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From: caf@omen.UUCP (Chuck Forsberg WA7KGX)
Newsgroups: comp.unix.xenix
Subject: Re: Adding ports to SCO XENIX system
Message-ID: <611@omen.UUCP>
Date: Thu, 29-Oct-87 03:57:10 EST
Article-I.D.: omen.611
Posted: Thu Oct 29 03:57:10 1987
Date-Received: Tue, 3-Nov-87 05:24:05 EST
References: <94@sda.atexrd.UUCP> <787@ssc.UUCP> <7433@dartvax.UUCP> <608@omen.UUCP> <7702@steinmetz.steinmetz.UUCP>
Reply-To: caf@omen.UUCP (Chuck Forsberg WA7KGX)
Organization: Omen Technology Inc, Portland Oregon
Lines: 39
Keywords: ports SCO XENIX

In article <7702@steinmetz.steinmetz.UUCP> davidsen@crdos1.UUCP (bill davidsen) writes:
:In article <608@omen.UUCP> caf@omen.UUCP (Chuck Forsberg WA7KGX) writes:
:|In article <7433@dartvax.UUCP> cmi@dartvax.UUCP (Theo Pozzy/R. Green) writes:
:|:We've pounded on it by using our software, MLINK, to do 9600 baud
:|:file transfers with one port looped back into another, with two
:|:loops running at once!
:|
:|A loopback test such as this is not necessarily a good prediction of real
:|world performance because the loopback configuration tends to regulate
:|the flow of data.  The result is an implicit, unexpected flow control.
:
:This is true. However, when running the loopback test, if the actual
:throughput is about right for a 9600 baud connection the system will
:handle one line at 9600, and unless the port controller buffers output
:but not input (I guess you could do it that way), it will handle two.

Close, but no cigar.  For openers, many serial IO controllers
do buffer or DMA output but not input.

When neither input not output are deeply buffered at the hardware level
(i.e., 8250, etc.) events (task switch, critical code, etc.) that
increase interrupt latency will not cause loss of data in a loopback
situation, but will cause loss of data in real world situations.

Why?  Because the interrupt latency inserts a short pause in
transmission that prevents the receiver from dropping characters beyond
the one or two that the UART can buffer.

A more realistic simulation is possible if high speed buffered modems
were inserted into the loops.  They would decouple the pauses of data
from the interrupt latencies, resulting in conditions representative of
actual applications.

Chuck Forsberg WA7KGX Author of Pro-YAM communications Tools for PCDOS and Unix
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