Installing PDP-11 UNIX w. no tape - solution
Steven M. Schultz
sms at moe.2bsd.com
Sat Jan 31 15:36:05 AEST 1998
I thought I'd chime in with my experience with "high" speed serial
> 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
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
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.
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