Modbus RTU on RS485 communication alarms

[Beth F]

First of all, let me say that I realize this is a long shot that somebody has experience with this same setup as mine, but any suggestions would be appreciated.  I'll just run with the theory that the whole is equivalent to the sum of its parts. 

I am trying to setup up an RS 485 loop between 4 temperature controllers, which connects to an Ethernet gateway.  In my PC, I am running a port redirector (to create virtual ports, so it appears to the software that the 4 controllers are hooked up directly to serial ports on the PC), the IA OPC Server with Modbus objects and LabVIEW DSC. 
I configured everything, and it works just perfectly... but it only works about 70% of the time.  Periodically, one of the controllers will return with a communications error message (often Garbled Communication or No Response).  This controller will stay in alarm for anywhere from 10 seconds to 10 minutes, and then clear by itself, then alarm again.  This wouldn't be too hard to troubleshoot if it was the same controller every time, but this alarm problem will occur in one particular controller off and on for a few hours, then later it will be a different controller that alarms and clears in the same manner.

 
I have tried using a different type of Ethernet gateway and using a different OPC Server, but neither seemed to make a difference.

Are there any special considerations when using RS485?  This is my first time trying it. 
Is there anything in the modbus protocol that might cause problems like this?  Any recommendations for timeouts and retry parameters? 
I am open to using direct VISA drivers instead of OPC, but have never gotten them to communicate at all.  
Thank you for any suggestions - I've tried everything I could think of. 

Beth

[Brian_Beal]

I don't have experience with your exact setup, but am using Modbus comms with an OPC server, 485, and a 485 to ethernet gateway.  They are not all on the same system, though.  I doubt that it is the OPC server.  It is probably the gateway, but you should narrow it down.  Check out the Modbus 485 first.  What controllers are you using?  What speed? What is the distance from the last controller to your gateway?  Is there a way to go directly to a computer with the 485?

I have tried 2 gateways, and had problems with both of them.  One I couldn't get to work, and the manufacturer spent much time trying to get it to work with Modbus.  They finally claimed that they found the right combination of settings, but I had already gone back to my original gateway.  That one acts like yours.  I lose comms with the instruments sproradically.