How to select marquee displays

The ballgame is about to start! As the first batter steps to the plate, the large electronic scoreboard provides name, number, and current batting average. As the train approaches the station, the pole-mounted sign over the platform scrolls the train number, time of arrival, and destination. Marquee displays are all around us, delivering consumer, retail, and entertainment information. Perhaps less obvious and more important are use of marquees in industrial manufacturing applications.

The importance of industrial message displays to improving plant productivity should be apparent. Communication in today’s manufacturing operations often needs to be instantaneous, interactive, and readily and continually available. Messages between employees and management need to be exchanged quickly and up-to-the-minute. Marquee displays provide one of the most effective ways for accomplishing these goals, keeping workers in touch with a process by monitoring productivity. They also communicate alarm and safety messages to employees, and provide continuous reports on plant conditions. Once expensive to use, advanced technologies have enhanced features and lowered costs.

Operation and application

Marquees are slave devices, presenting the information sent to them by a master: PLC, PC, or embedded controller. These master devices are used to create, store, and trigger information, which is then sent to the display in ASCII format over a serial connection (RS-232, -422, -485) or through a communications interface such as DeviceNet, Profibus, or Ethernet.

Marquees display preprogrammed messages or the contents of a scaled process variable. For example, a marquee affixed to a pressure transmitter might display a digital equivalent reading, or a display on the plant floor might be programmed to remind workers of a safety meeting time and date. Large LED displays may be visible up to 400 ft away to communicate vital information, or attached to machinery or production lines to monitor and display production rates, alarms, process conditions, critical data variables, operator instructions, and safety messages.

Messages on a marquee may be displayed as stationary, scrolling, or blinking messages, based on embedded codes in an ASCII string. For example, left-scroll messages feature smooth scrolling, with letters moving one LED at a time. Each portion of a letter will illuminate every dot in a row when it scrolls across the display. Non-scrolling messages can be highlighted with blinking characters, with on and off time intervals set, and controlled. Information may also be presented in mixed character sizes on the same line. A temperature zone and value might be identified on one line like this:

Temp. Zone 1: 454

Marquees come in sizes to meet almost any need. Units may be specified with red or tri-color LEDs in sizes to achieve readability at a range of distances. For instance, 4-in. characters are typically readable at 200 ft. Displays with 1 to 8 lines are common. Character capacity varies with size. Marquees may also be equipped with an international character set to allow multiple languages. Certain designs can withstand special plant conditions, including voltage fluctuations, high temperatures, shock and vibration, exposure to dust and water, and electrical noise. NEMA 12 and IP55 enclosures ratings are available.

Each LED may need only 10 mA of steady state current or 80 to 100 mA of peak current; however, the number of LEDs on a marquee may be large. Total power consumption levels should be considered, along with the level at which LED luminous intensity may diminish. Some systems automatically dim at certain temperatures, impacting visibility and clarity.

In some cases, software supplied with the display may be used to simplify the configuration of the unit, preview messages, and generate ASCII strings that then can be copied into PLC ladder logic.

Compiled by Jeanine Katzel, consulting editor , Control Engineering ( jkatzel@sbcglobal.net ), based on information provided by AVG/EZAutomation, www.ezautomation.net .

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