A non-text attachment was scrubbed... Name: not available Type: text Size: 1174 bytes Desc: not available Url : http://www.classiccmp.org/pipermail/cctalk/attachments/19990127/c62c68cc/at…

I no longer have the tape - didn't see the need to get it back since I have nothing that could read it. 3200 may be right. It was a Dec TU80(?) tape.

Bill Pechter <pechter(a)monmouth.com> wrote about 1/2 mag tapes and drives: 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. 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, all 8 data bits are zero, the parity bit will be one, so there is still one flux transition. If the head isn't aligned properly, the flux transitions on multiple tracks may not occur at exactly the same time. The drive electronics is designed to deal with a certain amount of skew. I think 3200 also uses PE, but I'm not certain. It is much less common than 800, 1600, and 6250. HP also has a proprietary 6250XC format that adds data compression. This was the forerunner of DDS-DC, the data compression used on 3.81mm DDS (DAT) cartridges.

At 10:49 PM 1/27/99 +0000, you wrote: 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 tight the bits are written to the tape. Gary

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. This is like a refresher training course... I loved tape drive work, except for the ^&^%$#@ 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@shell.monmouth.com|pechter@pechter.nws.net|pechter@pechter.ddns.org