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VMS control system
The VMS control system used in the Variable Message Sign consists of the following components:
•
NTCIP compliant controller
•
LED display board/driver board
•
Sign controller
•
Display adapter
General operation
The NTCIP controller interfaces with the "Host" computer system, and communicates with the Host through a CAT5 Ethernet cable
(standard) or an RS422 connection (option). Using this network connection or I/O port, the Host issues commands to, and requests
status information from, the NTCIP controller. The controller monitors and controls the entire LED VMS system. Communication by the
NTCIP controller to and from the Host is accomplished using industry standard 10/100 Ethernet communications circuitry and NTCIP
protocol.
Placement of controllers
The controllers are located in the lower left modular case of all AlphaXpress 2020 sign configurations. Single row signs have a remote
mounted controller.
Sign controller plate
LED brightness control system
The VMS is equipped with a brightness control system for the LEDs. The brightness control system ensures that the display is readable
in all lighting conditions. The photocells are enclosed to protect them from the environment. Three photocells continually monitor the
ambient light conditions. The photocell assembly is constructed in such a manner that adverse weather conditions (such as heavy
snow) should have no effect on its performance. The main components of the brightness control system are the following:
•
Sign Controller
•
Light sensor assembly
General operation
The photocells are enclosed in an assembly designed to protect them from the environment. The photocells continuously monitor the
ambient light conditions and the controller constantly polls the photocells for this information. The controller determines exactly how
much ambient light is detected through the light-sensing photocells and sets the light output of the display module for it to adjust duty
cycle of the LED current drive, thus dimming or brightening the LED modules according to the ambient light. (If the on time of the duty
cycle is 50%, then the light output will be at 50%; if the on time of the duty cycle is 75%, then the light output will be at 75%, and so on.)
Light sensor assembly mounting
Because the function of the photocells depends on ambient lighting, the light sensor assembly should be mounted in a location where
it will receive maximum light exposure throughout the day. When choosing a mounting location for the light sensor assembly, make
sure the assembly can receive ambient light on the front.
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