Taking on a refrigerator-sized minicomputer is not for the faint-hearted, but [Usagi Electric] has done it with a DEC PDP-11/44. He’s not doing it in half measures either, for his machine is tricked out with an impressive array of upgrades. Among them however is no storage, and with two co-processors there’s a meager 3U of rack space left. The plan is to fit a period 8″ hard drive in the space alongside a TU50 tape dive, and it’s this final component that’s the subject of his latest video.
DEC never did anything by halves, and a DECTape II cartridge is more than a simple container for tape reels. Instead it has a capstan of its own that engages with one in the drive, and an internal drive belt that moves the reels. All the rubber parts in both tapes and drive are thoroughly perished, and it’s impressive that he manages to find inexpensive modern polymer alternatives. The original drive is probably intended for a VAX system, thus it has the interesting feature of a second drive mechanism out of sight to hold a tape containing microcode.
Having reconditioned the drive, it goes in behind a custom front panel, and though there’s no useful data to test it with on the tapes he has, it appears all working. You can see it all in the video below the break, and if you’re interested further we’ve covered this machine in the past.
The PDP-11/44 I programmed in the early 80s – part time job while in undergrad – had a TS-11, better known as the Tape Stretch 11. It was a traditional fridge-sized reel to reel on which we did backups.
The setup we had was the TS-11 on the right, then the CPU and three removable pack hard drives, each with 26kwords (52kb) (maybe 52mb but I don’t think so). The OS sat on one drive, source data for one client on a second, results on a third. Running monthly reports involved multiple platter switching,
Fantastic video… always so engaging
cheers!
Andy
In case anyone else is curious like I was – 1978 TU58 DECtape II:
2 tracks.
Capacity 256 KB per tape, 128KB per track. 2 x 1024 records x 128bytes per packet.
PWM modulation. 0 is 1/3 duty cycle, 1 is 2/3.
800 bits per inch, 2400 flux reversals per inch, tape speed 30 ips, 41.7 us bit duration. 24Kbits = 3KB/s raw data speed.
After formatting and packetizing DEC promised real data transfer speed of 1280 Bytes/second.
~10 second latency, thats how long it takes for tape loop to repeat at 60 ips fast forward scanning speed.
Suffered reliability problems when first introduced.
$562 in 1979, thats surprisingly cheap for a 512KB (its a dual tape box) of ~random access storage in seventies.
Users: We want a floppy drive.
Sir Clive Sinclair: We have floppy drive at home.
Floppy drive at home: ZX Microdrive. ~$120 in 1985 delivering 85KB of storage at impressive 15KB/sec, but abuses tape stretching it over time until you lose all the data :-)
Another try: Rotronics Wafadrive (for Spectrum/Timex) and its offshoots like C64 Quick Data Drive Model 8500 and TRS-80 Model 100 (and clones) A&J MicroDrive System-100. $250 in 1985. 10 ips so tapes are not abused, <2KB/s and latency around 1 minute with 128KB tape. Apparently still unreliable to the point TI dropped the idea of approving it for their Compact Computer 40.
Only, this is not a loop of tape. It is reel-to-reel. See https://en.wikipedia.org/wiki/Quarter-inch_cartridge. The only loop is an elastic polymer band that wraps around both reels, and through the magic of stretching that band more on the side that’s pulling and less on the other side, it manages to tension the tape as it winds.
Yes, I edited that loop bit in when excitedly adding Microdrive/Wafadrive info :(
So DECtape II is a reused DC-100 from early seventies. Its fascinating the lengths those corporations went to just to save few bucks on mechanism cost versus ordinary Audio cassette transport. DEC even did standard (almost, no capstan/pinch rollers) Philips transport TU60 DECasette in 1972, but dropped it for this. As if saving $50 on a drive you are selling for $500-2000 would change anything.
That cartridge pictured is the 3M QIC (Quarter Inch Cartridge) (https://en.wikipedia.org/wiki/Quarter-inch_cartridge) which was available in many sizes. It was introduced in 1972, and its magic was that the drive could be made very cheaply because there was a wheel that protruded from the front of the cartridge that engaged with a rubberized direct drive motor, and this roller pulled an elastic polymer band that wrapped around both spools of tape, which both moved the tape in either direction and managed to produce some tension in the process. Which was also its greatest weakness since the more friction there was in the system overall, the higher the tape tension would get. I used DC300XL cartridges on a Tektronix 4052 computer in the 1980s when it was popular for minicomputers, and around the turn of the century an even smaller size became popular for a short time for backing up PC hard drives. These were a slooow way of backing up a hard drive, and if you ever needed to restore a drive, there was a good chance that an hour into the restore, it would come across an uncorrectable error, and you had to start over. And if THAT didn’t work, that was the end of the line. So you had to always have at least three tapes, which you cycled through so that if the latest backup failed, you had at least two more tries, although these would have older data. Still better than nothing, but then the price of hard drives got low enough that it made more sense to back up to other hard drives. Sad to say, we don’t really have a good option these days, aside from copying onto newer and newer hard drives. And when they stop making hard drives, who knows what we’ll do – SSDs may one day get good enough for archival storage, but the current focus is on more data into smaller chips, without much concern for longevity.
When I saw him put that green plastic band in there, I knew that he is heading for some heartbreak. That band needs to have quite a bit of tension in it to work properly, and his bands have some actual slack. So his real problem is not that these are VAX tapes, but that even once he figures out how to copy from the hard drive to the TU58, it’s still not going to work well. The good news though, is that these tapes generally don’t need to be formatted – when you write to them, they erase just ahead of the write head, so it’s like using a cassette tape.