Internet PLC Forum

General => Technical support => Topic started by: Brian Madsen on June 28, 2004, 03:31:35 AM

Title: rs-485 burnt
Post by: Brian Madsen on June 28, 2004, 03:31:35 AM

I'm running a T-100MD2424 with a single auto-485. Both 7805 regulators on the auto-485 and the plc itself overheated. I'm guessing that a loose wire sent a voltage spike into the rs485 line. Is the 75176 transceiver IC the only component that normally needs to be replaced in such an event? Is there a possibilty of any other collateral damage?

Title: Re:rs-485 burnt
Post by: support on June 28, 2004, 05:46:49 PM

Yes, if there is overvoltage on the RS485 line then it is possible that both the 75176 on the PLC and the Auto485 are fried. You can buy replacement ICs from Digikey.com or local electronic store rather easily. Alternatively you may want to buy a fault tolerant version of the RS85 driver made by Linear Technology. The part number is LT1785CN8 which can withstand up to +/-60V on the RS485 line. This chip is more robust if your RS485 line is long and has potential danger of experiencing high voltage spikes.

Title: Re:rs-485 burnt
Post by: PESPLC on July 29, 2004, 06:09:50 AM

I have fried two PLCs in the manner you describe.  I attribute this to a ground loop differential between comm1 and comm3.

I am now using an opto-isolated RS485 controller on comm1 to control digital devices and a similar opto-isolated RS485 on the PLC comm3 line to my PC's RS232 port.

At the same time I received a shock from my IBM thinkpad chassis (which was connected to PLC comm3 via an un-isolated 485 convertor)

Since switching to the opto-isolated converters I have had no problems.  I got then from www.rs485.com for about $70.00

Good luck!