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PLC Communications in a process control system

Communication has become a major part of any process control automation system. Today PLC communication is as much for data acquisition as plant control. The first thing the designer often asks is 'how'? But shouldn't he first be asking 'why'?

Before one can consider how to implement a communication system, one has to consider what the final objective is. What is the importance of the data, what is the amount or volume of data to be transferred and when or how often is the data required. All of these are factors of why the communication is needed. Once all this information is known, one is much better placed to decide how this is to be done.

In order to make this final decision however, we first need to look at the options.

S7-200 Tips : Using S7-300 as a Master

Special Hardware Requirements:

One S7-200 CPU with a EM277 DP Module
One S7-300 PLC, consisting of a PS 307 2A power supply unit, a CPU 315-2 DP, and an SM 374 digital input/output simulator module
One PC/PPI cable for programming the S7-200
One PROFIBUS cable (The cable should be terminated on both ends by means of the matching resistor integrated in the connector. The switch on each connector should be in the ON position.)
One MPI cable for programming the S7-300
One PC with MPI card (and the STEP 7 programming software, Version 3.1, as well as STEP 7-Micro/WIN Version 2.0)

S7-200 Tips : Using RS-485 Encoder

Special Hardware Requirements:
There are two types of encoders, each having different resolutions:

Encoder with 13 bit resolution: Type SVE10-06213
Encoder with 25 bit resolution: Type SHE86-10214

Manufacturer: Hohner Electrical Engineering, D-78532 Tuttlingen/Germany

It is theoretically possible to connect 8 encoders to an RS 485 interface.

S7-200 Tips : PROFIBUS/EM 272 Notes

Special Hardware Requirements:

One SIMATIC S7-200 CPU 222, CPU 224, CPU 226, or CPU 226XM
One EM 277 PROFIBUS DP expansion module

The S7-200 CPUs can be connected to a PROFIBUS DP network by using an EM 277 PROFIBUS DP expansion module. PROFIBUS stands for “Process Field Bus.” DP stands for “distributed peripherals,” that is, remote I/O. The EM 277 allows the S7-200 CPUs to be DP slave devices on a DP network. A DP master can write data to and read data from the S7-200 CPU over the DP network by using the EM 277 PROFIBUS DP module.

S7-200 Tips : Using Parallel Printer

Special Hardware Requirements:

One PC/PPI cable
One converter, 9-pin female to 25-pin male
One converter, serial-to-parallel
One null modem adapter for swapping lines 2 and 3 (if required)
One parallel printer

NOTE:
One problem might occur. Because the SIMATIC S7-200 and the printer both behave as slaves (data communications equipment, or DCE), the data transfer of each device tries to run in the same direction. This means that both the receive and transmit data lines end up connected to each other (lines 2 and 3). You can avoid this problem by setting up the converter properly or by using an appropriate wiring adapter (a null modem adapter).