Designing a protocol for the first time

CC · 2008-03-03T03:36:51+00:00

Greetings,I wish to transfer binary data in the simplest way possible from a PC to either an AVR or TMS320F2812 embedded target via USB.The PC will be running a Python/ program and use PySerial to send data out through the virtual COMM port produced by using a FTDI FT245BM USB-to-FIFO parallel interface chip on the target. Ideally this program will be useable on Linux as well as Windoze machines.I estimate that I need up to 120kB/s (without overhead) of data throughput to transfer 30000 points per second of 32-bit words containing vector graphic X,Y,RGB points for a laser show project.The FTDI chip says it can do 300kB/s with the virtual COMM driver, so this should work unless they are fibbing.I have gained some experience recently by writing a set of low-level serial driver functions for the F2812. Then I wrote a command parser for it so the F2812 can respond to text commands and send back various reports. This is all via text terminal. That was my first foray into communication interface programming. It was fun and not too difficult.Now I need to transfer binary data. The project here won't need any human readable command line. I also don't want to increase overhead by having to reserve the 8th bit for example to distinguish control codes from data, or use some text rendition of the data, etc. Just straight binary.Thus, I suspect the simplest way for the target to behave is to implement a state machine something like this:data: state: 1. wait for start code0x1234 2. (start code received) wait for size field0x0123 3. (16-bit size field received, specifying 291 words to follow) wait for data0xdddddddd 4. (data incoming) count data words until 2910xdddddddd received0xdddddddd...0xdddddddd 5. (291st word received) wait for CRC state0x3E5B 6. (16-bit CRC received) goto state 1Now of course, each state...

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Jim Stewart 18 years ago

Snip...

Brings back old memories. My first product had X-Modem download capability and I wrote and debugged it in assembly language on a Saturday afternoon. It was exquisitely ugly with software timing loops and no crc, but it worked for years. I finally got sick of it and completely rewrote it with crc.

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C

CC 18 years ago

That's the sort of thing I can deal with!

I wouldn't have considered Zmodem for my app.

Thanks for the input to the thread!

_____________________ Christopher R. Carlen crobc@bogus-remove-me.sbcglobal.net SuSE 9.1 Linux 2.6.5

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K

Keith M 18 years ago

Jim,

Sorry to bring the thread back from the dead -- just saw your reply now.

Yeah, old memories indeed. I co-wrote some (unpopular, basically unknown) BBS software years ago as a kid, and one of my tasks was implementing the file upload/download feature. I remember finding XMODEM.TXT(the xmodem specs) on some bulletin board, and being very excited about it. At the time, however, I didn't have enough RAM to store it, and I couldn't write real time to my tape drive. So what I did was attach my modem to my printer directly (daisy wheel 130 column

25cps), and lo-and-behold, I managed to get a printed copy of it! 300 baud is +/- 25cps, so a little flow control, and I was home free. Ingenious for not being yet in my teenage years! :)

Anyways, I ended up not understanding how the 8-bit checksum worked (I think it was just check=check+newbyte mod 256.), and so I picked up the phone and called the phone number at the top of the page.(of course!) Turned out to be either Ward Christensen or Chuck Forsberg at some university. I remember him telling me, "it's all in the spec" but I didn't know enough to understand most of what was in there. He provided a couple quick pointers and shoo'd me away back to my paper route.

Ahh well.

Keith

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Designing a protocol for the first time

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