Profibus or ethernet cable?

Remote PLCs

In some situations a number of PLCs may be linked together with a master PLC unit sending and receiving I/O data from the other units.

Cables

Twisted-pair cabling, often routed through steel conduit. Coaxial cable enables higher data rates to be transmitted and does not require the shielding of steel conduit.

Fiber-optic cabling has the advantage of resistance to noise, small size and flexibility.

Parallel communication

Parallel communication is when all the constituent bits of a word are

simultaneously transmitted along parallel cables. This allows data to be transmitted over short distances at high speeds. Might be used when connecting laboratory instruments to the system.

Parallel standards

The standard interface most commonly used for parallel communication is IEEE-488, and now termed as General Purpose Instrument Bus (GPIB).

Parallel data communications can take place between listeners , talkers , and controllers. There are 24 lines: 8 data (bidirectional), 5

status & control, 3 handshaking, and 8 ground lines.

Serial communication

Serial communication is when data is transmitted one bit at a time. A data word has to be separated into its constituent bits for transmission and then reassembled into the word when received. Serial communication is used for transmitting data over long distances. Might be used for the connection between a computer and a PLC.

Serial standards

RS-232 communications is the most popular method of plc to external device communications. RS 232 is a communication interface included

under SCADA applications. Other standards such as RS422 and RS423

are similar to RS232 although they permit higher transmission rates and longer cable distances.

 There are 2 types of RS-232 devices:

DTE – Data Terminal Equipment and a common example is a computer.

DCE – Data Communications Equipment and a common example is a modem.

 PLC may be either a DTE or DCE device.

 ASCII

ASCII is a human-readable to computer-readable translation code

(each letter/number is translated to 1’s and 0’s). It’s a 7-bit code, so we can translate 128 characters (2^7 is 128).

Protocols

It is necessary to exercise control of the flow of data between two devices so what constitutes the message, and how the communication is to be initiated and terminated, is defined. This is termed the protocol.

One device needs to indicate to the other to start or stop sending data.

Interconnecting several devices can present problems because of compatibility problems.

In order to facilitate communications between different devices the International Standard Organization (ISO) in 1979 devised a model to be used for standardization for Open System Interconnection (OSI).

Use OPC protocol, which work for different types of PLC also. 

Hi

As our friend el mouatez billah Messini said, there are several ways of communication between plc, but it is essential to know what types of communication that the plc model you are working on offers to choose the best method for the application in question. Because depending on the make and model, do not have many channels of communication

Hi

There are many different ways to communicate between PLC's. It all depends on the type of hardware you have on each PLC, their abilities and limitations. For example, some PLC's can only communicate serially, others can communicate via Ethernet, and others can do both!

What PLC's are you using?

Hi...

In my opinion, el mouatez billah Messini explained this question completely.

Also, you must know about hardware configuration.  For example in PLC S7-300 (Siemens's product) , it is necessary to use CP (Communication Process) module as hardware configuration which  communicates two CPU.

Good luck.

If there is only a few signals you need to communicate, you can also hardwire them (this it e.g. used for sending shutdown signals from a high level ESD PLC to lower level PSD PLCs).

Most suppliers of PLC's support socket communication. Using this method you only need the existing Ethernet network and no additional hardware.

If you can specify the vendors that you want to communicate between, you can get a more specific answer.

1. You can use different communication protocol such as STAR network ..using Ethernet 

2. Messaging can be used b/w PLC if they are from the same brand like eg AB

you can use DP/DP coupler in case of SIEMENS plc. It is a very very simplest way to communicate between two PLCs.

there are several methods, such as RS-232, RS-485 Modbus, ethernet ip. among others

el mouatez billah Messini sir explained very well, it will be very helpfull.

You must use industrial communication protocol e.g. modbus, profibus, profinet. You must determine requirements parametr of communication between to PLC

Jest to zależne od zastosowania. Czy ma być to komunikacja sterowania zdarzeniami czy w regularnych odstępach czasowych? Jaka będzie wymagana największa częstotliwość i rozmiar przesyłanych danych? Czy ma być to układ czasu rzeczywistego?

It depends on the application. Whether it's communication event control or at regular intervals? What will be required the greatest frequency and size of data transferred? Whether it's real-time system?

thanks to all of you for such kind response. finally  i decided  to have a DP/DP coupler.