Hi Forum Members,

I am in the process of designing a process control system which needs Network. The Network is needed so that a central PC in office can send parameters to several machines on the shop floor and machines can send their status and other relevant information to PC. Number of process control machines is expeted to be around 10 to 15.

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.

On the control boards of machine the MAX485 will be connected to P89V51RD2 UART. and the DE and /RE pins will be tied together and connected to a port pin say P3.7 These boards will use MAX812 power supervisor. 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 say if some other communication was on at that time there is a contention. How to avoid this,

One obvious solution is to use a transistor acting as an invertor connected between port pin and DE /RE.

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.

Now How to take care of the above both problems. I am not facing these problems since the product is still under design. I was thinking over contention issue and these things puzzled my mind.

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.


Thanks to all of you for reading my post.

Thanks & Regards,
Prahlad Purohit