SCADA Port Sharing is used when a Single Serial Communications Channel is shared among multiple SCADA Masters.

SCADA Port Sharing is also commonly termed  Modbus Muxing,  Modbus Arbitration when the SCADA Master is a Modbus Protocol Host; or SCADA Host Multiplexing when applied to other protocols.

SCADA Port Sharing is required when:

  1. Limited Number of Serial Ports on a RTU, PLC, EFM Devices
  2.  Simultaneous SCADA Host Access to a Multiple Drop Network
  3. Simultaneous Multiple SCADA Host Access to a Serial Radio Network

A. Sharing of Single RS232 and RS485 Device Serial Port between Multiple SCADA Masters

Frequently theree is need for multiple SCADA Host or Master devices   to communicate to Single Port of a PLC, RTU, EFM, or Analyzer or SCADA either directly or via wireless modems. This is the case with with legacy PLC, RTU, EFM devices that have limited number of serial ports. The device may already have HMI connected and a SCADA Host connected. When additional SCADA host connection is there be be be no more ports.  The following diagram shows use of a SCADALink SS20 Multiplexer to share a single R232 connection between two SCADA hosts.


B. Simultaneous Access to PLC,RTU, EFM’s on a Multidrop Network

Port sharing can be used when multiple SCADA Hosts needs to talk to PLC, RTU,EFMs on Multidrop Network.  RS485 multidrop network is common  wiring network for master to communicate to a network of PLC, RTU, EFM, or VFD devices.  If multiple hosts X needs to talk to Y devices independently, you would need X x Y connections if you were to use direct connection from X hosts.   Muxing allows multiple hosts (X) s to share a connection to devices on a RS485 network with a installation of an additional multiplexing device. The following diagram shows two SCADA host networks simultaneously communicating to the multi-dropped VFD on the Rs485 networks.


C.  Multiple SCADA Host Access to a Serial Radio Networks

SCADA Port Sharing is frequently used during SCADA Host Replacement/Upgrades OR when there is need to have separate SCADA and Measurement Hosts.

Simultaneous SCADA Host Access is used during Host Replacements to allow for testing prior to system switch over.  Alternatively, sometimes there is need for separate hosts for SCADA and Measurement. Using a Mux allows separate independents hosts to access the same RTU/PLC on the radio network.


On SCADA Port Sharing Techniques

Buffered Protocol Port Arbitration

With REQUEST- RESPONSE Protocols like Modbus RTU/ASCII, DNP3.0, or ROC Protocol, a port arbitration mechanism is used to perform port sharing.   The Port Sharing device will map X INPUT ports into one OUTPUT Port.  Each SCADA host is connected to an INPUT port. The arbitration device receives and buffers the input message. If the output port is free, it sends it to the output port and awaits for RESPONSE or TIMEOUT.  During this time if another INPUT is received, it is buffered into a QUEUE until the OUTPUT port is free.

Address Filtering/Blocking

Address Filtering is required when it desired that RTU messages addressed to devices NOT on the DESTINATION network are NOT  sent over  that network.  Muxing without filtering will cause all messages to be sent to the DESTINATION network, and even message NOT for the network would be required to TIMEOUT.  This function is import where Port Sharing is used on Slave side; when polling from SCADA Host Network A can interfere with polling SCADA Host Network B.  This is especially the case when both networks are very chatty with large number of RTUs.

DTE Port Splitting

When traffic from two or more SCADA hosts communicate infrequently to a DTE device,  a DTE Port Splitter can used. This is not recommended for applications where there is continuous polling from both SCADA hosts, as  there is no way to prevent message collisions.


SCADA Port Sharing Devices Comparison






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