Of course, you need here either buffers with 3-state or open-collector (open-drain). I do not remember whether the serial lines RX, TX are open collector or not for the 8052. If not then a buffer need to be inserted between the TX of the master and the RXs of the slaves. But in principle I should say the TX of the master is connected to RXs of the slaves, while the TXs of the slaves are connected together and to the RX of the master. In that case all TXs are in 3-state (high Z) or pulled-up (when open collectors are used). As I emphasized, the MASTER controles the "bus", that is he should talk always first, and ONLY the addressed SLAVE should answer. IN my project was a slitly complicated, because all TXs and RXs were connected together on the coaxial cable. In that manner, the master - after sending its message set its TXs in Z-high, and the ADDRESSED slave responded back after a while. The slave then sets its TX back to Z-high.