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Communication Efficiency


Introduction
This tutorial is intended as a supplement to the original product manuals. Therefore, only use it to familiarise yourself with the product and not as a guide to the complete installation.

 
Background

When it comes to communication between a PLC and a PC it is done in most cases over a serial line. The most commonly used of these is RS-232. Over this physical connection the information is exchanged via a defined language. Omron's PLCs have a language which is called Host Link. You can find the commands to this language in most programming manuals for the PLCs.

Whether we choose to programme(Syswin or CX-Programmer) a PLC or connecting to an operator panel or a SCADA solution (CX-Supervisor), it must be talking to the PLC via, for example, a serial port. When communicating via serial ports the communication speed (baud) and data format (databit, parity and stop bit) must be set up. The defaults for Omron PLCs are 9600 baud (bits / sec), 7 databit, equal parity, and 2 stop bit. At the start, we need 11 bits to transfer the characters / digits.

Example 1

If we are requesting from a PC the value of a data memory in the PLC (which consists of 4 characters), we need to send the PLC a question. This question will consist of 17 characters (or 178 bit). The PLC feedback consists of 15 characters (or 165 bit). For one question the PLC answers will "use the" 32 characters (or 352 bit) per value. Let us assume that this value will be updated every second. Let's further assume that we have many values that we want to monitor from the PC. With a communication speed  of 9600 bit / sec and 352 bits / value, we could theoretically have a maximum view of 27 values at the same time. At a faster update than every second, this number will be reduced accordingly. This can quickly give a strong restriction on how much information can be present.

Example 2

Let's now assume that we want to view the 14 data memories on your PC. We have scheduled programming so that all of these PLC memories are continuously within memory. We can then, with a command of 17 characters (the same as the previous example), ask about them. The answer back will consist of 67 characters. A total of 84 characters is used as (or 924 bit) per 14 values; giving 6 characters / value. In the previous example it was the same ratio 32; Therefore, we have achieved an improvement / efficiency by a factor of 5,3.
When we want to acquire a lot of information in a short time / shortest possible time, the answer is to have the information ready within the PLC using a 'smart way'. When a value consists of 4 characters, we can, by using large amounts of information, come down to this number but not under! Using CX-Supervisor as an example we can show how, in practice, do this.

Modifying points

 



The images above show how you can determine how many values you want to read simultaneously by the defining of a point / tag. Select the circled choice under the Data Transfer optimization. This also saves under communications. When you want to use the different values in a common data collection (array) use the option shown below.

Display values for array

The calculations in this document are based on theoretical numbers. In practice, it will be balanced between characters sent between a question and a subsequent response. Therefore, the practical examples of the figures will not be as good.
This is an additional factor in choosing structured communication exchange.
Remember! When a user CX-Supervisor, it is really CX-Server that performs the physical communication with the PLC.




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Created 2008-11-25
Modified 2008-11-25
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