On Friday 14 December 2007 14:19:06 Jules Richardson wrote: I don't think it makes a difference. The higher the speed, the larger the voltage from the flux transition, but that shouldn't be a problem (within reason). The head gap and medium speed dictates the "resolution" of the system. You can think of it as being like trying to write with different sizes of pen nib. Look at reel-to-reel audio recorders - they may run at a variety of speeds, giving a tradeoff between audio quality and recording time. You can use a higher flux density with a larger head gap (and a correspondingly larger drive signal), but you need to haul the tape through faster to maintain the bandwidth. On playback, you get the problem that higher frequencies produce a higher voltage, hence the need for equalisation (not unlike the RIAA curve for magnetic record pickups). In this case you probably just want to detect the presence or absence (or possibly polarity) of a pulse. Equalisation won't be a worry. Gordon

On Friday 14 December 2007 16:58:32 Dave McGuire wrote: That's true. Maybe pull some video recorder heads? Gordon

Some of the smaller drum-based computers from the late '50's and earlu '60's (transistorized) had drums that didn't rotate nearly as fast as 6000 RPM. In High School, we had a very interesting 24-bit machine made by 3M (did you know that 3M made computers for a time?) that was a dual-CPU machine, with a communications Channel between the two processors and shared access to peripherals. Each CPU was a 24-bit, bit-serial machine, each with an 8K-word magnetic drum, which used a pair of heads per track (one read, one write). The drum rotated at 3000 RPM. The drum was quite an assembly. One of the drums failed, leaving one CPU inoperative. I carefully disassembled the drum to figure out why it quit working. As it turned out, a bearing had failed, and the drum crashed into the heads. Irrepairable mess. A lot of brownish-colored powder greeted me when I opened the housing of the drum assembly, and the drum surface was scratched with some fairly deep grooves. The crash happened during the night, and there was a protection feature on the motor that if too much current was drawn (e.g., head crash or some kind of jam), the power to the motor was removed. When I came in the next day, that CPU was "dead". Fortunately, the other CPU could run without independently without its pair, and it was still running fine. It was clear that the machining of the drum assembly was very precise. The bearings were very high-precision, pin-type sealed bearings. The drive shaft and motor connection were precisely machined and clearly dynamically balanced. There was vibration isolation between the drive motor and the drive shaft for the drum. The heads flew at a fixed (but adjustable) height from the drum...they were not floating head The heads were mounted in a staggered fashion to enable them all to fit. They were very small head assemblies. The CPU was clocked by a master timing track on the drum. There was some kind of phase-locked loop that once the drum was up to approximately the correct operational speed, the PLL would monitor the clock frequency and make small adjustments to the motor speed to keep things all in synchronization. Spin-up time was about 20 seconds if I recall correctly. During spin-up, the CPU was held in HALT state, and the write amps were force disabled. IIRC, there were 40 (octal) tracks, 00-37. Each "sector" was 24 bits of data (with some additional timing/identification data that was invisible to the user). There were also some number of what were called "blocks" that were different groups of 40 tracks, but I can't recall the number of them (maybe 8? - though through some simple math that I don't care to do at the moment, it could probably be figured out). Instruction words had a 5 bit operation code, addresses in Block/Track/Sector format for operand and next instruction location, as well as some other bits for stuff like I/O instructions and immediate operations. I recall that the timing for most instructions was such that as the current instruction was fetched, the operand should be at the current track/sector address +3, and the next instruction should be at the current track/sector address +6 for optimal programming. This would mean that just after the instruction was fetched and decoded, the operand would be under the head ready to be read/written, and by the time the operation was complete, the next instruction would be under the head ready for reading. It was an interesting machine. No index registers. Doing table accesses required loading an instruction into the accumulator, adding a constant (or a calculated variable if you wanted to try to keep things optimized by interlacing table addresses), and storing the instruction back in its place, then branching to it. It had two main registers, the accumulator, and a "B" register, which could serve as a temporary register, and was also used for I/O operations. It had a hard-logic-based loader. Putting the machine in LOAD mode, would accept octal coded address and data information from the ASR33 Teletype and write it out to the drum. It could run fast enough to load pre-preared punched tapes from the ASR33's paper tape reader. I can't remember exactly, but I'd guess that the CPU had about 50 3x5 circuit boards, each with varying numebers of transistors and diodes. It was definitely discrete DTL logic. I'd guess the total transistor count to be something in the range of 1000-1500. The circuit cards were arranged in a horseshoe fashion, surrounding the magnetic drum. The CPU itself was about 12RU, and fit in a standard 19" rack. The two CPUs were in a small rack that housed them one atop the other. Each could slide out far enough for complete access to all components. The power supplies for the CPUs were in a separate enclosure, with an individual power supply for each CPU. There was a big I/O equipment rack that was 19", and about 6 ft. tall. The machine was originally used as a real-time data acquisition system (albeit at slow acquisition rates..the machine was not very fast), and once CPU did all of the data acquisition, and the other did post-processing and report generation. The I/O rack was full of various different types of modular units including A/D converters, programmable amplifiers, timers, D/A converters, counters, and simple contact closure inputs and relay outputs, as well as the interfaces for a real-time clock, the ASR-33 TTY, and a wide-carriage IBM (pre-selectric) output typewriter for typing out reports. Sure wish I could remember the model number of the machine. As I recall, it was manufactured sometime in 1965 or so. It vanished a few years after I graduated from High School...probably scrapped, sadly. I wonder ow many computers 3M built...can't have been many, as there's no reference to computer equipment as being products of 3M in anything I can find on the web. Wonder if there are any of these machines left at all? As for a 6000 RPM drum, I'd think that you'd definitely not be successful using a pop can with magtape glued to it. The tolerances and balance just wouldn't work at this kind of speed. In order to make a real practical drum, you'd need a pretty good machine shop, and access to high quality bearings, low vibration motors, and other materials (such as the coating for the drum and vibration damping materials) as well as a means to balance rotating assemblies precisely. Rick Bensene The Old Calculator Museum http://oldcalculatormuseum.com

