Interface circuit for use in a programmable message display

Abstract

An interface circuit for reading data to a programmable message display is provided wherein input power is reduced through sampling techniques and circuit complexity is reduced through multiplexing. Stable operation is providing in the presence of noise when the interface circuit is coupled to a device, such as a programmable controller. The circuit includes a plurality of input circuits which receive and condition a like plurality of parallel input data signals. A reference signal generating circuit is responsive to control signals from a microprocessor of the display to provide reference signals. A like plurality of comparator circuits are coupled to their responsive input circuits and to the reference signal generating circuit. Different banks of the comparator circuits sequentially compare the conditioned input signals with the reference signals. Isolation circuits are coupled to the outputs of the comparator circuits. In particular, each of the isolation circuits is coupled to the output of a single comparator circuit of each of the banks of comparator circuits to provide isolated output signals to the microprocessor which correspond to the level of the input data signals received by the input circuits.