Usually it happens when the transmitter and receiver baud rates or other communication parameters don't match.


First of all let me tell you that a multimeter is not a good enough instrument to check for rapidly changing port pin activity especially when the waveform is not continuous for a long enogh time for the multimeter to measure the average DC. Secondly I don't believe that on an AT89S51 (powered by 5V) you measure all pins 1-20 as high. Remember pin 20 is GND. How can it be high? Are you sure?

Ok if pin 20 is grounded and you see pin 9 as high, there must be something wrong with your Reset circuit. What are you using for Reset?

Also if you have MAX232C then 0.1 caps are fine otherwise you need 1uF capacitors as Peter already suggested.

I would suggest you to do as follows:

1) Remove micro from socket and power up the circuit. Does PC receive garbage?

2) For the time being forget serial communication and try running a very simple program to test your hardware. You can set TXD (P3.1 ?) high for 1 second and low for another second and repeat it in a loop. Now with your DMM check if TXD switches state from high to low and vice versa? If yes, now check at pin 14 of MAX232 (T1 Out). You should see a DC voltage with +10V and -10V values with the same timing, of course.

3) If you can't see any activity on TXD then check your Reset circuit, oscillator, state of EA, power supply, decoupling capacitor etc.