I know the answer is in the datasheets, and perhaps my book; but sometimes that information is too cryptic to understand.
When I started with the '51 I managed just fine with the documents I usually refer to (in book form, the internet did not exist), the data sheet of the actual chip I am working with, the compiler/assembler manual and nothing else.

So again, I did not mean to come off sharp to you Eric, and I always appreciate advice, but I assure everyone here that I do in fact read the manual before coming here. I know the answer is in the datasheets, and perhaps my book; but sometimes that information is too cryptic to understand.

Donald referred to the plethora of information available on the internet, maybe that is the problem - one get lost in the amount of information and when something is not found in a jiffy in a given document, on to the next. The result of this is than no document ever becomes an old friend.

When I gave channel 0 an analog voltage of 5V, all 4 LEDs came on briefly and then went off. It has never worked again. As the ADC is less than an inch away from the mPU, I don't think it is noise.
Two points: 0-5V it will take and awwful amount of noise to make 4 LEDs (I assume 5/8V in, 5/4Vin... stay dark. As for noise, the closer you get the adc to the uC, the more digital noise will be picked up by the ADC.

Have fun,

Erik