project plan

UNIVERSITY OF HUDDERSFIELD

SCHOOL OF COMPUTING AND ENGINEERING

PLANNING AN ADVANCED

TECHNICAL PROJECT REPORT

Topic:

PIC NETWORK USING 2-WIRE RS485

TRANSMISSION SYSTEM

Student: TRUONG TUAN VU

STUDENT ID: U1469803

Instructor: DR B.MEHRDADI

Project Plan: PIC network using 2-wire RS485 transmission system

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CONTENTS

CONTENTS ................................................................................................................ 1

LIST OF FIGURES ..................................................................................................... 2

LIST OF TABLES ...................................................................................................... 3

Chapter 1. Introduction ......................................................................................... 4

1.1. Aims ............................................................................................................... 4

1.2. Project Specifications .................................................................................... 4

Chapter 2. Background ......................................................................................... 5

2.1. RS485 Standard ............................................................................................. 5

2.2. RS485 Network ............................................................................................. 6

2.3. Cyclic Redundancy Check (CRC) ................................................................. 7

2.4. UART Connection ......................................................................................... 7

Chapter 3. Problem Analysis and Solutions.......................................................... 9

3.1. Problem Analysis ........................................................................................... 9

3.2. Proposed Solution ........................................................................................ 10

3.2.1. Driver Circuit ........................................................................................ 10

3.2.2. Programming Devices........................................................................... 12

Chapter 4. Project Plan ........................................................................................ 15

Chapter 5. Resources and Issues ......................................................................... 17

REFERENCES .......................................................................................................... 18


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LIST OF FIGURES

Figure 2.1: Signal on 2 wires of RS485 bus ............................................................... 6

Figure 2.2: Twisted pair wires .................................................................................... 6

Figure 2.3: RS485 network ......................................................................................... 7

Figure 2.4: Transmit status and control register[2] ..................................................... 8

Figure 2.5: Receive status and control register[2] ...................................................... 8

Figure 3.1: Network structure ..................................................................................... 9

Figure 3.2: Work breakdown structure ..................................................................... 10

Figure 3.3: MAX485 Top view [3] ........................................................................... 11

Figure 3.4: Typical application circuit [3] ................................................................ 12

Figure 3.5: Message format from master and slave .................................................. 13

Figure 3.6: Working principle of master ................................................................... 13

Figure 3.7: Working principle of slave devices ........................................................ 14

Figure 4.1: Gantt chart of project plan ...................................................................... 15


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LIST OF TABLES

Table 3.1: MAX485 Pin description [3] ................................................................... 11

Table 5.1: Project resources ...................................................................................... 17


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Chapter 1. Introduction

1.1. Aims

The main purpose of this project is to create a network of a number of

identical PIC boards using RS485 connection. A master PIC will control the traffic

across the network. Slave PICs will gather information and execute actions when

there is command from the master.

1.2. Project Specifications

The network needs at least 3 boards, perfectly 5 boards.

Devices on the network are connected to external modules such as LCD,

sensor, switches, LEDs.to be able to gather information or execute actions.

Line driver is used to convert data from microcontroller to suitable signal to

transmit on bus. MAX485 chip is a suitable driver.

Data words are transmitted in an asynchronous format using 9 bits to

transmit and receive data with bit 9 to distinguish between address and data

byte.

The message includes: 1 byte of address, 1 byte of command, data bytes and

Cyclic Redundancy Check (CRC) byte.


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Chapter 2. Background

This section describes the background knowledge about RS485 standard,

RS485 network and UART connection of PIC16F877A.

2.1. RS485 Standard [1]

RS485 connection is used for data acquisition at a long distance and control

the device. The significant features of RS485 is that it can support a network of up

to 32 stations on the same line, the baud rate can be up to 115.200 for a distance of

4000 feet (1200m). RS485 is used widely in industrial environments where noise is

high and the stability of the system is important.

With balanced transfer mode and the wires are twisted together, when noise

occurs in one wire, it also occurs in the others, i.e the two wires have the same

noise. This makes the voltage difference between the two wires change slightly.

Therefore, the receiver can receive signal properly by special features of the driver

that eliminates noise.

