Difference between RS485 and DH485


Thread Starter

Sandeep Shroff

Hello Automation List members,

At one of our site our client has provided AB's 1761-NET-AIC convertor (which the document says is a DH 485 to RS232 convertor) for doing a RS485 to RS 232 conversion. We are trying to talk using a modbus RTU protocol between a master and a slave device using this convertor. We are receiving junk data in this case.
We are just using this convertor for RS485 to RS232 hardware conversion.

I would like to know from my knowledgable friends whether DH485 and RS485 mean the same at physical layer. or is there any difference?

Is there any special settings or connection required?

Thanking you in anticipation.

Sandeep Shroff
DH485 is a 'protocol', or language like Modbus, or Unitelway is a 'protocol'. Rs232, and RS485 are the physical layer, how they are wired.

You need to check with A.B as to what electrical connection the output is. The protocol appears to be DH485 the convertor is converting the physical layer. If you want to talk modbus you need a 'gateway' (language translater) to convert DH485 (Rs232) to Modbus RTU RS232 or RS485, depending on what you require at the 'other' device. There are many company's that make gateways.

At the moment it sounds like you have a device talking Modbus trying to understand DH485 protocol, which it will not do as it is a different language.
The DH-485 is a protocol, RS485 and RS232 are electrical interfaces. The NET-AIC only converts the DH-485 from Isolated RS485 to RS232 so the protocol can be seen by a regular RS232 Device.
To Convert the protocol to Modbus you need the DL3500-Modbus/DH-485 and then you don't need the NET-AIC as the DL3500 also has isolated DH-485 and then it converts the DH-485 protocol to the Modbus Protocol.

Hope this helps...

Mike Bunker
www.datahighwayplus.com & www.datalinktek.com

Jerry Miille

DH 485 is a protocol, like Modbus and DF-1 are protocols. RS485 is an electrical specification for the physical layer connections. They are not related or compatible.

To be able to communicate with an Allen-Bradley device and a Modbus device you need a protocol converter. There are many converters available including one we make, the Omnii-Comm. You can get additional details at this link http://www.miille.com/din266-p00.pdf or contact me directly at [email protected]

Jerry Miille

Steve Myres, PE

People, people! Read the guy's post! He's not trying to bridge Modbus to DH485. In fact he goes so far as to say "We are just using this convertor for RS485 to RS232 hardware conversion."


You should really only use the NET-AIC for Allen Bradley DH485 Protocol.

The unit will not work for RS232 DF1 Protocol, so I assume it would also not work for MODBUS RS232.

Get a good RS232 to 422/485 converter. Not Allen Bradley. Their interfaces are for their protocols only. Even then I have had many problems with them.