Pete Turnbull wrote: It would take an examination of the magnet circuit that you are creating with the head material and the magnetic media. This is not a problem that can be attacked by comparing to other applications because the materials will be different. I don't know if one can obtain material that can be attached to a drum, but it probably wont be the same consistency as that of cassette or audio tape for instance. and the aluminum drum backing will be different in nature than the tape with the felt backing that you have with those heads. Also all the drums I ever saw had a sort of flying head with a horizontal head in a hole with a small spring mechanism that would hold it in position. I don't if the head normally was out of contact with the drum while the head was stopped and was then sucked in, or if it landed and was pushed back, but that whole affair was delicate as well. John Bohner has some drums that we could photograph and use for you to get some ideas on how to do the heads. I'm not aware of any drums that ever had heads that resembled cassette recorder heads though. Jim

On Fri, 14 Dec 2007, jim s wrote: Just for reference, here's some information about the drums used in the SAGE (AN/FSQ-7) system: Length: 12.6 inches Diameter: 10.7 inches Weight: 85 lbs Speed: 2914 RPM Recording Surface: nickel-cobalt alloy Word Length: 32 bits (typically, plus various control and parity bits) The heads were fixed, on 6 pairs of mounting bars, with a critical air gap between the head the drum surface. The Q7 drums had two sets of heads to support simultaneous r/w operations between the computer and drums, and outside equipment (radar info, xtel, etc). Here's a picture from the Computer History Museum: http://sturgeon.css.psu.edu/~mloewen/Q7/CHDrum.jpg ... and a very small picture of r/w head: http://sturgeon.css.psu.edu/~mloewen/Q7/parts.gif The head is in the lower right hand corner. Specifics were taken from pp 125-131 of the "Theory of Programming" T.O.: http://www.bitsavers.org/pdf/ibm/sage/31P2-2FSQ7-112_SagePgmNov56.pdf Mike Loewen mloewen at cpumagic.scol.pa.us Old Technology http://sturgeon.css.psu.edu/~mloewen/Oldtech/

In the machines made in Gottingen the procedure was to a cigarette paper as a shim... a machine made in the GDR seems to be the kind of machine the OP was thinking of : http://rechentechnik.foerderverein-tsd.de/d4a/d4avortrag/img6.html Jos Dreesen

On Fri, 14 Dec 2007, Gary Oliver wrote: As someone who had to replace those heads (while the drum was spinning, mind you), I can tell you that's NOT the procedure. :-) You hooked a scope to the read amplifier, then SLOWLY turned the screw until you started to see a level, then adjusted it for the proper output voltage. You're correct, it was not a flying head system. Mike Loewen mloewen at cpumagic.scol.pa.us Old Technology http://sturgeon.css.psu.edu/~mloewen/Oldtech/

On Fri, 14 Dec 2007, Mike Loewen wrote: And I am someone who had to replace (better: relocate) some heads on a drum this year ;-) This was because of a "squeal" that the drum suddenly emitted. And yes, you need your good old Tek 555 to properly adjust the head. One channel to the bit clock, the second to the read amplifier, the third to the read F/F and the fourth to the word mask. Surely not. I think that the OP was thinking of a fixed-head disk which sometimes was called a "drum", too. Christian

Is the massive SAGE complex close by in Newburgh, NY gutted out? I had heard (most likely in a thread on cctalk) that someone wanted to level the complex for commercial use - a process next to impossible without the aid of several thermonuclear devices ;-) Curt Richard wrote:

In article <4762FC0F.1070803 at atarimuseum.com>, "Curt @ Atari Museum" <curt at atarimuseum.com> writes: Isn't that the one that has a gutted interior that they wanted to make into a cold war museum? -- "The Direct3D Graphics Pipeline" -- DirectX 9 draft available for download <http://www.xmission.com/~legalize/book/download/index.html> Legalize Adulthood! <http://blogs.xmission.com/legalize/>

Here is a link a friend sent to me: http://www.smecc.org/sage_a_n_fsq-7.htm Man the pictures are spectacular.... I miss the Cold War :-) Curt Richard wrote:

On Fri, 14 Dec 2007, jim s wrote: All the drums that I have seen have fixed heads. There are adjusting screws for the distance between head and surface, but the head itself is then fixed with another screw after adjusting (we had to do this on the drum of our second LGP-30). Christian

I wonder if you could use those glass delay lines that were used in PAL colour TV receiver until fairly recently (and there must be plenty still around)? Alas they have a somewhat odd delay time (a little less than 64us -- one line period of the European TV signal), but I am sure you could fiddle the master clock to compensate. There were some similar dalay lines of exaclty 64us delay, used to store a picture line in the drop-out compensator circuit of early VCRs, but those are going to be a lot harder to find. -tony

On Fri, 14 Dec 2007, Jochen Kunz wrote: The Z22 is a bit 'bigger' than the LGP-30, although it's bit-serial, too. There's the Z22 in Karlsruhe which apparently runs (idle) several days a week. I have seen this machine when it still was at the FH some years ago. Yes, it should be easier although it is very sensible to temperature changes. Another, "more classic" memory devices would be Williams tubes. The big advantage of the drum is that it doesn't lose its contents when switched off. I think most drum based computers (at least the LGP-30) have permanently recorded timing and clock tracks (which can't be recreated in the field). The LGP-30, for example, derives its master bit clock from its drum because it was impossible to synchronize the drum to the computer. It's also a much simpler approach. Christian