Thus I am planning to use a RS485 network for this purpose each machines control board will have MAX485 or 75176 connected to P89V51RD2 micro. On PC side there will be a MAX485 + MAX232 going to serial port.
For this I would look at more modern transcievers with better ESD protection.

Now the problem I smell is bus contention on powering up a process control unit. When the control board is powered up due to reset controllers reset assertion for several 100 mili seconds + the 400ms period for which the P89V51RD2 waits for ISP program mode signal on UART. The port pins during this period will be all high. This will cause DE to go active and MAX485 will try to master the bus
There are modern transcievers that are short circuit safe. You may see the transcievers going into thermal shutdown for a brief period, but I have yet to see this for networks I have worked with (without the DE pin inverted).

But again I smell second problem, that is during this period for which micro waits for ISP signals on UART with all its port pin high. Due to high port pin the /RE will go active and say if by any chance there was an 'U' getting transmitted the micro will enter ISP mode.
You can not ISP with 485. You need 422 or 232. On my 485 boards I have a connector for an external "miniboard" with a MAX232 (equivalent) for ISP purposes. I have e-mailed umpteen Philips persons about a "direction pin" during ISP to no avail.

Another thing: According to NATIONAL APP NOTE "Bullet Proof RS485" The ICs can get damaged due to contention is it true. What are the chances of burning an IC due to this approx 500ms contention at power up.
quite good with older designs of the 485 transciever such as the -176, but with the newer derivatives with all kinds of protection very unlikely.

Erik