RS-485 Library
RS-485 is a multipoint communication which allows multiple devices to be connected to a single bus. The mikroBasic PRO for 8051 provides a set of library routines for comfortable work with RS485 system using Master/Slave architecture. Master and Slave devices interchange packets of information. Each of these packets contains synchronization bytes, CRC byte, address byte and the data. Each Slave has unique address and receives only packets addressed to it. The Slave can never initiate communication.
It is the user’s responsibility to ensure that only one device transmits via 485 bus at a time.
The RS-485 routines require the UART module. Pins of UART need to be attached to RS-485 interface transceiver, such as LTC485 or similar (see schematic at the bottom of this page).
Note: The library uses the UART module for communication. The user must initialize the appropriate UART module before using the
RS-485 Library. For MCUs with two UART modules it is possible to initialize both of them and then switch by
using the UART_Set_Active
function. See the UART Library functions.
- START byte value =
150
- STOP byte value =
169
- Address
50
is the broadcast address for all Slaves (packets containing address50
will be received by all Slaves except the Slaves with addresses150
and169
).
External dependencies of RS-485 Library
The following variable must be defined in all projects using RS-485 Library: | Description : | Example : |
---|---|---|
dim rs485_transceive as sbit bdata sfr external |
Control RS-485 Transmit/Receive operation mode | dim rs485_transceive as sbit at P3_2 |
Library Routines
- RS485Master_Init
- RS485Master_Receive
- RS485Master_Send
- RS485Slave_Init
- RS485Slave_Receive
- RS485Slave_Send
RS485Master_Init
Prototype |
sub procedure RS485Master_Init() |
---|---|
Returns |
Nothing. |
Description |
Initializes MCU as a Master for RS-485 communication. |
Requires |
UART HW module needs to be initialized. See UARTx_Init. |
Example |
' RS485 module pinout dim rs485_transceive as sbit at P3_2 ' transmit/receive control set to P3.B2 ... UART1_Init(9600) ' initialize UART module RS485Master_Init() ' intialize MCU as a Master for RS-485 communication |
RS485Master_Receive
Prototype |
sub procedure RS485Master_Receive(dim byref data_buffer as array[20] of byte) |
---|---|
Returns |
Nothing. |
Description |
Receives messages from Slaves. Messages are multi-byte, so this routine must be called for each byte received. Parameters :
The function automatically adjusts |
Requires |
MCU must be initialized as a Master for RS-485 communication. See RS485Master_Init. |
Example |
dim msg as array[20] of byte ... RS485Master_Receive(msg) |
RS485Master_Send
Prototype |
sub procedure RS485Master_Send(dim byref data_buffer as array[20] of byte, dim datalen as byte, dim slave_address as byte) |
---|---|
Returns |
Nothing. |
Description |
Sends message to Slave(s). Message format can be found at the bottom of this page. Parameters :
|
Requires |
MCU must be initialized as a Master for RS-485 communication. See RS485Master_Init. It is the user’s responsibility to ensure (by protocol) that only one device sends data via 485 bus at a time. |
Example |
dim msg as array[20] of byte ... ' send 3 bytes of data to slave with address 0x12 RS485Master_Send(msg, 3, 0x12) |
RS485Slave_Init
Prototype |
sub procedure RS485Slave_Init(dim slave_address as byte) |
---|---|
Returns |
Nothing. |
Description |
Initializes MCU as a Slave for RS-485 communication. Parameters :
|
Requires |
UART HW module needs to be initialized. See UARTx_Init. |
Example |
' RS485 module pinout dim rs485_transceive as sbit at P3.B2 ' transmit/receive control set to P3.B2 ... UART1_Init(9600) ' initialize UART module RS485Slave_Init(160) ' intialize MCU as a Slave for RS-485 communication with address 160 |
RS485Slave_Receive
Prototype |
sub procedure RS485Slave_Receive(dim byref data_buffer as array[20] of byte) |
---|---|
Returns |
Nothing. |
Description |
Receives messages from Master. If Slave address and Message address field don't match then the message will be discarded. Messages are multi-byte, so this routine must be called for each byte received. Parameters :
The function automatically adjusts |
Requires |
MCU must be initialized as a Slave for RS-485 communication. See RS485Slave_Init. |
Example |
dim msg as array[5] of byte ... RS485Slave_Read(msg) |
RS485Slave_Send
Prototype |
sub procedure RS485Slave_Send(dim byref data_buffer as array[20] of byte, dim datalen as byte) |
---|---|
Returns |
Nothing. |
Description |
Sends message to Master. Message format can be found at the bottom of this page. Parameters :
|
Requires |
MCU must be initialized as a Slave for RS-485 communication. See RS485Slave_Init. It is the user’s responsibility to ensure (by protocol) that only one device sends data via 485 bus at a time. |
Example |
dim msg as array[8] of byte ... ' send 2 bytes of data to the master RS485Slave_Send(msg, 2) |
Library Example
This is a simple demonstration of RS485 Library routines usage.
Master sends message to Slave with address 160 and waits for a response. The Slave accepts data, increments it and sends it back to the Master. Master then does the same and sends incremented data back to Slave, etc.
Master displays received data on P0, while error on receive (0xAA) and number of consecutive unsuccessful retries are displayed on P1. Slave displays received data on P0, while error on receive (0xAA) is displayed on P1. Hardware configurations in this example are made for the Easy8051B board and AT89S8253.
