USB-RS232 for Modbus communication


Thread Starter


I have to communicate throw Modbus and Unitelway from a handheld computer wich does not have comm port (9 pin standard)for RS232. My computer has only USB interface.

What interface can I use to make a communication link between the computer described and an external device (RS232 interface) throw Modbus and Unitelway communication protocols.

Thank you in advanced for your help and support.


Sandeep Navale

there a lot of USB to RS232 convertors available in market. you can use one of them.

Hi Dan,
Why not pick up a USB to RS232 converter? we use these in our projects and they work GREAT! The computer will just see this coverter as an extra "com" port.

Be careful which USB to RS232 adapters you use. I've seen a customer of mine not able to communicate with one (I think it was Belkin) but the one I bought (CP Technologies, 20 bucks at has worked with all of the various vendors PLC software I use.

Mark Hensley (Kepware Technologies)

Hi Dan,

We are currently selling the Digi International Edgeport USB to Serial converters. Digi's aren't the cheapest you'll find but we tested them with some of the more finicky idustrial protocols and they handled them just fine. The thing you have to watch for on Modbus RTU protocol is inter-character delays. Essentially the Modbud protocol is designed to sense gaps in a transmitted message. If a message has a gap bigger than about 1-2 character times( approximately 2 milliseconds at 9600 baud), then the device may see the message as an invalid message. So its crucial that regardless of which vendor you use that you do some testing to insure that the converter does not place any delays in the packets.

For us, Digi met those requirements and thats why we carry them.

Mark Hensley
Kepware Technologies