Modbus Converter connected in Multidrop Daisy Chain

[mafiasingh]

Hi Guys,

I am designing a redundant UPS System. For Each UPS module I am providing a Modbus Device to communicate with the DCS system. But my client wants me to loop these Modbus devices in Multidrop Daisy Chain configuration, so that they only have to use one Port at the DCS side, which I have already done. But now they are planning to use 2 ports at the DCS end, but at the same time they want to communicate with both the modules from each port at the DCS side. Is it Possible to provide redundant signal after Multidrop Daisy Chain looping?

Thanks
Manu

 

[djs]

To start off with, Modbus is a Master-Slave network. There is one master and one or more slaves. Slaves only talk on the network in response to the master. So your UPS modules will need to be Modbus slaves, and the DCS ports Modbus masters.

Depending on the DCS you might be able to connect the two DCS Modbus master ports to the same network. In order to do this, you will need to be able to completely disbale one or the other of the ports under program control.

The process goes Port A is primary and port B is secondary. Port A is enabled and polling the UPS devices. Port B is disabled. Communication failures are detected on Port A and the decision is made to switch to port B. Port A is disabled, Port B is enabled and polling resumes on Port B.

 

[mafiasingh]

So if I want to provide two slave ports (one for Master Port A and other for B). Can I just connect 2 slave ports in Parallel, without adding any additional device?

 

[djs]

Every slave on the network will need to have an address. I believe that Modbus limits the number of slaves to 127. The master will also need to have a node address. As far as connecting two slave ports for the same device on the same network, you run into some problems. First that idea is that no two devices try and talk at the same time on the network. So if you have two ports on the network for the same device and those two ports have the same node address, then they will try and talk at the same time. Big no-no. If you are goinfg to have two master ports and two slave ports on each device, then you will be better of having two seperate networks.

 

[mafiasingh]

But as you mentioned earlier that at a time, on the DCS side, only one Master Port works and other has to be disabled. Then there should not be a chance of both the master ports trying to access same slave port(address) at the same time. or do we still need to provide different addresses for both slave ports?

Thanks for your help
Manu

 

[djs]

Say for example, UPS #1 is node 10. It has two serial ports connected to it. Both ports will respond to node 10. So when master A polls node 10 , both serial ports will respond. Unlike ehternet networks, Modbus will not otlerate nor is it set up to handle collisions.

 

[LionelHutz]

Every device on a daisy chained RS485 or RS422 modbus network needs it's own address.

Every device on a daisy chained RS485 or RS422 modbus network appears the same in the idle state. All the devices go to a tri-stated mode where they are just listening.

It makes no sense to connect the redundant slave ports to the same wiring. If (when?) a node fails and shorts the buss you have no redundancy. Every slave that is active and will respond needs it's own address, even if it's the same device with 2 ports.

The difference between a master and a slave is that a master will initiate communications and a slave will only respond to a communications request. So, you can put 2 master connections as long as only one is working at a time.

I suppose you can have both master ports use the same address as long as one is left tri-stated and does not do anything while the other one is communicating. You could also have 2 slave ports at the same address as long as only one is active at a time.

If you have 2 slave ports on each device and 2 master ports on the DCS then run 2 RS485 networks.