Introduction to Industrial Automation

Figure 8.8  Three different networks (corresponding to machine, production control, and 

supervisory levels) are interconnected through network devices.

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  Introduction to Industrial Automation

on the status of the PLC1 input. The response time or the total communication time is calculated 

by the equation,

 

R

I

ST

T

T

=

+

+

+

+

+

+

+

2

2

1

1

2

2

PT A

T

PT

ST O

T

T

T

where

 I

T 

= Input delay time, which is the time that elapses from the moment that an input contact 

closes until the moment that this 1-bit information is available for reading in the PLC1.

 ST

1

 = The scan cycle time of PLC1.

 ST

2

 = The scan cycle time of PLC2.

 PT

1 

= The processing time in the PLC for the preparation of data to be transmitted.

 PT

2 

= The processing time in the PLC2 for receiving arrived data and preparation in order to be 

available for the automation program executed in the PLC2.

PLC 1

PLC 2

CPU

Communication

module

Communication

module

Transfer memory

buffer A from

communication

module to PLC

central memory

Transfer memory

buffer A from

communication

module to PLC

central memory

Read

Inputs

Read

Inputs

User program

Instructions for

variables that

will be send to

other PLC

User program

Instructions for

variables that

will be send to

other PLC

Sc

an c

ycle

Sc

an c

ycle

Update

outputs

Update

outputs

Transfer specific

memory words

of PLC to the

memory buffer

B of the

communication

module

Transfer specific

memory words

of PLC to the

memory buffer

B of the

communication

module

Physical medium

Network line

CPU

I/O

bus

I/O

bus

A

B

A

B

Figure 8.9  The scan cycle in relation to the communication task of two PLCs.

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Industrial Networks of PLCs 

◾

 

367

 A

T

  = Waiting time of a PLC to access the physical network.

 T

T

  = Data transmission time for the given data transfer rate (baud rate) of the network.

 O

T

  = Output delay time, the time that elapses from the moment that 1-bit information is avail-

able in the output module until the moment that the corresponding output is energized 

in the PLC2.

The numeric values for all the above parameters make sense only in the case that these refer 

to a specific network and with a certain PLC. To give a sense of the time correlation between a 

PLC and a corresponding network, it is reported that, in many cases, the access cycle time of the 

network may be faster than the scanning cycle of a PLC that is connected to the network. For 

example, in a Profibus network with the smallest possible speed (selected and programmed based 

on the length of the network) and a usual number of nodes, it is possible to have a 2–3 network 

operation cycles per scan cycle of a PLC.