PLCs and PCs

When PLCs were introduced over 30 years ago, relay ladder logic (RLL) was the standard programming language. Ladder diagrams identify the order of operation of devices. The logic can be rewired by programming on the CRT display and tested, without any physical rewiring of the banks of relays. The displays show a ladder diagram with contacts, coils and circuit connection very similar to a schematic diagram. RLL was not designed to support a wide range of control functions. The main input devices...

Conventional Alarm Systems

Conventional alarm systems are typically advisory in nature. They are tag-centric. They provide discrete alarms based on an absolute limit that is defined for a single processing value. They mirror the function of microprocessor-based annunciators by generating messages, annunciating the alarm conditions and logging the event with a data and time stamp. The alarm is usually identified as HiHi, High, Low, or LoLo to advise about the alarm condition urgency. This identification does not take into...

Wearable Electronic Displays

A plant supervisor may soon be able to receive vital information and see multiple areas of the plant through special safety glasses. A technician may be able to probe the circuit and see the output of an oscilloscope in the same view, eliminating the need to look back and forth. Wearable electronic displays can be mounted on or integrated into most frames or safety glasses, letting users obtain and view data while wearing normal vision aids. These new eyeglass displays use a biaxial scanning...

HVAC Intelligence

Most improvements to HVAC systems involve increasing their intelligence. Setback thermostats, which are also known as clock or electronic thermostats, automatically raise or lower indoor temperatures to meet the building's schedule. Indoor temperatures can setback to lower than average comfort levels while the building is unoccupied, then they are automatically brought back up to the comfort level before it is fully occupied. Other smaller setbacks might start at mid-morning when the outside...

Device Net Future

U.S. device sensor buses are projected to grow to 2.7 million nodes in 5 years. DeviceNet should account for over one-third of this total. More than 300 ODVA members direct the future of DeviceNet through special interest group activities. These include alternative media options such as noninsulated flatwire cable, to reduce costs, and gateways and high-speed routers to other networks. When two or more nodes try to access the network simultaneously, an arbitration scheme resolves the possible...

Device Net Troubleshooting Tools

DeviceNet users have a tool to troubleshoot their networks. The hand-held DeviceNet Detective can quickly assess network problems. The unit has the following capabilities anonymously eavesdrop on a network, register on the network as a slave, monitor voltage levels on the network, report errors and diagnostic messages, indicate which node numbers are being used, and report which nodes are masters or slaves. Another unit called a DeviceNet Message Display uses an LCD display to provide...