DECtape begins to become unreliable. The operating system is usually programmed to keep retrying a failed read operation, which often succeeds after multiple attempts. Experienced DECtape users learned to notice the characteristic "shoe-shining" motion of a failing DECtape as it is passed repeatedly back and forth over the tape heads, and would retire the tape from further use. On non-DEC computers Computer Operations Inc (COI) of Beltsville, Maryland offered a DECtape clone in the 1970s. Initially, COI offered LINC-tape drives for computers made by Data General, Hewlett-Packard and Varian, with only passing reference to its similarity to DECtape. While DECtape and LINC tape are physically interchangeable, the data format COI initially used for 16-bit minicomputers was distinct from both the format used by the LINC and the format used on DECtape. When COI offered the LINC Tape II with support for the DEC PDP-8, PDP-11, Data General Nova, Interdata 7/32, HP 2100, Honeywell 316 and several other computers in 1974, the drive was priced at $1995 and was explicitly advertised as being DECtape compatible. In 1974, DEC charged COI with patent infringement. COI, in turn, filed a suit claiming that DEC's patent was invalid on several grounds, including the assertions that DEC had marketed DECtape-based equipment for over a year before filing for the patent, that they had failed to properly disclose the prior art, and that the key claims in the DEC patent were in the public domain. The US Patent and Trademark Office ruled DEC's patent invalid in 1978. The court case continued into the 1980s. DECtape II DECtape II was introduced around 1978 and has a similar block structure, but uses a much smaller tape (the same width as an audio compact cassette). The tape is packaged in a special, pre-formatted DC150 miniature cartridge consisting of a clear plastic cover mounted on a textured aluminum plate. Cartridge dimensions are . The TU58 DECtape II drive has an RS-232 serial interface, allowing it to be used with the ordinary serial ports that are very common on Digital's contemporary processors. Because of its low cost, the TU58 was fitted to several different systems (including the VT103, PDP-11/24 and /44 and the VAX-11/730 and /750) as a DEC-standard device for software product distribution, and for loading diagnostic programs and microcode. The first version of the TU58 imposed very severe timing constraints on the unbuffered UARTs then being used by Digital,