Dear All

Thanks for all your comments, i will explain you the exact condition.

The project is for railway station where we have to connect 10 LED display boards of 16X288 LED matrix double faced for train information. All the 10 boards have 18 (9 double faced) 16X32 Led boards as i have explaind in the last diagram. These boards are controlled by one master controller attatched to the PC kept at around 600-700 feet away from the last board. i have used RS485 @ 4800 BPS for communication. all these boards are working fine independently, in loop upto 6 boards are working fine with out any problem. The problem i am facing is in the last four boards. Where the data received is not proper.

I am using normal computer SMPS (as used in PC) and my one line consumes arround 2.7-3A so both line together is 6 A max this is the total load on 5 V of the SMPS. I first thought that the RS 485 signal is creating problem, we tried all the options but did get any results. Then finally i changed the SMPS of 6th board with liner and tried to maintain 5 V on all the LED card, The boards started working fine absolutely fine. So i started to look arround the voltages. Another interesting thing i found is the difference in 75176 lot i have used some are 75176AP and some are 75176BP. I searched the data sheet the difference is input hysterisis loop AP version have 50mv and bp has 70 mv. I have to verify this also sine the boards having BP are functioning properly.

I will keep you guys updated.

Thanks for your patience

Santanu