[Info-vax] TK50 - this is annoying...

[Johnny Billquist]

On 2012-10-19 20:05, Bob Koehler wrote:
> In article <k5rnkc$7os$1 at Iltempo.Update.UU.SE>, Johnny Billquist <bqt at softjar.se> writes:
>>
>> The fastest tapes on a PDP-11 is the TU77, which writes at 125 ips. At
>> 1600 bpi, that means it writes 200,000 bits per second per track, so
>> approx 200 KB/s. And this is disregarding the block gaps, where it will
>> pause.
>>
>> Are you saying that no disks you had could exceed that?
>>
>
>     We had RM03, I think, and TU45.  Out engineer insisted the TU45
>     was faster.

It was not.

>> A normal RP05 or RP06 will transfer 806 KB/s, just as comparison.
>
>     What kind of delay do you get when the disk does a seek?  That
>     could kill our application.  It was hard real-time, get behind once
>     and you failed.

Seek will probably kill you, I bet. Unless you write to the disk without 
a file system, you have little control over placement and other issues. 
But assuming you could just write to disk blocks, in sequence, then that 
would have given you better performance than a tape. And, as I said at 
the beginning, you could keep the disk writing without missing the next 
block, and having to wait a rotation.

>> And both the TU77 and the RP drives hang off massbus, so the speed
>> outside of the drive have the same restrictions, so no way the disk
>> could ever be slower than the tape.
>
>     MASSBUS?  We din't have no stinkin' MASSBUS.  These were UNIBUS
>     systems.

:-)
Both the RM03 and TU45 are also massbus devices. Massbus is (was) an I/O 
bus, not a computer bus. For a PDP-11, you used either an RH11 or RH70 
to connect your massbus devices to the Unibus. For VAXen it was things 
like the RH750 or RH780.

Oh, and the RH11 was the only one which actually sat on the Unibus. All 
the others had their own connections more directly to the cpu and memory.

	Johnny