RS485 Master code:
program RS485_Master dim dat as byte[20] ' Buffer for receving/sending messages counter, j as byte cnt as longint ' RS485 module connections dim rs485_transceive as sbit at P3_2_bit ' Transmit/Receive control set to P3.2 ' End RS485 module connections ' Interrupt routine sub procedure interrupt org IVT_ADDR_ES EA_bit = 0 ' Clear global interrupt enable flag if(RI_bit) then ' Test UART receive interrupt flag RS485master_Receive(dat) ' UART receive interrupt detected, ' receive data using RS485 communication RI_bit = 0 ' Clear UART interrupt flag end if EA_bit = 1 ' Set global interrupt enable flag end sub main: cnt = 0 P0 = 0 ' Clear ports P1 = 0 UART1_Init(4800) ' Initialize UART module at 9600 bps Delay_ms(100) Rs485master_Init() ' Intialize MCU as RS485 master dat[0] = 0xAA ' Fill buffer dat[1] = 0xF0 dat[2] = 0x0F dat[4] = 0 ' Ensure that message received flag is 0 dat[5] = 0 ' Ensure that error flag is 0 dat[6] = 0 Rs485master_Send( dat, 1, 160 ) ES_bit = 1 ' Enable UART interrupt RI_bit = 0 ' Clear UART RX interrupt flag EA_bit = 1 ' Enable interrupts while TRUE ' Endless loop ' Upon completed valid message receiving ' data[4] is set to 255 Inc(cnt) ' Increment loop pass counter if (dat[5] <> 0) then ' If error detected, signal it by P1 = 0xAA ' setting PORT1 to 0xAA end if if (dat[4] <> 0) then ' If message received successfully cnt = 0 ' Reset loop pass counter dat[4] = 0 ' Clear message received flag j = dat[3] ' Read number of message received bytes for counter = 1 to j P0 = dat[counter-1] ' Show received data on PORT0 next counter Inc(dat[0]) ' Increment first received byte dat[0] Delay_ms(1) Rs485master_Send(dat,1,160) ' And send it back to Slave end if if ( cnt > 10000 ) then ' If loop is passed 100000 times with ' no message received Inc(P2) ' Signal receive message failure on PORT2 cnt = 0 ' Reset loop pass counter Rs485master_Send(dat,1,160) ' Retry send message if (P2 > 10) then ' If sending failed 10 times Rs485master_Send(dat,1,50) ' Send message on broadcast address end if end if wend end.
RS485 Slave code:
program RS485_Slave dim dat as byte[20] ' Buffer for receving/sending messages counter, j as byte ' RS485 module connections dim rs485_transceive as sbit at P3_2_bit ' Transmit/Receive control set to P3.2 ' End RS485 module connections '-------------- Interrupt routine sub procedure interrupt() org IVT_ADDR_ES EA_bit = 0 ' Clear global interrupt enable flag if ( RI_bit ) then ' Test UART receive interrupt flag RS485Slave_Receive(dat) ' UART receive interrupt detected, ' receive data using RS485 communication RI_bit = 0 ' Clear UART interrupt flag end if EA_bit = 1 ' Set global interrupt enable flag end sub main: P0 = 0 ' Clear ports P1 = 0 UART1_Init(4800) ' Initialize UART module at 9600 bps Delay_ms(100) RS485Slave_Init(160) ' Intialize MCU as slave, address 160 dat[4] = 0 ' ensure that message received flag is 0 dat[5] = 0 ' ensure that error flag is 0 dat[6] = 0 ' ensure that error flag is 0 ES_bit = 1 ' Enable UART interrupt RI_bit = 0 ' Clear UART RX interrupt flag EA_bit = 1 ' Enable interrupts while TRUE ' Endless loop ' Upon completed valid message receiving ' data[4] is set to 255 if (dat[5] <> 0) then ' If error detected, signal it by P1 = 0xAA ' setting PORT1 to 0xAA dat[5] = 0 end if if (dat[4] <> 0) then ' If message received successfully dat[4] = 0 ' Clear message received flag j = dat[3] ' Read number of message received bytes for counter = 1 to j P0 = dat[counter-1] ' Show received data on PORT0 next counter Inc(dat[0]) ' Increment received dat[0] Delay_ms(1) Rs485slave_Send(dat,1) ' And send back to Master end if wend end.
HW Connection
Example of interfacing PC to 8051 MCU via RS485 bus with LTC485 as RS-485 transceiver
Message format and CRC calculations
Q: How is CRC checksum calculated on RS485 master side?
START_BYTE = 0x96; ' 10010110 STOP_BYTE = 0xA9; ' 10101001 PACKAGE: -------- START_BYTE 0x96 ADDRESS DATALEN [DATA1] ' if exists [DATA2] ' if exists [DATA3] ' if exists CRC STOP_BYTE 0xA9 DATALEN bits ------------ bit7 = 1 MASTER SENDS 0 SLAVE SENDS bit6 = 1 ADDRESS WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE 0 ADDRESS UNCHANGED bit5 = 0 FIXED bit4 = 1 DATA3 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE 0 DATA3 (if exists) UNCHANGED bit3 = 1 DATA2 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE 0 DATA2 (if exists) UNCHANGED bit2 = 1 DATA1 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE 0 DATA1 (if exists) UNCHANGED bit1bit0 = 0 to 3 NUMBER OF DATA BYTES SEND CRC generation : ---------------- crc_send = datalen ^ address; crc_send ^= data[0]; ' if exists crc_send ^= data[1]; ' if exists crc_send ^= data[2]; ' if exists crc_send = ~crc_send; if ((crc_send == START_BYTE) || (crc_send == STOP_BYTE)) crc_send++; NOTE: DATALEN<4..0> can not take the START_BYTE<4..0> or STOP_BYTE<4..0> values.
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