Path: utzoo!attcan!uunet!nuchat!sugar!karl
From: karl@sugar.uu.net (Karl Lehenbauer)
Newsgroups: comp.sys.amiga
Subject: Re: Multiple Serial Ports (Re: vt100 v2.9)
Summary: not sure you must have an offboard CPU anymore for UART boards
Message-ID: <3256@sugar.uu.net>
Date: 10 Jan 89 03:13:41 GMT
References: <8812150227.AA09671@postgres.Berkeley.EDU> <14049@oberon.USC.EDU> <147@ziggy.UUCP>
Organization: Sugar Land Unix - Houston, TX
Lines: 43

In article <147@ziggy.UUCP>, scotty@ziggy.UUCP (Scott Drysdale) writes:
> first of all, anyone who develops a multiport serial board that doesn't have
> it's own CPU on board is absolutely nuts.  using interrupts from the UARTs
> is also grounds for the electric chair.  

It's not as necessary as it used to be.  With the chip I mentioned, the NEC
16550A, it can buffer up to sixteen bytes of input data.  That means you
can have as many as 16 times fewer interrupts...only 60 a second for one port
at 9600 baud.  You can set a lowater mark, too, to where the chip won't 
generate an interrupt until that number of chars (or a timeout?) has been
received.  You also reduce costs a lot and obviate the need for a superhacked
custom driver -- they're tricky.  I think a 4-port board based on 16550s for,
say $300 retail, would be desirable to most people over a 4 or 8 port smart
board for $600-1000, and I'm cutting some slack there because PC/AT smart
boards all seem to be over $1000.  

Plus, I still need MIDI and my stuff is already ready for it from a dumb 
port -- the default in the machine is one, after all, and the smart board
will probably need a precision clock and internal specific intelligence
for MIDI -- sort of like a multiport version of the Roland MPU-401.  The
trouble is, too, it's more expensive plus the programming model is totally
different.


i've done this before - the board was
> an intel SBC547, which is a multibus I card with and 80186, 4 zilog SCCs, and
> some rom and ram.  
...remains of serial I/O performance story deleted.

I too worked on a system that didn't have enough serial I/O throughput, an
8 MHz 8086 Multibus system (this was a few years ago) running the JMI C Exec
couldn't receive 9600 baud data on even one port using 8251s.  We switched
to Zilog Z8530s, which have a 3 byte typeahead buffer, and it could receive
all the data it needed to and not lose bits, a line in at 9600, in at 2400
and out at 9600.  I never tested to see how far it could go.

I guess the point is that I had a really favorable experience with adding
typeahead buffer in the UART, and it's less expensive and easier -- the
16550As are 8250 pin-compatible, by the way.
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