Two wires and an address

A write, frame by frame

Press play and read the sentence under each frame. Green is your board driving SDA, orange is the chip.

Your board calls 0x50 with a 0 on the end, which means "I am going to write". The chip pulls SDA low in the ninth clock, its acknowledge, and the other devices on the bus stay quiet. Then come two bytes of memory address, high then low: this chip has 16,384 places, and one byte only counts to 255. Then the value, then the stop.

The stop is the important frame. Nothing has been stored until it arrives. Then the chip spends up to 5 ms writing, and for that time it answers nobody, not even its own address. Five milliseconds a write is about what that does to a sketch.

A read starts as a write

Switch the figure to the read. It begins exactly like a write, 0x50 and the two address bytes, but sends no value. That "write" only moves the chip's pointer to the byte you want.

Then a second start with no stop in between, so nothing else can take the bus, and 0x50 with a 1 on the end: "now you talk". The chip sends the byte. Your board answers with no acknowledge, which means "that is all", and a stop.

If your board acknowledges instead, the chip sends the next byte, and the next, for as long as it is asked, across the whole chip. That is how the sketches read a block of text in one go.

In Arduino

The same frames, in the Wire library:

Wire.beginTransmission(0x50);   // START, 0x50 + W
Wire.write(0x01);               // address, high byte
Wire.write(0x00);               // address, low byte: byte 256
Wire.write(42);                 // the value
Wire.endTransmission();         // STOP: the write begins now

Wire.endTransmission() returns 0 when every byte was acknowledged, and a number when one was not. The sketches check it.