Thank you all for the advice.

The OE lines are already tied together in hardware. The boards will all have same 8051 code on them. They will be programmed the same in production. Each board will have a 6 byte string in it which acts like to slave id. The six byte string is unknown to the master device. So there is no way master device will know the slave devices id and vice versa.

When the power is turned on, all 3 units come up as slaves.
The unit that is connected to Ethernet will have a port pin that goes low when Ethernet activity is detected. Hence he becomes the master. This port pin (ex. p3.6) will not go low on the slave devices. The 2 devices that have p3.6 high will remain as slaves.

The plan is to pull the master device's OE line high to transmit a hex string like AA 01 CC. After sending this string the master will pull OE line low. This means that the master has switched to receive mode.

The slave devices will all have OE line low to start with. That means they are in receive mode. The slave devices will receive AA 01 CC simultaneously.
The slave devices will turn the OE line high and switch to transmit mode.



[B]The slave devices will all try to send their acknowledgement back. As there can be only one transmitter on the RS-485 line, the device that succeeds to pull its OE line high first will get control of Transmit line and send a byte like '55h' to the other slave device. The slave device that did not succeed will receive byte 55h and just shut up. [/B]

I see some confusion here as the slaves are already in transmit mode and do not have receive mode enabled (OE line low).

I am stuck here. After this I do not know how to proceed.

Frankly speaking this idea came up after all the hardware was implemented in our design. Our hardware Engineer had provided 2 dip switches on slave devices as well as master devices that could be set to 00,01,10,11 in binary. The original plan was to use the dip switches as slave ids.
This seemed pretty feasible in the beginning.

Now the sales guys are asking the Engineering to take out the dip switches and make the slave recognition automatic.

I am looking for options in software.

Thanks.