Computer Interfacing
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Multidrop Communication using RS 485


 

       Computer Interfacing Forum Index -> RS422 and RS485
Author Message
Kaushik
Guest







Oct 17, 2009 7:21 am

We have made 8051 micro controller based computer interface boards for SMPS (80 KHz switching frequency)power supply.The scheme is as follows:

1.The reference signal and the read-back signal from power circuit section of the power supply is directly (without any isolation) connected to the DAC and ADC.

2.DAC and ADC are directly connected to the AT89C51 microcontroller.

3.For remote opeartion RS 485 is adopted. The signal between MAX485 IC and AT89C51 is optically isolated (PC 847 is used as optocoupler) and GNDs of them are separted.

Now, we r using 12 such power supplies in multidrop fashion.PC acts as master and connected to the supply through Rs-485 to-Rs232 converter powered by +24V Dc supply.We r using BELDEN cable
Two wire (D+ & D-)communication is adoted.

NOW THE PROBLEM IS THAT WE FIND THAT POWER SUPPLIES COMMUNICATE WITH PC FOR RANDOM NUMBER OF CONTINUOUS HOURS.SOMETIME THEY WORKS FINE FOR 4-5 HRS AND EVEN FOR DAY LONG,BUT SOMTIMES THEY FAIL IN EVEN ONE HOUR.

Some points to be noted as follows:
i)we r using labview for PC side application.Sometimes PC hangs due to communication failures.

ii)The power supplies are located in back ground of RF,microwave radiation.
iii)Load of power supply is inductive and length of the load cable is around 50 mtr.
iv) The local display of some of the supplies (not all)also give incorrect display.The local display is a 16X2 LCD display which is also directly connected to the AT89C51 controller and having same ground of microcontroller.
v)We have not used any terminal resistor as our baudrate is 9.6kbps and distance is 50 mtr.
vi) we have not used any pull-up or pull down resistor externally to the D+ and D- pins.

Considering all the points mentioned above if anybody give me some solution so that there a completely reliable communication can be established.

Any quiry for further clarificatin is appreciated.

Thanks in advance

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