Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!ncar!ames!netsys!ziggy!scotty
From: scotty@ziggy.UUCP (Scott Drysdale)
Newsgroups: comp.sys.amiga
Subject: Re: Multiple Serial Ports (Re: vt100 v2.9)
Message-ID: <155@ziggy.UUCP>
Date: 14 Jan 89 05:53:57 GMT
References: <8812150227.AA09671@postgres.Berkeley.EDU> <14049@oberon.USC.EDU> <554@sunkisd.CS.Concordia.CA> <3241@sugar.uu.net> <147@ziggy.UUCP> <373@madnix.UUCP> <150@ziggy.UUCP> <385@madnix.UUCP> <154@ziggy.UUCP>
Reply-To: scotty@ziggy.UUCP (Scott Drysdale)
Organization: Un*x Link,Frederick Md.
Lines: 13

oh, one more little thing about using DMA on serial ports - it gets tough
to track errors, since they're not part of the data stream.  the other
problem with DMA is doing handshaking (either the hardware kind or XON/XOFF).
if your UART does the proper hardware handshaking on it's own (the SCC has
a mode to allow this) then that's not too bad, but you end up with a
non-configurable hardware handshake.  if you don't let the UART do the
hardware handshake, you have to poll the silly UART for state changes, or
add more interrupts to handle it.  now that i think about it, XON/XOFF
shouldn't be too awful in terms of the latency of receiving the XON/XOFF
and reacting to it by pausing DMA, if the host gets the bytes relatively
quickly (say, allow for 8 bytes of slop once a XOFF has arrived at the UART).

  --Scotty