Messages

Examples of command/answer protocols over RS485. (to be elaborated)

This is part of the RS485 class - https://github.com/RobTillaart/RS485

Guidelines

Designing a protocol is most often not trivial and might need more than one iteration.

Keep protocols as simple as possible.
Make sure that baud rates match between the nodes in the network.
Software Serial can work when time between requests is long enough. It depends on how much processing is needed.
Be aware of implications of future expansion. Design your protocol with room for additional commands. Or even a version number in it.
In designing a protocol be aware of ambiguities. An request or answer may contain bytes (sequences) that might trigger other nodes.
If you request data from a node, it might happen that it won't answer or that the answer fails to arrive in correct state (missing bytes, flipped bits etc.)

ASCII codes

Possible useful char codes.

commando	value	meaning
SOH	0x01	start of header
STX	0x02	start of text
ETX	0x03	end of text
EOT	0x04	end of transmission
ACK	0x06	ACKnowledge
NAK	0x15	Not Acknowledge
CAN	0x18	CANcel

example 1

A minimal message protocol consisting of 1 byte commands

command = (0x80 | 7 bits command) answer = (0x00 | 7 bits answer)*

All command are coded in a single byte with 0x80 bit set. All answers bytes 0 or more (sender knows how many to expect, or a specific end char.

requirement:

All devices listen to a disjunct command set.

example 2

Using a device id to send the command to.

command :=

SOH start of header, (attention new command) deviceID to (is it for me?) command (if so, exec command) bytes expected (to generate the bytes)

answer :=

bytes requested bytes (send them back)

example 3

Command and answer have same layout. Uses device ID's to address specific device.

SOH start of header 0x01 deviceID to deviceID sender length length of message message idem in ASCII

optional extend with: checksum optional EOT optional (end of transmission)

example 4

More complex package with multiple fields and crc per message.

SOH start of header deviceID to deviceID sender fields 0 or more length length of field 1 STX start of text message idem CHECKSUM idem, message only! ETX end of text length length of field 2 STX start of text message idem CHECKSUM idem, message only! ETX end of text ... EOT end of transmission

example 5 (still alive)

Maybe most simple protocol, just send an ID, and if the slave is alive it responds with its ID. Think of it as the PING command

command = ID answer = ID

Could be embedded in other protocols too,

Future

binary protocols

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