Date   : Sun, 15 Apr 1990 03:00:14 EDT
From   : dg%pallio.UUCP@XAIT.Xerox.COM (David Goodenough)
Subject: Z80 serial comm speed (was Re: Why Bother With CP/M?)

Dave Close says:
> dg@pallio.UUCP (Who, me?) writes:
>> On the other side though, it still chugs along nicely: a 4MHz Z80 can
>> do the CRC and data transfer to keep an Xmodem transfer going at up to
>> about 22000 BPS, and that's doing the CRC the hard way (i.e. bit banging),
>> with a table driven CRC, it'd probably exceed 38400 BPS.
>
> Actually, it will do even better.  I know of a 4MHz Z80 pumping 56Kbps SDLC.

You'll forgive me for playing devil's advocate, but we're comparing apples
and oranges here. The SDLC is probably done with an SIO, which is capable
of doing all the CRC and framing trickery itself, all the Z80 has to do is
stuff bytes into the SIO, and with a DMA, the Z80 could actually spend a
good proportion of the time asleep.

I'm not sure you can do sync prtotocols with an async only uart: the hunt
phase would be a real nightmare, and you'd get in all sorts of trouble with
the incessant start and stop bits. Hence I can see a Z80A very easily
achieving 56KBPS doing SDLC (probably faster if the SIO could be clocked
fast enough). However, the Xmodem needs to have the Z80 do the CRC
evaluation as well, so it's got to do a little more work.

Not that I'm condemning 56K SDLC or anything, it's kinda nice to hear that
the Z80 is up to talking to the big guys as a peer. I'm sure that with a
little creative genius, it would be possible to do such wonders as HASP
and RJE from a Kaypro (look out BITNET - here come the CP/M'ers :-> )
-- 
       dg@pallio.UUCP - David Goodenough               +---+
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