Re: TI-H: Data Transfer Idea
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> C.J. Oster wrote:
> >
> > Ok, all this time, we've been trying to use both lines for data
> > transfer. If we use both, then we are esentially using sterio sound, but
> > modems work great on mono. Sure if you are hard wired to gether, then the
> > connection goes red-red, and white-white. But in a radio transmitter, we
> > can transmit on one, and recieve on the other. Or we could just use one
> > line and employ a protocol that modems use to transfer. This is where you
> > guys come in... The most efficient transfer protocol is by far Xmodem CRC,
> > right (assume that were leaving out isnd and T3 lines). Does somebody know
> > EXACTLY how it takes care of errors? Don't reply saying that it has
> > something to do with a checksum bit, I know that. Then you could leave you
> > calc on waiting for a signal, and when you get a request, you could play a
> > game, or chat, or whatever.
> >
>
> Uhh.. no.. Xmodem is ancient. There a million (well, not really) better,
> newer, faster protocols. Ymodem and Zmodem, for example.. ISDN and T1/T3
> are entirely different.. They're digital. No Xmodem, Ymodem, Zmodem, or
> anything like that is even involved.. They've got their own entirely
> different system.
>
> Checksums add up all the bits (or bytes) in a packet, run it through a
> math formula of some sort (different formula for each type of checksum)
> and you come out with an answer. That's the checksum. No, I don't happen
> to know any checksum formulae off hand...
Well, we have to keep in mind that we don't want a huge and slow
transmission program to compress/error-correct the data. Therefore,
the best protocol would be something with small packets and a
checksum. (A checksum is just that -- a sum of all the bytes, MODded
down to one byte. A CRC is what you are talking about, I think.
CRCs are computed in many different ways and are slightly more
reliable. We don't have the space required to do full CRC.) With
small packets, if an error is found, the data can be resent quickly.
Also, the possibility of an error getting past the checksum check is
lowered. Maybe regular Xmodem without CRC, but a simple checksum
instead, is the best protocol, or maybe we should create our own.
But Zmodem would be _extremely_ hard to implement in the space of the
calc's memory.
shakal@ns.net
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