Two MODBUS IO_Servers talking to One Device


Thread Starter


Can I have two Modbdus IO Servers (IE: Wonderware) talking to the same device?

Do I need a MUX between the two HMI systems and the one device, if so who makes devices like this?
I haven't used it, but would check out the following if I had your task:

Modbus to Ethernet Bridge, part 174CEV30010. Associated documentation, 890USE15500, available at Modicon's website.

Good luck

William Hinton Sr. Engineer Delphi

The Modbus Plus network allows multiple processors to read and write to the same I/O. You simply traffic cop it in both controllers. The documentation is at "":

We have done this for various monitoring applications with good results and we have also written and read the same registers by more than one master using this same proceedure.

Lynn August Linse

Easiest answer is to use Modbus/TCP drivers. Then purchase a simple Modbus/TCP Ethernet to Modbus/RTU Serial bridge (as made by Digi and
Modicon and many others) can be used to 'share' your Modbus serial slaves amongst many master (or OPC servers). This type of product has been
maturing for almost 5 years, so works fairly well as a type of 'MUX' front-ending serial devices. Our product is called the DigiOne-IA-RealPort.

Best Regards

Lynn August Linse, alist (at) linse (dot) org
IA Firmware Specialist, Digi Int'l (
26741 Portola Pkwy, Suite 1E #242
Foothill Ranch CA 92610-1743 USA
Ph/Fx: 949-916-1524 (voice message usually)

Henrik W Maier

Modbus RTU and Modbus ASCII are pure Master/Slave protocols which allow only one master to operate on the link.

MODBUS/TCP is different and because of its underlaying TCP/IP allows multiple connections on one single link.

I assume that your device is classic RTU or ASCII serial which means you need a multiplexer or data concentrator device in between.

You could realize such a multiplexer with a Modbus/TCP->RTU bridge and run two Modbus/TCP connections from your IO server to the bridge.

The other option is to have the second IO server retrieving its data from the primary IO server.

With kind regards,

Henrik W Maier
FOCUS Software Engineering
Brisbane, Australia

Eddie Hague - FieldServer

There are a number of solutions to your problem. Some higher performance than others, and some offer hot standby capabilities in addition.
FieldServer Technologies makes a multi-port unit that can handle hundreds of devices, while other manufacturers are aimed at low cost solutions with single serial ports.

Edward Hague
Chief Technical Officer
FieldServer Technologies
408 262 2299 x 128
[email protected]