hmi plc communication


Thread Starter


hai all

Can two independent hmi stations can acess information fro ma single source plc ie from the same db or same bit information for datalogging andcontrol of the process if it is so whatare pro/cons

thank u
I just finished a project last week with two RSview32 HMIs and a single SLC5/05 plc. No problems whatsoever with having two HMIs access the same PLC.

You just have to make sure they are different node addresses on whatever network you're using.
The PLC comms overhead will sort out the traffic. If operators at both mmi's are trying to set the same tag or address to a different value at the same time, the last one in wins.
It's not a problem as long as the number of HMIs and PLCs are low. As the number of stations grows, the number of conversations goes up exponentially. Two HMIs and one PLC will send 2 conversations over the bus, but 4 HMIs and 4 PLCs will send 16 conversations over that same bus. You can see how just adding 1 more workstation to a network of 30 PLCs will affect your throughput.

Zan Von Flue

yes 2 or more hmi's can connect to the same plc. In the plc a good structure has to be programmed not to have the 2 HMI's conflict with each other. Displaying the same DB is ok, but I would avoid writting to the same DB word or bit.

This is not an uncommon design practice and easily accomplished with Siemens products. Some of the PLCs actually allow up to 64 HMI connections via PROFIBUS or MPI.

Ronald Powers
SR Product Specialist-HMI
Validation Center
Siemens Energy & Automation
5300 Triangle Pkwy
Norcross, GA 30092

Piotr Kowalski

There is possibility in InTouch to configure kind of redundancy having 2 InTouch nodes - when one of them has comm problem another can switch to its
local data server and connect to PLC.

Piotr Kowalski
There are several ways of achieving data redundancy in HMI's connoted to PLC's. If the PLC's or other equipment are on a multidrop network like ethernet or rs 485 etc., then several PC's running HMI software's can be
simultaneously connected to these devices and there is no data loss. You can have redundant data servers, in case you would like to use a
distributed architecture and then get the data from the data servers. In case the devices are serial and you are forced to connect in a RS232
type network, then there is definitely a problem, which requires custom solutions depending on the systems in question.