Most common connector for Modbus cabling?


Thread Starter

Klaus Kragelund


We are developing a new product to be hooked up on a Modbus cable as a Slave

I have done a lot of searching for modbus application to find which Modbus connector is the most common one. As far as I have found, Screw terminals are the most common, followed by RJ45 and lastly DB9 connectors

Is this a valid assumption?

Or is the trend the last couple of years towards RJ45?

The advantage of RJ45 is easy cabling to the product, but then one needs a splitter box or another way to do the daisy chaining of the Modbus unit. Another advantage of RJ45 is that the connector can be used in an Ethernet/LAN system also, without changing the product enclosure etc.

MODBUS serial is a <b>SOFTWARE</b> specification only. It does not require any particular type of serial port hardware. You are free to choose any type of port, but a standard DB9 serial port wired for RS-232 is probably still the most common. RS-232 cables are cheap and easily available and most end users can find a spare cable at the bottom of a drawer somewhere. Some suppliers are moving towards using RJ45 because its smaller and neater. Typically, RJ45 serial cables are more expensive and users dont have one lying around already so if price is a consideration, stick with DB9.

Modbus TCP is an ethernet protocol and almost always uses RJ45 as that allows the use of off-the-shelf ethernet cables.

Lynn August Linse

> I have done a lot of searching for modbus application to find which Modbus connector is the most common one.

Ta-Da - the specification "Modbus Over Serial Line V1_02" actually includes recommendations for RS-485 naming, grounding, as well as RS-232/485 pin-outs for RJ45 and DB9.

One could easily include 2 RJ45 sockets on an RS-485 product to allow easy daisy-chaining. The main problem with RJ45 is one won't find 'serious' 1mm/22awg wires for long distance.

You can find this on the 'tech reference' > 'Specifications' area.
I suspect you are actually thinking of a DE9 connector, not a DB9. This is a common error made by people who do not realize that the second letter designates the shell size. Size "B" D-shell connectors are about two inches long. For example, the parallel printer ports on our old computers used DB25 connectors. Size "E" connectors are about one inch long. DE9 connectors are the 9-pin ones commonly used for serial ports and DE15 connectors are the 15-pin ones commonly used for VGA ports.