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The 8574A/PT provides general-purposeremote I/O expansion via the two-wire bidirectional I2C-bus (serial clock(SCL), serial data (SDA)). The devices consist of eightquasi-bidirectional ports, 100 kHz I2C-bus interface, three hardware addressinputs and interrupt output operating between 2.5 V and 6 V. Thequasi-bidirectional port can be independently assigned as an input to monitorinterrupt status or keypads, or as an output to activate indicator devices suchas LEDs. System master can read from the input port or write to the output portthrough a single register. The low current consumption of 2.5 A(typical, static) is great for mobile applications and the latched output portsdirectly drive LEDs. The 8574A/PT are identical, except for thedifferent fixed portion of the slave address. The three hardware addresspins allow eight of each device to be on the same I2C-bus, so there can be upto 16 of these I/O expanders 8574A/PT together on the same I2C-bus, supportingup to 128 I/Os (for example, 128 LEDs). The active LOW open-drain interrupt output(INT) can be connected to the interrupt logic of the microcontroller and isactivated when any input state differs from its corresponding input portregister state. It is used to indicate to the microcontroller that an inputstate has changed and the device needs to be interrogated without themicrocontroller continuously polling the input register via the I2C-bus. 
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