RS-485 Library

RS-485 is a multipoint communication which allows multiple devices to be connected to a single bus. The mikroC PRO for FT90x 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).

Library constants:

START byte value = 150
STOP byte value = 169
Address 50 is the broadcast address for all Slaves (packets containing address 50 will be received by all Slaves except the Slaves with addresses 150 and 169).

Important :

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 multiple UART modules it is possible to initialize them and then switch by using the UART_Set_Active routine.

Library Dependency Tree

External dependencies of RS-485 Library

The following variable must be defined in all projects using RS-485 Library:	Description :	Example :
`extern sfr sbit RS485_rxtx_pin;`	Control RS-485 Transmit/Receive operation mode	`sbit RS485_rxtx_pin at GPIO_PIN57_bit;`

RS485Master_Init

Prototype	void RS485Master_Init();
Description	Initializes MCU as a Master for RS-485 communication.
Parameters	None.
Returns	Nothing.
Requires	External dependencies of the library from the top of the page must be defined before using this function. UART HW module needs to be initialized. See UARTx_Init and UARTx_Init_Advanced.
Example	sbit RS485_rxtx_pin at GPIO_PIN57_bit; // set transcieve pin ... UART1_Init(19200); // initialize UART1 module Delay_ms(100); RS485Master_Init(); // initialize MCU as Master
Notes	None.

RS485Master_Receive

Prototype	void RS485Master_Receive(char *data_buffer);
Description	Receives messages from Slaves. Messages are multi-byte, so this routine must be called for each byte received.
Parameters	`data_buffer:` 7 byte buffer for storing received data. Data will be stored in the following manner: `data_buffer[0..2]:` message content `data_buffer[3]:` number of message bytes received, 1–3 `data_buffer[4]:` is set to 255 when message is received `data_buffer[5]:` is set to 255 if error has occurred `data_buffer[6]:` address of the Slave which sent the message The routine automatically adjusts `data[4]` and `data[5]` upon every received message. These flags need to be cleared by software.
Returns	Nothing.
Requires	MCU must be initialized as a Master for RS-485 communication. See RS485Master_Init.
Example	char msg[8]; ... RS485Master_Receive(msg);
Notes	None.

RS485Master_Send

Prototype	void RS485Master_Send(char data_buffer, char* datalen, char slave_address);
Description	Sends message to Slave(s). Message format can be found at the bottom of this page.
Parameters	`data_buffer:` data to be sent `datalen:` number of bytes for transmition. Valid values: 0...3. `slave_address:` Slave(s) address
Returns	Nothing.
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	char msg[8]; ... // send 3 bytes of data to Slave with address 0x12 RS485Master_Send(msg, 3, 0x12);
Notes	None.

RS485Slave_Init

Prototype	void RS485Slave_Init(char Slave_address);
Description	Initializes MCU as a Slave for RS-485 communication.
Parameters	`Slave_address:` Slave address
Returns	Nothing.
Requires	External dependencies of the library from the top of the page must be defined before using this function. UART HW module needs to be initialized. See UARTx_Init.
Example	sbit RS485_rxtx_pin at GPIO_PIN57_bit; // set transcieve pin ... UART1_Init(19200); // initialize UART1 module Delay_ms(100); RS485Slave_Init(); // initialize MCU as Slave
Notes	None.

RS485Slave_Receive

Prototype	void RS485Slave_Receive(char *data_buffer);
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	`data_buffer`: 6 byte buffer for storing received data, in the following manner: `data_buffer[0..2]:` message content `data_buffer[3]:` number of message bytes received, 1–3 `data_buffer[4]:` is set to 255 when message is received `data_buffer[5]:` is set to 255 if error has occurred The routine automatically adjusts `data[4]` and `data[5]` upon every received message. These flags need to be cleared by software.
Returns	Nothing.
Requires	MCU must be initialized as a Slave for RS-485 communication. See RS485Slave_Init.
Example	char msg[8]; ... RS485Slave_Read(msg);
Notes	None.

RS485Slave_Send

Prototype	void RS485Slave_Send(char data_buffer, char* datalen);
Description	Sends message to Master. Message format can be found at the bottom of this page.
Parameters	`data_buffer:` data to be sent `datalen:` number of bytes for transmition. Valid values: 0...3.
Returns	Nothing.
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	char msg[8]; ... // send 2 bytes of data to the Master RS485Slave_Send(msg, 2);
Notes	None.

Message format and CRC calculations

Q: How is CRC checksum calculated on RS485 master side?

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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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