Re: Designing a protocol for the first time

Jim Stewart wrote:

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.

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
.