DHV (was: RL01 drive select plug and power supply questions)

[Johnny Billquist]

On Fri, 21 Oct 2005, Allison <ajp166 at bellatlantic.net> wrote:

> >Subject: Re: RL01 drive select plug and power supply questions
> >   From: Paul Koning <pkoning at equallogic.com>
> >   Date: Fri, 21 Oct 2005 10:58:55 -0400
> >     To: cctalk at classiccmp.org
> >
> > Allison> Yes I do.  However for a single user system the load is not
> > Allison> an issue.  If your running a timeshare system such as RSTS
> > Allison> or RSX with more than one user then DHV11 sense as well.
> >
> >Not true.  9600 baud is 960 interrupts per second, on a character I/O
> >device.  That's a big number for a PDP-11.  Output will be
> >significantly less of a burden with a DH type output controller than
> >with other types -- even for just one active terminal.
>
> Depends somewhat on the OS.  It dont know about you but most people
> can't type much faster than 100WPM (less than 10 chars sec).

Um, that's irrelevant. A DH type controller also interrupt on each
character input. It's output that differs. And filling a full screen is
1920 characters, which at 9600 bps will take about 2 seconds. Do that on
one terminal will the system will definitely notice. If you have a serial
printer of some speed (such as an LN03) we're talking about a lot more
than about 2k of data sometimes. So even with a single user system, it can
make a big impact. And of course, if you have anything else running at the
same time, it will make it even worse. Multi-user systems definitely
suffer if you use DL11 controllers for users. It's more or less a no-no.

I'm not even sure you can drive one single line at 9600 bps at full speed
on a DL11, let alone 19200.

> For RT11 it's mostly unimportant.  For unix (the most sensitive
> to interrupt loading you _may_ care as a single user).

It matters for both, as I've pointed out above.

> > Allison> For most of my 11s four lines is the limit for what I can
> > Allison> seem to keep busy.  Figure a user terminal, LA100 Printer
> > Allison> and serial line for modem or data line to another system. At
> > Allison> the extreme I've run two terminals for OSs that support that
> > Allison> but, I can only type on on at any instant. ;)
> >
> >Sure, if you're mostly doing editing, then the CPU burden of high
> >speed output may not be obvious.  If you had an LN03 or similar
> >printer, you might see it more easily.  An LA100, of course, isn't
> >much of a problem because it is quite slow.
>
> Actually printers are a bursty load (fill the buffer and go away)
> and I've found that in practice the faster you fill the buffer
> the better (high line rates or use a parallel interface).

Yes, and that burst will drop a PDP-11 to it's knees if it's on a DL11 at
high speed. Sure, if you're running 2400 bps then you'll live. But not
many does these days.

> Performance was good enough that we used that over the VAX
> (line lengths limited us to 2400baud in our part of the mill).

Yes, interrupt performance on the VAX was not pretty. One or two DZ11 on a
VAX-11/750 would kill it.

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt at update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol