pull up and pull down resistors

Consider a bus(a line) of microprocessor/controller to which various peripherals are connected. If any of these require a service from processor, they will 'pull down' the line to ground potential(or to -5v sometimes). The processor checks this line periodically and if found low(i.e logic 0) it then proceeds to serve the particular peripheral. Once the task is complete the peripheral ceases to pull the line(typically it brings the line to 0 potential by shorting to ground or negative of supply terminal). Now in order to bring this line back to logic 1(to raise it +5v), it will be connected to a supply voltage through a resistor. This resistor is called pull up resistor. All it has to do is to keep the line in logic 1 state provided none of the peripherals are pulling it down to logic 0. This is necessary in order to prevent the processor from getting the wrong signal though none of the peripherals require service,because the line remains at 0 logic until the resistor pulls it. Similarly a pull down resister will keep the line in logic 0 state by default except when any of the interrupting device pulls it to logic 1 state.

Nice explain Ajit,
I'm other simple language you can Pull up and Pull down resistors are required when there is more current required like in case of microcontroller Chip can provide 20mA maximum and you want to draw more current then you can use Pull up resistor.

Dear Friend... Considering a CMOS Transistor u may understand this concept of Pull Up and Pull Down. The Pull up Transistor Pulls the Output to Vdd (That is the supply given)
The Pull Down one Pulls output to Zero...
So u have answer in ur que. itself.

Then if u dont have these transistors means what else will be there? For building any circuit we need transistors only. Without pull up or Pull Down Transistors how can u balance the output to Vdd and 0 when Required!!!

So, Go on with Uyemura Text, U can understand this Beautiful Concept...
The Resistor is one used to prevent Transistor and Based on Bias Concept (Dono u r aware are not at this moment) the Transistor is same as Resistor...

And the Concept of Pull Up and Pull Down Remain Same... 👍

Any Glitches in my words kindly let me know...

Any Help Regarding VLSI or Electronics Related... Leave me a Post...

All The Best...