Path: utzoo!attcan!uunet!lll-winken!ames!amdahl!oliveb!tymix!antares!jms
From: jms@antares.UUCP (Joe Smith)
Newsgroups: comp.sys.amiga
Subject: Re: Multiple Serial Ports
Summary: XOFF must be processed quickly
Message-ID: <347@antares.UUCP>
Date: 17 Jan 89 03:25:58 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: jms@antares.UUCP (Joe Smith)
Followup-To: comp.sys.amiga.tech
Organization: Tymnet QSATS, San Jose CA
Lines: 32

[I've changed the title and sent followups to comp.sys.amiga.tech]

In article <154@ziggy.UUCP> scotty@ziggy.UUCP (Scott Drysdale) writes:
> ... doing a write from DMA to serial port would be deliciously
>efficient - but input's another story.  how are you going to have the DMA
>controller stop waiting for bytes from the UART and inform the host that
>the transfer is complete without periodically polling the DMA to see if
>anything came in, or adding timers on the serial board to generate interrupts
>if no bytes have arrived after a certain (programmable) delay?  
> --Scotty

One killer problem that I have come across is when the terminal at the other
end demands immediate response to XON/XOFF.  For instance: Lets say you have
smooth-scrolling VT100 terminal running at 9600 baud.  As the host tries to
send a bunch of consecutive linefeeds, the VT100 will send a Control-S to
the host, indicating that the host should stop sending.  Some older terminals
will lose data if the host sends more than 3 characters after the terminal
sends XOFF.  Here is where having a FIFO on the input side of the UART can
work against you.  And it's even worse if the DMA on the output side of the
UART cannot be quickly disabled.  Very nasty to have 16 characters sent to
the terminal after it says to stop.

I don't know if they make any UARTs that can be programmed to recognize
Control-S (with and/or without parity) and have the hardware disable the
transmit flag automatically when the XOFF is detected.  If hardware won't
do it, then there is an argument for using multiple dedicated microprocessors
for multiple serial ports.
-- 
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