RS485

 

RS485 is used for serial communication and well suited for transmission over large distances.

Similar to RS232 we need a level shifter.

 

The sample above uses a MEGA161 or MEGA162 which has 2 UARTS. This way you can have both a RS232 and RS485 interface.

The RS232 is used for debugging.

In order to test you need 2 or more similar circuits. One circuit would be the master.

The other(s) would be a slave.

The same hardware is used to test the MODBUS protocol. The bus need to be terminated at both ends with a resistor. 100 ohm is a typical used value.

The GND of both circuits may not be connected ! Only connect point A and B from both circuits. For industrial usage it is best to use an optical isolated level shifter.

 

 

Simple MASTER sample

$regfile = "m162def.dat"                                    ' specify the used micro

$crystal = 8000000

$baud = 19200                                               ' use baud rate

$hwstack = 42                                               ' default use 32 for the hardware stack

$swstack = 40                                               ' default use 10 for the SW stack

$framesize = 40                                             ' default use 40 for the frame space

 

$lib "modbus.lbx"

Config Print1 = Portb.1 , Mode = Set              ' use portb.1 for the direction

Rs485dir Alias Portb.1

Config Rs485dir = Output

Rs485dir = 0   ' go to receive mode

Portc.0 = 1    ' a switch is connected to pinc.0 so activate pull up resistor

'             TX    RX

' COM0   PD.1   PD.0   monitor

' COM1   PB.3   PB.2   rs485

'            PB.1           data direction rs485

 

 

Config Com1 = Dummy , Synchrone = 0 , Parity = None , Stopbits = 1 , Databits = 8 , Clockpol = 0

Config Com2 = 9600 , Synchrone = 0 , Parity = Even , Stopbits = 1 , Databits = 8 , Clockpol = 0  ' MUST MATCH THE SLAVE

 

'use OPEN/CLOSE for using the second UART

Open "COM2:" For Binary As #1

 

'dimension some variables

Dim B As Byte

Dim W As Word

Dim L As Long

 

W = &H4567   ' set some values

L = &H12345678

 

 

Print "RS-485 MODBUS master"

Do

 If Pinc.0 = 0 Then                                       ' test button

    Waitms 500                                            ' delay since we want to send just 1 frame

    Print "send request to slave/server"            ' to debug terminal

   ' Print #1 , Makemodbus(2 , 3 , 8 , 2);                  'slave 2, function 3, start address 8, 2 bytes

   ' Print #1 , Makemodbus(2 , 6 , 8 , W);                 'slave 2, function 6, address 8  , value of w

     Print #1 , Makemodbus(b , 16 , 8 , L);                'send a long

 End If

 If Ischarwaiting(#1) <> 0 Then  'did we got something back?

   B = Waitkey(#1)  ' yes so get it

   Print Hex(b) ; ",";  ' print it

 End If

Loop

 

A slave would simply listen to data, and once enough data received, send it back.