re-wire for new sensors.
• Laying down new wiring from each freezer to the control room would require a major overhaul to the facility's infrastructure.
• Directly connecting every sensor to the control center would cause the wiring at the control to be very complex.
• Sensor signal integrity would be difficult to maintain over the required distances due to line corruption and signal loss.
• XYZ Inc would be committed to the new wiring infrastructure, and future modifications would be even harder to implement.

Power that comes at a price: PLCs and Ethernet Modules

Instead of directly wiring the freezer sensors to the control center, XYZ Inc decided to install PLCs (Programmable Logic Controllers) with Ethernet modules. With PLCs, wiring was much simpler since sensor wires were grouped at each PLC instead of strung throughout the facility. PLCs also provided high performance and versatility, since sensor information could be easily shared with other devices and network hosts within the SCADA system.

When XYZ Inc faced a need for additional refrigerated storage facilities, they wondered if other options were available. The new facilities would all be similarly equipped with multiple freezers and a central control station. However, several disadvantages had been encountered with the PLC and Ethernet module approach:

• Specialized ladder programming would be required for any installation or configuration change.
• Ethernet modules for PLCs were expensive.
• A proprietary design was used for the PLC, which would make XYZ Inc dependent on a single vendor for expansion options.

Taking advantage of cheap PCs: PCs and I/O Servers

XYZ Inc was able to meet their needs by using PCs and I/O servers. PCs provided flexibility and cost-effectiveness by being easily replaced, easily programmed to perform any function, and easily integrated with XYZ Inc's other IT systems. Monitoring software could be purchased or custom-developed without requiring additional protocol conversion or programming in ladder logic. Furthermore, PCs were an open platform that would not become outdated when the existing Ethernet architecture was eventually upgraded.

As high-speed IP networks began to connect almost every aspect of XYZ Inc's operations, they continued to search for more effective ways to connect sensors over Ethernet. Even though communication was theoretically possible between any two points on the network, XYZ Inc still encounctered some drawbacks with the PC-based approach:

• PCs and I/O servers introduced a 100 ms lag time to sensor readings, which was not acceptable for certain critical processes.
• When a large number of sensors were connected, performance suffered since the PC had to poll every sensor repeatedly.
• Substantial programming time and effort was still required in order to manage the sensors and display panel from the PC.
• For certain locations, physical limitations made it impractical or impossible to install a separate PC.

A new and easier alternative: Peer-to-Peer I/O Servers

Since XYZ Inc already had an Ethernet infrastructure in place, they were able to use peer-to-peer I/O servers to connect sensors to remote display panels. Due to its simplicity, this option offered significant advantages over both direct wiring methods and other Ethernet-based connection methods. A peer-to-peer I/O server could be installed at each end of the connection with just a few minutes of configuration and no specialized training. No programming was involved, and no additional controllers or modules were required.

The effect was as if the sensors were physically connected to the display panel, with a response time of only 20 ms. Even better, low overhead and space requirements meant that XYZ Inc could install sensors in previously inaccessible locations. They were now able to monitor items and processes that would have been impossible to reach using direct wiring or other methods.

Peer-to-peer I/O servers offer the following advantages over other methods:

• Almost no additional overhead is required in terms of equipment, space, time, and effort.
• High monitoring performance is achieved at a fraction of the cost and effort required by other methods.
• Installation is the easiest of all available methods and requires no programming and no changes to the infrastructure.

Conclusion
As we can see from the experiences of XYZ Inc, sensors can be connected to remote display panels using a variety of methods. As the distance between sensor and display panel increases, direct wiring becomes more difficult, and connecting over Ethernet becomes more attractive. Among the options available for connecting sensors over Ethernet, using peer-to-peer I/O servers requires the least overhead, yet provides the best performance and the easiest installation. For simple, direct sensor connections over long distances, peer-to-peer I/O servers can save companies a tremendous amount of effort and expense, and can make new monitoring applications possible.

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