The balanced transmission system includes two signal wires A and B but no

ground wire. The reason is called the balanced system is that the signal in one wire

is reverse with the signal on the other. This means that if signal in wire A is at high

level, the signal in wire B must be at low level and vice versa.

With 2 balanced wires, the high TTL level signal is defined when the voltage

of wire A is higher than the voltage of wire B at least 200 mV. The low TTL level

signal is defined if the voltage of wire A is smaller than wire B at least 200 mV. If

VAB is in range from -200 mV to 200 mV, the signal is considered as in unstable

area.


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Figure 2.1: Signal on 2 wires of RS485 bus

= ( + ) ( + ) =

As the name implies, twisted pair (Twisted-pair wire) is simply a pair of wire

with equal length and are twisted together. Using twisted pair wire can reduce noise,

especially when transmitting at a long distance and with high speed.

Figure 2.2: Twisted pair wires

2.2. RS485 Network

This protocol allows a number of microcontrollers to be linked using a

twisted pair of wires. It is a half-duplex network. It means that at a time, there is

only one device transmitting or receiving. RS485 uses 9 bits to transmit and receive

data. The 9th

bit is also called the wake up bit. If the information is address, the 9th

bit is 1. If not, it will be 0. One microcontroller will be the master, others are slaves.

Each micro is allocated a unique address.

The master will send the address byte with bit 9 set. It interrupts all the slave

microcontrollers on the network. The only device which matched the transmitted

address responds. Others on the network ignore the incoming data.


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Figure 2.3: RS485 network

2.3. Cyclic Redundancy Check (CRC)

CRC (Cyclic Redundancy Check) is a method to detect errors by appending

a check block behind the data blocks. CRC is a powerful technique to detect faults,

so it is widely used in all data communication system. The error detection using

CRC is very simple. In the transmission, the CRC is calculated based on the

information bytes and sent along. When the receiving ends, the receiver calculates

the CRC from information byte and compares with received CRC byte. If 2 CRC

bytes are same, the information is considered to have been received correctly. If the

two do not match, it indicates that there are errors in transmission.

Generally, CRC calculation is like arithmetic division where the quotient is

not cared and the remainder is the result. The difference is that CRC calculation

uses carry-less arithmetic. However, this calculation method is computationally

intensive. The easiest CRC is to exclusive OR each data byte with the next.

2.4. UART Connection

The UART connection of microcontroller PIC16F877A is configured and

controller by some register: TXSTA (Transmit status and control register), RCSTA

(Receive status and control register) and SPBRG (Baud rate generator register)

The detailed functions of each register are shown in pictures below.


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Figure 2.4: Transmit status and control register[2]

With this register, the asynchronous and 9-bit transmission mode that are

required by the network can be chosen.

Figure 2.5: Receive status and control register[2]

The baud rate of UART is calculated by the below formula:

=

16 ( + 1)

Where: X is the value of SPBRG register.


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Chapter 3. Problem Analysis and Solutions

This chapter describes how the big project is split into smaller targets to

solve and the proposed solution for them.

3.1. Problem Analysis

The main purpose of this project is to design a PIC network using RS485

connection. The general structure of network is shown in the figure below

Figure 3.1: Network structure

According to the network, there are 2 smaller tasks that must be achieved to

finish this project:

Building the driver circuit

Programming devices on the network

The development boards in the laboratory already contain microcontroller

and other external modules such as LCD display, sensor, LEDs, switches.

Therefore, they can be used as the individual device on the network. The driver

circuits are required to connect them together.

Each device on the network should be programmed to use all external

modules when required. Otherwise, it is able to transmit/ receive signal to/ from

other devices. To do that, switching mode mechanism (transmitter/receiver) needs

to be developed, and the devices have to distinguish between address and data

bytes. Generating and checking CRC are also required to detect transmission errors.


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Figure 3.2: Work breakdown structure

3.2. Proposed Solution

3.2.1. Driver Circuit

To transmit and receive data, the signal from devices needs to be converted

to suitable voltage level. This task requires an external chip. For RS485 network,

MAX845 chip is a suitable driver.

