Installing PDP-11 UNIX w. no tape - solution

[Steven M. Schultz]

Hi -

	I thought I'd chime in with my experience with "high" speed serial
	transfers...

> From: allisonp at world.std.com (Allison J Parent)
> 
> Try slowing down.  You may be overflowing the input buffer.  This was
> a common problem on TU58s hooked to the 2nd DL on some systems at speeds
> above either 4800 or 9600.  It only happend in the TU58 to host direction

	The TU58's lack of flow control (unless you were on the Vax-750 with
	something I believed was called the MRSP roms) made them all but
	useless except in a 'standalone' environment.  As a boot device they
	were just "slower than molasses in January".  As a data storage device
	to be used while the system was up and doing other stuff the TU58 was
	quite poor.

	I tried to use the TU58 on an 11/44 once and it just wouldn't work
	reliably when trying to transfer a file from TU58 to disk.  The first
	time the system had to tape a couple milliseconds to write a block to
	disk you had a DL11 overrun and the transfer was corrupt.

> (read) as the opposite path expected a handshake every 128 bytes(to allow 
> the tu58 to actually do the write to tape).  It seems the tu58 would send 
> a 512byte block as 4 128byte packets at a sustained rate fast enough to 
> overrun the PDP-11 host input buffer; before it could be emptied.  You 

	The DL-11 to which the TU58 was attached (could it be hooked up to
	something a bit better?  I would think so but don't know for sure)
	had no buffering/silo - at 9600 there was only 1 millisecond to get
	the character and that's cutting things a bit too fine on a ~.5 mips
	machine, especially if other things are going on at the same time.

> may be emulating a similar problem.  PCs do not service interrupts all 
> that fast and OS overhead can make that longer.

	Ummm, 'PC's I'm used to don't seem terribly upset at 10 or 20 thousand
	interrupts per second - that should be sufficient to handle any 9600
	baud serial line I'd think.

> Note PDP-11s can have enough overhead and higher priority stuff ahead of 
> the 2nd DL that it cannot take data at greater than 4800 baud (sustained 

	Not 'overhead' as much as just 'slowness'.  An 11/44 is about .6 mips
	(an 11/73 is about 15% less) - that's quite a bit less than even a
	286.

	The biggest problem I ran into was the fact that the disk systems
	all used SPL-5 while the serial ports (DL11,etc) were at 4.  A disk
	interrupt would (and did) come in and would delay things just enough
	that the DL running at 9600 with no flow control would overrun.

> rate) without some kind of handshake to allow processing in between.
> If the system is basically unloaded like my minimal 11/23 it can run at 
> 38.4!  The most likely time when this overrun can happen is while doing 

	If it's not doing too much else.  I don't see an 11/xx handling high
	serial line rates without some form of RTS/CTS flowcontrol while a
	kernel recompile is going on ;-)  If you're using a DHV-11 the
	data flow rate is quite a bit less than 38.4k - the bit timings are
	that fast but the board can't handle it and the effective rate is
	lower.  A DHQ-11 is quite a bit better but all in all anything over
	9600 requires hardware flow control, especially if the data has to
	make its way to disk.

	Steven Schultz


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