DHV (was: RL01 drive select plug and power supply questions) - test-drb@ccmp.vtda.org

22 Oct 2005

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