Basics on PLC-DCS interfacing


Thread Starter


When interfacing a PLC and a DCS system, I had never been able to fully understand the communication terminology. Is RS485/RS232 serial communication, and ethernet connection two different ways (options) to interface these equipments and accomplish the same thing? Do they in instances use the same type of electrical connectors? I know that RS232 is limited to 50' distances, while RS485 goes further.

I'll appreciate you all input. Thanks.
RS232 goes 50 ft (15 m) and is serial. It typically uses a 9-pin DB9 connector. It uses 3 or more wires, often 5. Sometimes it has a 25 pin connector. Speed is usually no more than 38 kbit/s although more is possible. Typical is 9.6 kbit/s. You can only connect two devices together,
although 3 or more is also possible depending on the protocol. There are thousands of different incompatible protocols using RS232 as a media. Such protocols include Modbus/RTU. Old computers have RS232 ports so you can connect directly. Modern computers need a USB-RS232 interface converter.

RS485 goes as far as 1.5 km (nearly a mile) and it is serial. It uses 3 wires put people often run just 2 and it works anyway. It has no specific connector, often just simple screw terminals, but some devices use DB9. E.g. Profibus-DP specified DB9. Speed can be up to 1 Mbit/s or so, but can be pushed to 12 Mbit/s using special connectors, e.g. in the case of Profibus DP. Typical speed for many is 9.6 kbit/s in many cases. You can connect up to 32 devices on the same wire in parallel (multidrop),although more is possible using modern devices. There are thousands of different incompatible protocols using RS485 as a media. Such protocols include Modbus/RTU and Profibus-DP and many other industrial protocols. Computers don't have RS485 ports. There are many simple interfaces that convert from RS232 to RS485 - and this works well with Modbus. Profibus has special timing issues so you need to buy a special Profibus card for your PC.

Ethernet in copper media goes only 100 m but goes 2-20 km using fiber optics. There are also wireless solutions. Ethernet is also serial, but for some reason it is not called "serial" (to distinguish it from RS232/RS485). It uses 4 wires. It uses the RJ45 connector. Speed can be 10, 100, 1000 Mbit/s. Ethernet connects one device per wire to a central hub/switch. There are thousands of different incompatible protocols using Ethernet as a media. Such protocols include Modbus/TCP, Foundation fieldbusT HSE, PROFINET and
many other industrial protocols. Modern computers have Ethernet so you just connect.

Ethernet is clearly the preferred choice but if both PLC and DCS do not have it you may have to go RS485.

However, note that RS232, RS485, and Ethernet are only the electrical aspect. Even if you connect both PLC and DCS to Ethernet they will not simply talk and you will not automatically get data from one into the other.

The reason is that RS232, RS485, and Ethernet there are thousands of different protocols. All modern PLCs and all modern DCSs use Ethernet as a backbone, but they all do so using a different application protocol.

Therefore RS232, RS485, and Ethernet is not sufficient, you must make sure that both ends are Modbus/RTU, Profibus-DP, Modbus/TCP, or Foundation
fieldbusT HSE etc. If the protocol is not the same they will not talk even if they are electrically the same.

Thus, if you want to hook up a PLC and a DCS you must start by figuring out which protocols they support. If you can find a match, possibly Modbus, then you can hook them up. Even so, there will be lots of configuration work to be done. The interface and OPC servers you can get from the PLC/DCS vendor or third parties.

If there are no matching protocols between the PLC and the DCS then another solution is to use OPC. You need a computer with an interface hardware and OPC server software for the PLC protocol and interface hardware and OPC server software for the DCS protocol. Lastly an OPC bridge software to bridge the PLC server to the DCS server.

If you want to know more about Ethernet, protocols, and OPC then refer to the two book links below.

Jonas Berge
[email protected]
Learn fieldbus and Ethernet at your own pace:
Learn OPC and automation software at your own pace:
In general, RS232/422/485 serial interface and Ethernet interface differs in the following ways:

RS232/422/485 Serial Interface
As the name says, RS232/422/485 is a serial interface, meaning that it is a common *point-to-point* communication interface. (It CAN be used as a networking interface, ie. Modbus, but not an option as popular.)

RS232 interface usually uses the DB9 cable - or widely known as Null-Modem Cable - as its physical connection. It is exactly the same as the COM port on your PC.

RS422/485 interfaces can also use DB9 connections, but with different wiring schemes, and the maximum distance at which the signal can travel is longer than RS232.

Note that DB9 is not the only option for RS232/422/485 interfaces. I have seen products out there that utilizes 5-pin connectors, DB15, DB25, and some other customized connectors as RS232/422/485 physical connector as well.

So, generally speaking, when you are using RS232/422/485 serial interface to establish a connection between your PLC and DCS, you are applying a point-to-point communication between the two devices.

Ethernet Interface
During my internship at Equustek, I noticed that many people in the automated control industry are not very familiar with the term "Ethernet". As a Microsoft Certified System Engineer, I sure can give you a satisfying clearification on this question.

Generally, Ethernet is a networking interface, which means it is a fundamentally different interface from RS232/422/485 Serial Interface.

The main idea that distinguishes a serial interface from a networking interface is that, a networking interface deals with communications among a collection of nodes interconnected through one backbone of networking line. And as you can imagine, the algorithm with which a networking interface can coordinate the communications on the network is MUCH MORE complicated than that of serial interface.

But, before I jump into detail, I have to clearify one critical concept that many people have misunderstood: Ethernet is NOT a protocol but a predefined physical networking structure on which all sorts of protocols can be utilized.

IEEE standard defines Ethernet as a physical BUS networking structure and a logical RING structure, ie. CSMA/CD. (only applied up to 100-base-T)

IEEE also defines the hardware standards of Ethernet, such as the standard Ethernet cables, connectors, transceiver nodes, and so on.

10/100-base-T is the most common Ethernet structure in industrial Ethernet. It uses standard RJ-45 connector and Cat.5 Unshielded Twisted Pair(UTP) cable as its communication medium.

The nodes on 10/100-base-T are collectively connected to a physical "center" of the network, which may be a Hub, a Switch, a Router, or a Gateway, all of which serves different purpose on networking traffic control.

However, although the appearance of 10/100-base-T is a STAR, it is internally connected to a BUS inside the "center" device. This means IEEE stardards still apply.

Then why IEEE defines Ethernet a logical RING structure? It all comes down to how the nodes on the Ethernet coordinate with each other in order to effectively share a single-lined network backbone.

Ethernet implements an algorithm named CSMA/CD, and the way it works is:

A token is passed on the network; whenever a node receives the token, it has the previlige to send data onto the network. Once it completes an action, it passes the token to the next node on the network. Thus, the token is cycled around on the network, and every node can, in theory, equally shares the network.

Nevertheless, if the node detects that others are using the network line, it will stop its attempt and give up the token for this "round", and wait for the next time it gets the token.

Therefore, when is a large number of nodes on the network, you may expect a slow communication because of the heavy traffic.

Above information summerizes a rough outline of Ethernet. And the protocols such as Modbus TCP, Ethernet IP, and the ever-popular TCP/IP, are working on top of the Ethernet Structure.

So, what is the difference between connecting PLC and DCS through RS232/422/485 serial interface and with Ethernet networking interface?

The difference is, with RS232/422/485 interface, you are establishing a point-to-point connection that is entirely dedicated to communication between PLC and DCS. However, the highest bandwidth you can obtain through serial interface is in Kb/s range. (Kb as Kilo-baud)

With Ethernet interface, you are basically putting PLC and DCS both onto the network. PLC and DCS thus have to share the networking bandwidth with other nodes on the network (if any). Fortunately, the bandwidths of Ethernet interface are either 10Mb/s or 100Mb/s for 10-base-T and 100-base-T respectively. But please keep in mind that the "real bandwidth" that PLC and DCS can use is dependant of the networking traffic.

