throughput using CTS flow control on a Trailblazer+

[pri=-10 Stuart Lynne]

In article <871 at spdcc.COM> dyer at spdcc.COM (Steve Dyer) writes:
>I'm using a Telebit Trailblazer+ on one of my dialin ports, fixed at 19.2kb
>using hardware flow control (gettydefs has EXTA and CTSFLOW defined).  I'm
>finding that effective UUCP throughput ranges between 80-100 bytes per second
>when a UUCP neighbor without a Trailblazer dials in at 2400 baud, completely
>negating any benefit of the neighbor's >1200 baud speed.  The CTS light seems
>to blink on every SD (send data) blink.  People who dial in to this line at
>2400 baud also report jerkiness (although this seems to be hidden and not
>observable at 1200 baud.)
>
>Note we don't appear to be losing data; the low transfer rate isn't due to
>retransmissions, but due to (excessive?) throttling of the output stream.
>
>Any ideas of what to try?

I'm assuming that you are running on an IBM AT with SCO Xenix (as evidenced
from cross posting and use of CTSFLOW).

Do you have SCO's fix's for Telebit Support? Phone and ask for it if you
don't.

Why are you running with a fixed speed? SCO's software all seems to work very
well using the auto baud feature. 

What I would think is happening is that you have turned hardware flow
control on in the Telebit. When you do this and connect at slower speeds the
Telebit seems to use CTS to regulate your dataflow down to the connect speed
instead of bufferring it (which it will do if you just ignore CTS).

SCO has implemented CTS flow control in their serial driver. But remember that 
CTS is not supported by the 8250/16450 Serial chip. It must be implemented 
higher up in the interrupt service routine.

Meaning for every CTS state change you must service an interrupt, and do some
work to get things stopped or started. This is very inefficent, you will see
*two* state changes for every character you send. This increases your
interrupt loading from 1 to 3 for each character sent.

As an aside, here at home I *do* run with a fixed speed to solve other
problems with my serial drivers and getty. I don't run into the same problem
as seen here because the serial chip (8274) supports CTS directly. So even
though I see the CTS light blinking madly away in tandem with the SD light
it doesn't affect performance at all.

I would suggest that you don't use hardware flow control unless you are
connected in FAST mode. It does work well there.

An alternate suggeston would be for Telebit to not worry about CTS for each
character. Perhaps make it configurable, ATS??=n, where n is the number of
characters to buffer before worrying about dropping CTS.

-- 
{ihnp4!alberta!ubc-vision,uunet}!van-bc!Stuart.Lynne Vancouver,BC,604-937-7532