Fast parallel driver for Unix/Xenix AT?

[Rick Richardson]

In article <FZL6GM at b-tech.mi.org> Jon Zeeff writes:

>I've given up on getting good parallel port performance and have 
>switched to a 19200 bps serial link to a HP Series II.  You might look 
>into serial->parallel adapters if your printer only supports 
>parallel.  

I missed the first part of this discussion, and am surprised that
you find the serial interface faster.

I measure 52,000 bits per second through the parallel interface.

The environment is 386/ix 2.0.2 talking parallel thru a Taiwan, Inc.
parallel port.  6 feet from the parallel port is a "Bravo Communications"
parallel line extender.  25 feet from that is a Taiwan, Inc. PB-64-2
parallel in/parallel out 2 port 64K printer buffer.  And 6 feet from that
is the HP LaserJet Series II.

The test is to take the printer off-line and then "lp bigfile".
I measured the time from issuing the command until the printer
buffer LEDs indicated it was full.  This worked out to 52,000 bps.

Even at 52,000 bps, the bottleneck is still the link to the printer
for printing bitmaps and images.  I don't know how anybody could
get by with less than half of that speed.

FYI, my Series II manual lists "typical" printer BUSY times
of 143 microseconds for font and image data and 2 milliseconds
for text data.  This works out to 55,000 bps and 4,000 bps,
respectively.  The Series IIP lists only one "typical" BUSY
time of 200 microseconds, which is 40,000 bps.

When I first got the printer buffer, I recall that it had
two effects, both good.  During parallel printing, 386/ix
performance suffers noticeably.  The first effect was that the
buffer shortened the elapsed time that 386/ix performance was
poorer.  The second effect, though, was that the total throughput
to the printer went up tremendously.  This was back when I had
386/ix 1.0.6, and I haven't done any testing without the buffer
since.  I suspect that the reason for this is that the buffer
takes data with BUSY asserted only for a very brief time,
no matter what kind of data is being transmitted to the printer.
Apparently, this time is short enough that the 386/ix printer
driver keeps blasting data down the pipe.  But if BUSY exceeds
some (unknown) length of time, then the driver appears to take
a clock tick or some other timeout before starting to transfer
again.

Anyhow, I'm happy, and the buffer was cheap.  If I had to do it
again, I think I'd get a 256K buffer with automatic switching
between the two ports (the one I have has a pushbutton to
switch the source).

Oh yeah, the Taiwan, Inc. parallel port couldn't reliably
drive the most significant data bit the 25 feet to the buffer,
so that's why I have the parallel line extender.  Don't ask
how I know it was the MSB that was getting smashed...

Finally, if I read my parallel interface spec's correctly,
the maximum theoretical throughput should be around 600,000 bps.
But don't try this at home, kids.

-Rick

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