However, if you want to control both your PLC and DCS through the PCs in your office network, putting them on Ethernet is your ONLY option.

Hope that helps!
Hi Scott,

In context of your mail. I just have some queries.

1) You wrote that when one Node gets the TOKEN, only then it is able to send data onto the network. But you also wrote that when one receives token & also sense that other node on the network is transmitting it gives up that token. (A token could be at only one node at a time as you wrote that "one when gets token, starts transmitting & when finishes transfers that token to the other", then how is it possible that one is still transmitting & on the same time other gets token." From where that token come from which node has transfered that token to it, THAT node which is still transmitting or any one else, although there is no chance that other could.)

For me (as far as I know) this method of TOKEN passing is used in TOKEN RING (IEEE 802.5).

2) I think in Ethernet (IEEE 802.3) CSMA/CD is used. The nature of data transmission is BROAD CAST. Every node SENSES the "BUS" (as you also mentioned inside central device there is Bus topology) when bus is IDLE (determined by voltage level on the BUS) every one tries to transmit (that is why it is BROADCAST) but one which transmits first is succeded & as other senses that any else node is succeeded first & has started transmitting they give up & wait that node to finish transmitting. there is always a chance that TWO (2) or more nodes may transmit at one time so "COLLISSION DETECTION" is done.

I think this is Carrier sense for multiple access with collission detection.

I expect you may surely be updating me on that matter.
Dear Scott Chen,

Although I agree Ethernet is the way to go, there are a few things in your message which I believe are not correct.

RS232 is point-to-point but RS485 is made to link together many devices. In typical application RS485 link together many devices. Just like Ethernet. In principle the networking is the same, just achieved slightly differently.

DB9 and "null modem cable" is two different thing. A null modem cable means that the transmit signal is connected to the receive signal and vice versa.

Fundamentally Ethernet and RS485 are the same - they both network many devices. In general you can say modern Ethernet is faster and one-to-one while RS485 is longer distance connecting many on the SAME wire.

RS485 also "deals with communications among a collection of nodes interconnected one backbone of networking line" just like Ethernet. The data link layer handles the coordination, e.g. Profibus or DF1 etc. similar to the data link layer in Ethernet.

Ethernet is not a logical ring.

Ethernet is not token passing. There is no token circulated in a ring. The concept of CSMA/CD is first come first served.

TCP/IP is not a protocol like Modbus/TCP and EtherNet/IP. Both Modbus/TCP and EtherNet/IP use TCP/IP as a platform above Ethernet. TCP/IP corresponds to layers 3 and 4 while Modbus/TCP and EtherNet/IP include protocols up to layer 7.

Again, RS485 is not just point-to-point

Ethernet is not the only option even if you want a PC connected as well, but it is the simpler option. Having said all that, I do think that Ethernet is a better option today than RS485. I just wanted to straighten out some of
the points.

Jonas Berge
[email protected]
Learn fieldbus and Ethernet at your own pace:
Learn OPC and automation software at your own pace:
Mr. Khan is correct. Ethernet does NOT use tokens. However, CSMA/CD Ethernet is not often used at all. Rather 10/100/1000BaseT Ethernet is most commonly used in which there can be no collisions. All Ethernet messages originate at one node on a line not shared with any other node other than a port on an active Ethernet switch. The
switch buffers all traffic and forwards the entire message to either another node on that switch if the MAC address for that node is the destination address, or to its "network" port for all other MAC addresses. Most modern Ethernet switches operate in full duplex mode allowing bidirectional traffic.

The description of token passing in Mr. Chen's response is somewhat similar to IEEE 802.4, Token Bus rather than token ring, but this is of no real interest since both of these protocols are now generally regarded as obsolete, although they are still used in some legacy systems.

Dick Caro
Richard H. Caro, CEO
CMC Associates
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Automation Network Selection
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