How PLC Communicates with DCS


Thread Starter


Hello there guys,

The question is how PLC relates to the DCS. I read so many articles on the internet. As i understood PLC and DCS are 2 different systems of process control, but still it is possible to communicate PLC to the DCS. It is a little bit hard for me to understand, please can anyone clarify?

Thank you in advance, and whoever will read this post have a nice day :)
Hello Mans92,

From communication protocol point of view, DCS and PLC basically is a same entity. So they are definitely will be able to communicate each other as long as, at least, below conditions are satisfied:

- There are compatible communication physical layer, example:
RS-485 port should be connected to RS-485 port/network. Ethernet port should be connected to Ethernet network. so DCS with RS-485 port will be able to connected to PLC with RS-495 port.

- After have compatible ports, they should have compatible protocol or they are talking the same language. As example, device (PLC, DCS, etc) with Modbus RTU protocol only able to communicate with other device using Modbus RTU protocol.
Thank you very much! But I am a bit confused cause you considered a DCS as a device while it is a control system.
Can you please clarify that?

Thank you!
Hello Mans92,

Whatever their functionality or hierarchy on control system, From electronic point if view a DCS or PLC or Computer or LCD monitor, they are just electronic equipments or electronic devices consisting of electronic component such as resistor, transistor, integrated circuit, that was why I considered DCS as a device.

Another consideration, from network point if view, on the peer to peer communication network, which is each node or entity will be able to initiate communication to other entity or node they would like to ask, all of entity have the same position, same right and same access, whether they are DCS  or PLC or just smart instrument etc, nothing special for DCS compared with other, so they are just device or just node.


Most popular and de facto mode of communication is Modbus RTU. Almost all DCS and PLCs support this communication. The interface is typically RS-485 as it supports multidrop. It means that one DCS acting as a Master can talk to multiple PLCs acting as Slaves connected on the same RS-485 network. All communication is managed and initiated by the DCS (Master) which can fetch or distribute data to one or more slaves. Slaves talk to Master only and don't talk to each other.

You can also use RS-232, but it's a point-to-point interface for short distances. You can typically use it for communicating the DCS with one PLC only.

Another common variation of Modbus RTU is Modbus TCP, in which the serial interface is replaced by Ethernet. Implementing communication with Modbus TCP is easier, as the devices are star connected (using an Ethernet switch). It is easier to add new devices or remove an existing device without disturbing the communication with other devices.

One the other hand, RS-485 network is a bus network and adding or removing a slave (PLC) is a more difficult. This requires disturbing the whole network and communication of all devices might be affected while adding or removing a device.

Anyways, if you are not modifying your network, both interfaces work equally good.