The History About The Multipoint Communications

Published Date: 02 Nov 2017

CHAPTER 1

In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels. Serial communication is used for all long-haul communication and most computer networks, where the cost of cable and synchronization difficulties makes parallel communication impractical. Serial computer buses are becoming more common even at shorter distance, as improved signal integrity and transmission speeds.

RS485 is an Electronics Industry Association (EIA) standard for multipoint communications. It supports several types of connectors, including DB-9 and DB-37. RS-485 is similar to RS-422 but can support more nodes per line RS485 meets the requirements for a truly multi-point communications network, and the standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. RS-485, which specifies bi-directional, half-duplex data transmission, is the only EIA/TIA standard that allows multiple receivers and drivers in "bus" configurations.

MODBUS is a serial communications protocol published by Modicon in 1979 for use with its programmable logic controllers (PLCs). Simple and robust, it has since become a de facto standard communication protocol, and it is now amongst the most commonly available means of connecting industrial electronic devices.  The main reasons for the extensive use of MODBUS in the industrial environment are:

It has been developed with industrial applications in mind

It is openly published and royalty-free

It is easy to deploy and maintain

It moves raw bits or words without placing many restrictions on vendors

MODBUS allows for communication between many devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. MODBUS is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Many of the data types are named from its use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact. Devices can be setup to communicate on standard MODBUS networks either of two transmission modes: ASCII (American Standard Code for Information Interchange) or RTU (Remote Terminal Unit). The mode must be the same for all devices on a MODBUS network.

In this project, RS485 communication network will be develops using PIC microcontroller and MODBUS protocol to communicate between two PICs, PC with PIC microcontroller and PLC with PIC. The master device sends the data to slave device and the slave device will sends the data back which is its I/O status of the slave device to the master. Going to more advances, the device may able to replace PLC to perform real time machine control system with relatively low cost.

Problem Statement

In any automation industry, it is difficult that in order to integrate any PIC microcontroller into any serial communication network which comprises PLC, HMI running in RS485. Therefore, a PIC application with MODBUS connectivity function via RS485 will be develop in order to integrate into the serial communication network.

Some of the slave remote I/O devices are using serial communication line i.e., RS485 and using MODBUS protocol in order to integrate into this network, microcontroller has to conform the required communication protocol standard. So the master MODBUS device using PIC will be used in order to integrate into MODBUS communication of slave remote I/O devices for data sharing purpose.

In PIC microcontroller UART communication module with RS232, it allows only 1:1 interface without multi drop availability. Hence, it is unable to connect it into any serial communication network. Normally in the automation industry, a serial line i.e., RS485 will be used for the communication network. It is implementing with MODBUS protocol for multi devices in the data sharing purpose.

Objectives

The main objective of this project is to demonstrate and apply the standard serial communication protocol of the industrial. One of the common standard serial industrial communication protocols is the MODBUS protocol. MODBUS protocol was chosen for this project because it is freely available, easy to implement and it is thus supported by many manufacturers. The next objective of this project is to develop a RS485 communication network using PIC microcontroller and MAXIM 485.

Furthermore, the objective of this project is to build a master device and slave devices which build up of PIC16f877A and some electronic components such as LEDs, pushbutton, LCD display and it is running in a small scale of communication network.

Scope of project

This project focuses on a serial communication between PIC devices and PC with PIC device. The program code that wrote in MPLAB software is compiled and downloaded into microcontroller. The code is implemented with MODBUS protocol to communicate in the RS-485 communication network.

During serial communication between PIC devices, the master device will send the data to the slave device. If the slave device receives the correct code, it will proceed and send its status as data back to the master device. After that, master device will display the information that get from the slave device on the LCD display. Same method applies to serial communication between PC and microcontroller. The master PC will show the result in Microsoft Visual Studio GUI program. This project aims to receive or modify the microcontroller pin status of the slave device. This project can act as a small Flexible Manufacturing System (FMS) using RS-485 communication network and standard industrial protocol when go to the advance level.