Image Map Image Map
Page 1 of 5 12345 LastLast
Results 1 to 10 of 43

Thread: A new G888 manchester read/write module

  1. #1

    Default A new G888 manchester read/write module

    Well, this is something I have been struggling with for almost four years! The history starts in 2014 when I was given a PDP-9 computer. Almost immediately I realized that the paper tapes and magnetic tapes must be preserved. I also realized that I had a long way to go if I would use the PDP-9 computer to do that.

    The paper tapes are secured since 2015: http://www.bitsavers.org/bits/DEC/pd...pe_Collection/

    The DEC tapes has not yet been preserved.

    So first I started a collaboration with Mattis Lind on a standalone TU55 tapecontroller, this never worked. I had one single G888 board and a couple of TU55's to work with. I got really frustrated, but realized that if I only got a working setup I would have a better chans to succeed.

    Then I decided to change the way forward and I built a clone of the G888 with modern components that is easy to find. This since the G888 is the key, it takes small signals from the read/write head and produces a digital pulses. It will also do the other way around to write data on the tape. It takes five G888 in one tapecontroller since there are five logical tracks on the tape (and ten physical). Got the first prototype built and compared it with the G888 that I got and it looked promising. Then Mattis got a tape controller TC11 and a TU56 going with this PDP11/04?. He tested my prototype, it worked somehow but didn't pass the maindec's.

    Then I got a chance to borrow a TD8E and a TU56 drive in unknown condition that I got to work. After some time I started to think about those boards again and tested around a bit. I realized that the board actually could read tapes and the maindec's passed the tests when the board sat in a position where only the read circuitry was exercised (mark and timing track). After measuring on the writing circuitry it was just an easy patch to get it to work.



    Now it passes the TD8E maindec! But I have still some work to do to get the tapes preserved...

    I wish I had my own TD8E, because then I could hook up my own TU55 to my PDP-8... Well a TU56 would also be a nice find.

  2. #2

    Default

    Very nice!, I guess a lot of other DEC collectors will be happy, as they might miss one or more of these cards for their equipment.

  3. #3
    Join Date
    Jul 2014
    Location
    Rhode Island
    Posts
    1,069

    Default

    I have a large number of LINC tapes that need to be preserved. I have a working TU56 and a working TD8E. I think that the only hardware change that I need to do is to add a switch so I can select the timing track from TT(0) and TT(1) because the LINC tapes go backwards and use a timing track that is opposite phase from a DECtape. Once I can image a LINCtape, I will have to do a lot of bit fiddling to get the tape image into SIMH format.

    The PDP-12 at the RICM has M888 modules in the TC12 controller. Unfortunately our rented office space has a lot of RF noise from somewhere in the building. The RF noise is getting coupled into the TU56 tape heads, and then causes problems for the G888 modules. Do you think that your G888-mk2 modules work better than the original G888 modules?
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  4. #4

    Default

    Quote Originally Posted by m_thompson View Post
    The PDP-12 at the RICM has M888 modules in the TC12 controller. Unfortunately our rented office space has a lot of RF noise from somewhere in the building. The RF noise is getting coupled into the TU56 tape heads, and then causes problems for the G888 modules. Do you think that your G888-mk2 modules work better than the original G888 modules?
    I have no idea! The card itself might be better since it has a proper ground plane. On the other hand there are long cables between the TC and the tape drive. Those can probably pick up som RF noise depending on which cable type it is. From the tape read/write head to the relay board it is a shielded coax cable which is good. How does the cabling from the drive to the TC look?

  5. #5
    Join Date
    Mar 2004
    Location
    Wilmette, IL (north of Chicago)
    Posts
    613
    Blog Entries
    1

    Default

    Congratulations! Now, about that TU55 controller ... :>)

  6. #6
    Join Date
    Jul 2014
    Location
    Rhode Island
    Posts
    1,069

    Default

    Quote Originally Posted by anders_bzn View Post
    How does the cabling from the drive to the TC look?
    From the back of the TU56->side of the cabinet->back of the cabinet->TC12, total length about 1.5 meters.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  7. #7
    Join Date
    Sep 2006
    Location
    Silicon Valley
    Posts
    1,757

    Default

    Quote Originally Posted by m_thompson View Post
    From the back of the TU56->side of the cabinet->back of the cabinet->TC12, total length about 1.5 meters.
    You might try moving the G888s to their slots inside the TU56 and re-cable

  8. #8
    Join Date
    Jul 2014
    Location
    Rhode Island
    Posts
    1,069

    Default

    Quote Originally Posted by Al Kossow View Post
    You might try moving the G888s to their slots inside the TU56 and re-cable
    That would be a bunch of work, but if we can't find another solution that might be a way to get LINCtapes to work.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  9. #9

    Default

    Quote Originally Posted by jackrubin View Post
    Congratulations! Now, about that TU55 controller ... :>)
    Thanks. Well give it a couple of years...

    Well, my goal is really to get those tapes archived.

    Quote Originally Posted by Al Kossow View Post
    You might try moving the G888s to their slots inside the TU56 and re-cable
    Long cables can be a problem. I guess this is ordinary flat cables, works great as antennas. Aluminum foil around the cables can also work, grounded off course.

  10. #10
    Join Date
    Sep 2006
    Location
    Silicon Valley
    Posts
    1,757

    Default

    Quote Originally Posted by anders_bzn View Post
    Long cables can be a problem.
    In particular the TC-12/TC-08/TC-11 where the preamps are in the controllers, not at the drive,
    and the preamp and limiter are high gain.

    In the case of the TD8-E, the G888s are in the TU56, so you have relatively low frequency digital data on the cables.

    I really need to get back to DECtape/LINCtape archiving.
    I may just use the same setup this time as I'm using with 7-track tape and just stay in the analog digitization domain.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •