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Typically most people dont require the use of R/W. Most of the time you will only be writing. I personally have not really found a need to read anything back from the lcd, so my R/W is always grounded.
RS is as the name suggests. Register Select. Its used to switch between certain registers. You need it to inialize, write data, create custom characters etc...
Enable is like a 'switch' that basically says, "do something now". In order for the lcd to respond, it has to go high, delay for a certain amount of time, and then go low. Only then does it perform some function (depending on whats D7-D0)
I have connected the R/W line on my current project as I wanted the ability to scroll the display when I reached the bottom line (4x20 display). However, as tresca states it is normally acceptable to tie R/W low and have a short delay after each write.
RS is for selecting registers between Instruction register and Data register.When RS=0 the signal on the data interface (D1~D7 ) will be write to Instruction register.The device will perform the commands writed.When RS=1 the device will desplay the character corresponded .The pin R/W is for selecting read or write the device.R/W=0 is for writing and the other is for reading.We connect it to GND of circuit usually.Changing pin E from "1" to "0" will make the device to start read/writing (depending the state of pin R/W) the interface.