What should I expect at UART Tx and Rx pins of 8051 when the pins are not connected, i.e hanging. Using multimeter, I found Rx pin high, and Tx pin low. Is it OK?
A stop bit (and therefore an inactive UART Tx pin) is high by default. So the voltage on that pin will be somewhere near Vcc when unloaded and when no data is sent out of the chip.
Rx is an input and therefore carries no voltage. Depening on the input circuitry of the chip the input may be pulled low, high or floating high impedance. In either case it can barely source/sink any current.