Differences between USB-485 adapters?

[JK-74]

Hello,

I'm working with an application collecting data from a gas sensor via Modbus. I'm using a USB-485 adapter for the Modbus communication and found out a strange problem. I have several USB-485 adapters that should be identical but only one of them works fine. With the other ones, I get error messages indicating that Modbus connection can't be opened or Modbus registers can't be read. I even bought two new adapters so I don't think the non-working adapters are just broken.

I've checked that the port settings for each adapter are correct, both in Windows and in MAX. I'm using LabVIEW 2016 and NI Modbus 1.2.1 library.

Have any of you dealt with a similar problem and managed to solve it?

Kind regards,

Jari

[jay0909 · Accepted solution]

Hi,

       Normally if you use FTDI USB RS485 it works good for all the cases. I also have used 1 or 2 adapters from Chinese manufactures and it works good only upto certain Baudrates. Try FTDI.  if you need any help executing your program i can help with it.

[JK-74]

Hi,

Thanks for your reply. I don't know the manufacturer of the functioning adapter but the new ones are from China. In my application, I use baud rate of 9600 which shouldn't be that high.

[pincpanter]

It may seem a silly question and probably it is, but... do your adapters automatically manage the echo suppression?

[Hooovahh]

FTDI and Prolific are two popular options for USB to serial adapters.  These chipsets have tons of counterfeit chips on the market.  As a result both FTDI and Prolific have release drivers that cause the counterfeit chips to stop working, and in some cases stop functioning permanently.  First make sure when you plug in your device Windows recognizes it and it shows up in Device Manager without any errors.  If it doesn't have errors it should be good to use in LabVIEW.  If it does it won't work in LabVIEW or a any other application.

http://www.totalcardiagnostics.com/support/Knowledgebase/Article/View/92/20/prolific-usb-to-serial-f...

https://hackaday.com/2016/02/01/ftdi-drivers-break-fake-chips-again/

[JK-74]

To clarify the problem a bit, all adapters I've used are from National Instruments. So for me they should all work in a similar way.

http://www.ni.com/fi-fi/support/model.usb-485.html

[pincpanter]

I never used such adapters, but it seems that it's possible to change the wire mode, either by MAX or programmatically.

I guess the wire mode you need is 2 Wire - Auto.

[JK-74]

Yes, the correct mode in my case is the 2 Wire Auto. I've checked that the settings are correct both in Windows Device Manager and MAX. Still only one adapter works fine...