Applies To:
  • CitectSCADA 3.xx, 4.xx, 5.xx
Summary:
I have multiple devices that support a common protocol. I want to know if this protocol supports a multi-dropping arrangement.

Solution:
Multi-dropping is where multiple devices can be linked via one physical link such as RS422 and RS485 serial communications. However, to make use of multi-dropping the protocol that is used over the physical medium must be capable of allowing the device to respond appropriately without conflicting with others.

In Master - Slave arrangements, this is normally done with an address field within the frame sent by the Master. The Slave knows that it must respond only if the address sent by the Master matches it's own address.

Other arrangements may include token passing, time slicing etc.

As an example, Citect's MODBUS driver to the MODBUS protocol is a Master - Slave arrangement. It has an address field which is used to coordinate multi-drop communications.

 
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