2.2 KiB
Messages
Examples of command/answer protocols. (to be elaborated)
Guidelines
- Keep protocols as simple as possible.
- be aware of implications of future expansion.
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
Future
- binary protocols
- example code