please tell me to what the pull up resistor is connected... i mean whether something is connected to the port 0 or just a pull up to the supply is used...
The IR Tx is operated in forward bias and Rx in the reverse bias...
black color is just the absence of all colors and hence it doesn't absorb radiations and hence black line is used.... \]
1-port 0 of 89c51,2(or s51,2) is open collector(drain) port.(the collector of internal transistors not connected to supply )
so we must use of external pull up resistor and so this port can supply current more than other ports.
Port 0 is open drain / open collector and as such needs to be connected to the supply. This has the advantage of being able to supply more current to the circuit.
it has open drain output so as to interface with external memory..
We need a true bidirectional port for interfacing of memory.. and PORT 0 serves as both data bus ( D0-D7) and lower address bus( A0-A7).. these are time multiplexed
and lower byte of address is latched with the help of an octal latch and ALE signal.