> At 10:49 PM 1/27/99 +0000, you wrote:
> >Bill Pechter <pechter_at_monmouth.com> wrote about 1/2 mag tapes and drives:
> >
> >> Tape Density Encoding method DEC Tape Drive
> >>
> >> 200BPI (NRZ?)
> >> 556BPI (NRZI?) TU10?
> >
> >200 and 556 were only used for 7-track. Even from a single vendor such
> >as IBM, some 7-track systems used odd parity and others used even. I'm
> >not sure what recording mode the even parity systems used, as NRZI wouldn't
> >work with even parity unless you could otherwise guarantee that there are
> >no all-zeros data conditions. See below.
> >
> >> 800BPI NRZI (Non-Return to Zero Indescrete) TU10/16/TE10/16
> >
> >800 BPI was used for 7-track and 9-track. NRZI is Non Return to Zero
> >Inverting. On a 9-track, each "line" on the tape consists of 8 data bits and
> >an parity bit. The "I" (Inverting) means that a flux transition is recorded
> >on the tape if a data bit in the line is different than the same bit position
> >in the previous line. So unless the data is all zeros, each line will have at
> >least one flux transition. Unlike DECtape, there is no "clock" track, so it
> >is essential that there is always at least one track with a flux transition,
> >from which a read clock can be recovered. The odd parity guarantees that if
> >all 8 data bits are zero, the parity bit will be one, so there is still one
> >flux transition.
> >
Been a long time since my tape maintenance classes and days going through
the DEC TM03 bit fiddler board prints 8-)
Thanks for the refresher course.
It's been 13 years or so since I last worked on a TU77 repair.
>
> It was conventional on 7 track drives to use the modes "BCD" and "Binary"
> to describe the parity of the written characters. In BCD mode, each character
> was written in even parity, while "binary" mode wrote odd parity. One
> interesting side effect of this was the character code for '0' (usually six
> bits of 0) couldn't be represented, since. as described above, it would have
> been encoded as no flux changes for that field - i.e. a "blank" spot which
> would have been completely skipped by the read electronics. For this reason,
> some other character (often an octal 12 - often ':' in BCD) was chosen as the
> replacement character, so in BCD mode, often the ':' couldn't be represented.
> This code chosen varied from manufacturer to manufacturer.
>
> In "binary" mode with odd parity, all 6 bit codes could be represented since
> there would never be a field of all 0's. I've never found out why there
> needed to be both of these modes rather than just use binary mode for
> everything. Anyone know the reason for this?
>
> I should point out that my experience on this was on Control Data 3300
> equipment (and some early CDC 6000 equipment with 7 track drives as well.)
> We also used a number of Kennedy (sp?) small stepper-motor 7 track 200BPI
> drives and, as far as I can tell (I have an old 7 track drive I use for
> reading legacy tapes) all densities 200, 556 and 800 BPI use NRZI. In fact
> for my small drive, setting "read" density is a No-op. It only affects
> the clock generator for writing. So I assume the only difference is how
> Gary
This is like a refresher training course...
I loved tape drive work, except for the ^&^%$#_at_ DEC TU45 abortion
(the Pertec drive from HELL).
Bill
---
Bill Gates is a Persian cat and a monocle away from being a
villain in a James Bond movie -- Dennis Miller
bpechter_at_shell.monmouth.com|pechter_at_pechter.nws.net|pechter_at_pechter.ddns.org
Received on Wed Jan 27 1999 - 20:01:12 GMT