" I'm using a 19 channels serial ADC, TLC545. The PC select one of the 19 channels of the ADC. The converted data of the selected channel is received by 51 and sent back to PC using 51's UART.

The PC outputs multiplexer address to 51, and 51 sends data back to PC."

You yourself mentioned that 51 receive channel address from PC and its send back data of that channel to PC. So whatever channel address PC has sent it receives data of that channel only

Like
PC to 51 -> 07 ; pc wants 51' to send data of
channel 7
51' gets data of channel 7 and send it to pc

51 to PC -> 3fh ; acquired data value of channel 9

Can't understnad 1,2 1,2 ..?

Anil