Bit of DEC Trivia

On 2/24/11 2:58 AM, Philipp Hachtmann wrote: It shouldn't matter over the range that the G888A can decode it. DECtape uses Manchester encoding, so it is self-clocking. Head skew relative to the timing tracks becomes more of an issue, but even that was mitigated by putting redundant tracks on different parts of the tape and or-ing the heads together. The trick with holding your thumb on the reel became necessary when DEC started shipping less tape on the reel than certain PDP-8 operating systems expected. G888's are odd beasts. They expect the head amplifier to oscillate when the tape isn't moving. That is needed for the up to speed circuit to work in the TC-11 (and probably in the TC-08) A while back, I added some background information on the Wikipedia entry for DECtape, going back to the Raytheon tape drives used on Whirlwind. The direct ancestor to LINCtape was the TX-2 high speed tape system designed by Tom Stockebrand. DEC patented the encoding used on DECtape, so the earlier LINC Tape was copied by a couple of other companies, in particular Computer Operations, Inc. in Beltsville, MD and sold interfaces for DEC, HP, and DG. Sept, 69 Datamation: minicomputer tape deck The C0600 LINC Tape System, a minitape system for minicomputers, was named in deference to the MIT computer project called LINC which first defined a need for such a peripheral. The C0600 consists of two tape drives, each with 150 feet of ~4 inch tape, which hooks up with a mini-cpu. Each reel of tape is capable of storing over 100,000 16-bit words in blocks of 256. The data transfer rate for the device is roughly 4.2KC, and the time required for a pass of a full tape is listed as 27 sec. The system approximates a random access storage device. It is not used as a standard tape deck, but more as a disc. The 256-word blocks are given permanent, pre-recorded numbers. The numbers are used to retrieve the 256-word data elements as though they were recorded on addressable disc tracks. Each block is given a checksum, and tape searching may be done moving the tape forward or backward. Two models are available, an "A" model which uses an I/O channel of a Varian 620/i and costs $9,950, and a "B" model which uses 620/i direct memory access, transfers through a double buffer on a cycle stealing interrupt basis, and costs $10,500. Software issued includes utilities, a loader, assembler, FORTRAN with math and I/O routines, and bootstrap. Although the Varian-compatible version is ready, models for the Honeywell 516 and 316 and the General Automation SPC 12 are expected. COMPUTER OPERATONS, INC., Silver Spring, Md. The photo in the announcement looks identical to the drives on the LINC There is a later single transport version which is shorter and the tape path is "S" shaped.