25 Feb
2021
25 Feb
'21
9:14 a.m.
On Feb 25, 2021, at 2:40 AM, Eric Smith via cctalk
<cctalk at classiccmp.org> wrote:
On Wed, Feb 24, 2021 at 7:06 AM Chris Zach via cctalk <cctalk at classiccmp.org>
wrote:
I know most DEC OS don't support this but I'm pretty sure that at least one of
them does. Not sure which.
A given Massbus can only do one transfer at a time, so yes, a tape transfer in progress
would mean you can't start a disk transfer (though you can start the seek). For that
matter, if one disk is transfering another can't start either. So if you want max
throughput, you want to spread the disks over several massbus adapters, and keep the tape
separate.
But if you need lots of disk space, you may have to put tape and disk on the same Massbus
since any given system has a limit on Massbus adapters (4 for the PDP-11/70?) and unit
numbers are 3 bits. For PDP-11 systems, not likely to be an issue. For mainframe
(PDP-10), it might be. I know mainframe systems at times had dozens of disk drives. One
I used (CDC not DEC) had 22 RP04 class drives.
paul
Yes, the TM02 and TM03 formatters allowed MASSBUS
to connect to Pertec
drives, but I don't think you could run a tape drive and a disk drive on
the same MASSBUS channel anyway. Wonder why, technically the MASSBUS
cable is just an extension of the Unibus, so it shouldn't matter much
what combination of cables you used. Maybe they just wrote different
drivers or something.
There's nothing in the hardware that prevents it from being done, but DEC
OSes don't support it, and it's generally not a good idea. Tape activity
would block disk activity, because it doesn't have anything like modern
"tagged command queuing".