Simple485

Simple multipoint communication protocol using RS-485 for Arduino.

This library implements simple communication protocol for multiple devices connected to serial RS-485 port. It includes addressing, error detection and simultaneous transmission prevention.

Tested on Arduino Uno and Arduino Nano. Python implementation here.

Basic Usage

Simple485 rs485 = Simple485(&Serial, addr);
rs485.send(dst, len, bytes);
rs485.loop();
if (rs485.received() > 0) {
  message = rs485.read();
}

Features

1 byte addressing (up to 255 points)
broadcast address 0
transmitting payload up to 255 bytes
XOR checksum
simultaneous transmission prevention

RE/DE pin

Some RS-485 transceivers use RE and DE pins to turn receiver and transmitter on (RE = receiver enabled, DE = driver enabled). When a pin number provided, Simple485 sets the pin high when transmitting and low otherwise. Connect both RE and DE to the selected pin together.

Packet format

(LF) SOH dst src len STX payload ETX crc EOT (LF)

Segment	Length	Description
dst	1 byte	destination address
src	1 byte	source address
len	1 byte	payload length
payload	2N bytes	encoded payload
crc	1 byte	checksum

Payload code

Payload is encoded to consists of antisymmetric bytes only: byte is separated into two nibbles and each is complemented by its inverse nibble. This makes the payload twice as long.

Simultaneous transmission prevention

If multiple devices transmit at the same time, packets are corrupted. To avoid this, Simple485 checks whether data is incoming: If the period since last data arrival is greater than LINE_READY_TIME (default 10 ms), protocol starts transmitting its buffered packets. Otherwise, it does nothing until next loop.

Note that if for any reason (e.g. extremely synchronized transmission) the packets are corrupted, the receiver ignores faulty packets, but no retry is implemented.

Library description

Simple485(Stream * serial, uint8_t addr) constructor, serial stream object, addr this device's address
Simple485(Stream * serial, uint8_t addr, uint8_t pin) constructor with RE/DE pin (see above)
void receive() process serial buffer
void transmitt() transmit packets in buffer
void loop() { receive(); transmitt(); }
void send(uint8_t dst, uint8_t len, uint8_t * bytes) send bytes of length len to destination dst
void send(Message message) same as above, variables in struct
void send(uint8_t dst, String text) send text message to dst
int received() number of received messages in buffer
Message read() get first message in buffer as struct

typedef struct Message {uint8_t addr; uint8_t len; uint8_t * bytes;} Message;

Testing

Tested with Arduino Uno and Arduino Nano.

rzeman9 / Simple485