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Are there any advantages to installing a parallel port in a C1541-family drive and using a parallel cable to a PC to image disks, other than increased speed (i.e. since the standard IEC protocol is serial, one bit at a time)? I got the impression years ago that some nibblers (i.e. very low-level copiers, for things like copy-protected software) only work with parallel, but I can't reason out why.

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    I think you may be mis-remembering something, or confusing two different things (or maybe it's just me who's confused). I know you can connect a 1541 drive to a PC using a specially-wired parallel cable but it still does data transfer serially. Theoretically you could install a parallel port on the drive itself and move the special wiring inside the 1541 case but it would still be using the IEC protocol. The only reason the parallel port is used is because it allows the PC-side software to control the signals on multiple IEC pins simultaneously (i.e. clock and data pins). – Ken Gober Jun 10 '16 at 20:25
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Nibblers that are utilizing opencbm on a PC need a faster-than-stock connection to read the GCR encoding in real time. There are a couple of solutions, the most common of which requires a ZoomFloppy. To nibble from a 1541 requires a parallel port. 1571 drives are much faster due to better hardware, allowing ZoonFloppy to nibble over serial.

  • That's probably what I was thinking of. But why can't it just read GCR data and transmit it at any rate? Does it have to do with the amount of time the read head can stay at any given point? – echristopherson Jun 17 '16 at 2:48
  • There is more GCR data to encode and send over the connection than standard IEC (even with fast load code) can send. – Payton Byrd Jun 17 '16 at 6:07
  • Right, but I'm failing to understand why the drive can't get all the data at one point, send it over the wire, then move on to the next point -- at the expense of it being "real time". I assume there is either a restriction on read heads moving slowly enough to do that, or the amount of RAM in the drive is insufficient. – echristopherson Jun 17 '16 at 13:20
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    As I recall, the issue is that the GCR encoding encompasses more data than the nominal track width and is how protection using half-tracks is accomplished. ALL of the data must be cached and sent down the wire simultaneously. – Payton Byrd Jun 23 '16 at 12:37
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    @echristopherson: It may be possible to read all the data from a track using such an approach, with the caveat that stitching together all the different pieces may be non-trivial. Writing data would be another story. Any time one starts and stops writing data will generate a slightly-unpredictable "glitch" on disk. Normally, those would occur at places where they won't affect anything, but copy-protection code may write one track [likely with some glitches], then read back exactly where those glitches were, and then write information about where they were onto another track. – supercat Jun 1 '18 at 17:05
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For PC connectivity you win absolutely nothing. Various devices that allow connecting a 1541 (and others) to PC will still talk IEC to the drive, regardless whether they use USB, serial or parallel to PC.

There are two devices that are praised for their data transfer abilities - ZoomFloppy and KryoFlux - the former uses IEC as well but has optional connectors for the drive (and you could use it with a 1541), the latter talks directly to the drive.

There exists a parallel interface for 1541 that connects to CIA chip in a C64 - that allows for much faster transfer because it bypasses the IEC bus completely. But there are no devices on the market that would use such connection to a PC.

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    Just to be exact (to avoid unnecessary confusing), the C64 has two CIA (MOS 6526) chips handling parallel and complex i/o. The VIA is usually known as 6522 (as found in a 1541). – Johann Klasek Jun 16 '16 at 21:44
  • This answer is simply wrong about using the ZoomFloppy without a parallel port. – Payton Byrd Jun 17 '16 at 0:44
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A parallel cable helps to copy special formatted disks with an original drive connected to a C64 or even a PC. The problem is the limited RAM (2 KB) on a 1541 that prevents to store a full track keeping all details of the format. With a parallel cabling it's possible to transfer on the fly of a full track. An example for such a picky format is GEOS with its original disk-set. The gaps between the well-ordered sectors are carrying special information to prove the disks are genuine (sorry, I have just a german reference GEOS-Kopierschutz).

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