Simple serial point-to-point communication protocol

by

I would use HDLC. I have had good luck with it in the past. I would for a point to point serial just use the Asynchronous framing and forget about all of the other control stuff as it would probably be overkill.

In addition to using HDLC for the framing of the packet. I format my packet like the following. This is how options are passed using 802.11

U8 cmd;
U8 len;
u8 payload[len];

The total size of each command packet is len +2

You then define commands like 

#define TRIGGER_SENSOR 0x01
#define SENSOR_RESPONSE 0x02

The other advantage is that you can add new commands and if you design your parser correctly to ignore undefined commands then you will have some backwards compatibility.

So putting it all together the packet would look like the following.

 // total packet length minus flags len+4
 U8 sflag;   //0x7e start of packet end of packet flag from HDLC
 U8 cmd;     //tells the other side what to do.
 U8 len;     // payload length
 U8 payload[len];  // could be zero len
 U16 crc;
 U8 eflag;   //end of frame flag

The system will then monitor the serial stream for the flag 0x7e and when it is there you check the length to see if it is pklen >= 4 and pklen=len+4 and that the crc is valid. Note do not rely on just crc for small packets you will get a lot of false positives also check length. If the length or crc does not match just reset the length and crc and start with decoding the new frame. If it is a match then copy the packet to a new buffer and pass it to your command processing function. Always reset length and crc when a flag is received.

For your command processing function grab the cmd and len and then use a switch to handle each type of command. I also require that a certain events send a response so the system behaves like a remote procedure call that is event driven.

So for example the sensor device can have a timer or respond to a command to take a reading. It then would format a packet and send it to the PC and the PC would respond that it received the packet. If not then the sensor device could resend on a timeout.

Also when you are doing a network transfer you should design it as a network stack like the OSI modle as Foredecker points don’t forget about the physical layer stuff. My post with the HDLC is the data link layer and the RPC and command handling is the Application Layer.

More Related Contents:

  1. How do you design a serial command protocol for an embedded system? [closed]
  2. Looking for an efficient integer square root algorithm for ARM Thumb2
  3. High delay in RS232 communication on a PXA270
  4. using serial port RS-232 in android?
  5. Process for reducing the size of an executable
  6. Cycle counter on ARM Cortex M4 (or M3)?
  7. C pointer to another struct
  8. Difference between const & const volatile
  9. Why can’t I pass a Protocol.Type to a generic T.Type parameter?
  10. Serial Comm using WriteFile/ReadFile
  11. Calling protocol default implementation from regular method
  12. How to pause a pthread ANY TIME I want?
  13. Is there any alternative to using % (modulus) in C/C++?
  14. Serial Port Polling and Data handling
  15. Using JSONEncoder to encode a variable with Codable as type
  16. Serial to USB cable from a scale to PC ,some values are just question marks
  17. Reading and writing to serial port in C on Linux
  18. How to define a protocol as a type for a @ObservedObject property?
  19. How do I use dataReceived event of the SerialPort Port Object in C#?
  20. Explaination of ARM (especifically mobile) Peripherals Addressing and Bus architecture?
  21. C++, can I statically initialize a std::map at compile time?
  22. Serial Port (RS -232) Connection in C++
  23. What to do with errors when streaming the body of an Http request
  24. Finding information about all serial devices connected through USB in C#
  25. Why do we need a bootloader in an embedded device?
  26. Reasons to include function in protocol definition vs. only defining it in the extension?
  27. Multiple assignment in one line
  28. Compiling an application for use in highly radioactive environments
  29. What’s the difference between a protocol extended from AnyObject and a class-only protocol?
  30. GWT RPC data format

Leave a Comment