The MAX485 is low-power transceivers for RS-485 and RS-422

communication. The IC contains one driver and one receiver [3]. Some features of

this chip are listed below [3]:

Low quiescent current: 300A

-7V to +12V Common-Mode input voltage range

30ns propagation delays, 5ns Skew

Operate from a single 5V supply

Allows up to 32 transceivers on the bus

Data rate: 2.5 Mbps


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Figure 3.3: MAX485 Top view [3]

Table 3.1: MAX485 Pin description [3]

No Name Function

1 RO Receive output: if A > B by 200mV, RO will be high; if A < B

by 200mV, RO will be low.

2 RE Receiver Output Enable. RO is enabled when RE is low; RO is

high impedance when RE is high.

3 DE Driver Output Enable. The driver outputs are enabled when

DE is high. They are high impedance when DE is low. If the

driver outputs are enabled, the parts function as line drivers.

While they are high impedance, they function as line receivers

if RE is low.

4 DI Driver input. A low on DI forces output A low and output B

high. Similarly, a high on DI forces output A high and output B

low.

5 GND Ground

6 A Driver Output and Receiver differential input.

7 B Driver Output and Receiver differential input.

8 Vcc Positive Supply: 4.75V Vcc 5.25V

The typical circuit of MAX485 is shown in the figure below.


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Figure 3.4: Typical application circuit [3]

This chip receives signal from microcontroller through pin DI and send

signal to microcontroller through pin RO, so these two pins are connected to pin

RC6/TX and RC7/RX of microcontroller 16F877A respectively. Since the RS485

network is half-duplex, there is only one device transmitting or receiving at a time.

Therefore, 2 pins RE and DE should be connected together and controlled by one

pin from port C of microcontroller.

3.2.2. Programming Devices

Because of the simplicity of programming other modules, in this section, I

focus on programming UART.

The message from master will consist of: address byte, command byte, data

bytes and CRC byte. After receiving, the slave will calculate CRC and compare

with the CRC byte from master. If there is no error, it will send back a message that

consists of: address byte, data bytes and CRC byte. If there is error, slave will

indicate and send the request to master to retransmit the message.


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Figure 3.5: Message format from master and slave

The master will start with transmission mode. After finishing sending

message, the master will switch to receiver mode to receive the feedback message

from slave and then switch back to transmission mode. This process is repeated

continually.

Figure 3.6: Working principle of master


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In contrast, the slave devices start with receiver mode to listen from the

master. After finishing receiving message, if the address is matched, they will

execute the request and switch to transmission mode to send the message back to

master device and then switch back to receiver mode. This process is also repeated

continually.

Figure 3.7: Working principle of slave devices


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Chapter 4. Project Plan

This part describes the detail plan of this project including logical sequence

of activities and the duration of those activities.

This project was started at the early of May and is planned to finish in July.

The detail plan is shown below.

Figure 4.1: Gantt chart of project plan


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Until now, some activities were already finished such as researching relevant

knowledge, purchasing MAX485 chip and building test circuit on project board.

This test circuit will be used to test the connection between 2 devices. If this circuit

works, the PCB will be designed. After testing with 2 devices, the program will be

developed and tested for 3, 4 and 5 devices.

The planned finish date is 14th

July, so there is a great deal of time for coping

with unexpected events before the submission date.


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Chapter 5. Resources and Issues

This part describes the resources that will be used in this project and the

issues that might be relevant to the project.

The table below lists all software and hardware resources, their source and

cost.

Table 5.1: Project resources

No Name Function Source Cost

1 PIC development

boards

Devices on the network Embedded

system

laboratory

Free

2 MAX485 Driver chip Purchase from

Ebay.co.uk

0.895 each

3 Electronic

components

Components of driver

circuit

Electronic

laboratory

Free

4 MikroC PRO for

PIC

Programming

microcontroller

Embedded

system

laboratory

Free

5 Proteus software Simulate the action of

whole system

Design PCB

Internet Free

There is not any ethical or legal issue that might be relevant to this project.


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REFERENCES

[1] RS-485. Retrieved from http://en.wikipedia.org/wiki/RS-485

[2] Microchip PIC16F87XA datasheet. Retrieved from

http://akizukidenshi.com/download/PIC16F87XA.pdf

[3] MAX485 datasheet. Retrieved from

http://3egadgets.com/attachment.php?id_attachment=23

project plan

Documents

rs485 network

pic network

rs485 standard

list of figures figure

network structure

wires of rs485 bus

mehrdadi project plan

project specifications