i am trying to design a microcontroller based fault detection system where a single receiver will receive any data from multiple transmitters. it is assumed that at a time only one transmitter will send the data. all microcontrollers belong to 8051 family, and have a 11.059MHz crystal. the baud rate is 9600bps. i have used max487 for the networking. now when a single transmitter is attached to the network, the receiver works. but when i add another transmitter (tot 2 transmitters & 1 receiver in the network), the receiver does not respond. i checked with logic probe that no data is reaching at rx pin of receiver. the number of transmitter will be upto 127. please suggest.

 

With the MAX487 DE pin pulled high ALL transmitters are on at once, so they are fighting each other to drive the network. As stated in the Wikipedia Article "RS-485 drivers need to be put in transmit mode explicitly by asserting a signal to the driver. This allows RS-485 to implement linear topologies using only two wires. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations and devices."

So you need a little hardware change so each slave controls DE, then a software change where the master unit requests each slave in turn to report status.