Connecting IO on S908 Remote IO to Modbus


Thread Starter

Pankaj Joshi

We have Quantum IO and 800 series IO drops on S908 remote IO bus connected to Quantum processor. We need to remove this processor and
connect IO to a Modbus Plus bus that is available in the same location. How can I do it? If there is a solution to hook these onto Modbus
then we can perhaps use BM85 to connect to Modbus Plus.

I would suggest that you contact Modicon tech support directly for this, but to the best of my knowledge there is not a product avail that
will allow you to do this, especially with the 800 series I/O. For the Quantum I/O you could possibly replace the 140CRA93XXX with a
140CRA21XXX distributed I/O adapter for MB+.
A link to tech support can be found at "":

Barry Dupuis

> The Quantum racks may be able to be converted over to run on Modbus Plus. This can be done by replacing the RIO drop adapters with DIO
drop adapters.

You will have to check the amount of I/O words consumed per drop are in the limit of DIO. DIO supports less words per drop that RIO.
Usually only an issue when you have alot of analog. ( Can add extra DIO drops )

Doing it this method lets you keep your I/O and racks etc, need to replace the one card and run Modbus PLus network to replace the RIO. You
will also have to change your I/O mapping ( easy ).

Check the distances to ensure Modbus plus network will be ok.

DIO works very well , with little to know maintence as compared to Coaxial cable.

The 800 series does not support DIO, >

Paul Murnaghan

Is the Quantum not already connected to the MB+ highway! If so, I'm not sure what the benefit is in putting this I/O onto MB+. A Coax cable network performs just as well as a twisted pair network if its installed properly. The biggest thing to watch out for is that the S908 Remote I/O network (1.544 MHZ)is dedicated and therefore predictable of when the inputs and outputs are
being controlled. Going to MB+ the predictability is now affected by the Token
rotation time of all devices plus the scan time of the Master devices which will be doing the reading and writing. HMI devices and programming devices can greatly affect the token rotation time especially if they are being
connected/disconnected periodically such as a Laptop PC. It really depends on what your Process is, but if it was me I would stay with the Remote I/O highway and reap the benefits of both highways.