The First Magnetic Random-Access Memory with Interchangeable Media

By George Comstock and Len Shustek, March 2015

John T. Potter, who founded Potter Instrument Company in 1942, was a prolific inventor with over 80 US patents. On September 1, 1948 he applied for a patent on a "Three-Dimensional Selector and Memory Device", and it was issued as patent number 2,620,389 on December 2, 1952.

The invention was for a digital storage device that used steel wires strung on a set of two-dimensional frames that were packed together in rows. The machine would access one of the frames, then a particular wire in the frame, and then the data recorded somewhere on the wire. It was three-dimensional memory.

In 1955 Potter negotiated a contract with the of the Univac military computer division of Sperry Rand to develop and build three of the three-dimensional memories. The Project Engineer was George Comstock, who began work on January 2, 1956.

Just sixteen months later, in April 1957, the three working completed units were delivered to Univac in St. Paul, Minnesota. But there were several significant differences from the invention as disclosed in the patent:

1. Instead of wires, the storage medium was strips of the ½" mylar-backed magnetic tape that was commonly being used in reel-to-reel computer tape drives.

2. Instead of using rotating shafts and electromagnets, the three-dimensional positioning was done using hydraulic cylinders with binary-coded displacements. This was a variation of a positioning system using binary-coded rotating rings that Potter had filed a patent on in 1951.

3. The storage bin containing all the tapes could be taken out of the unit and replaced with another, making the total amount of storage unlimited. This interchangeable media provided true removable data storage. None of the other random-access magnetic storage devices of the time, like IBM's RAMAC disk drive, had removable media.

The detailed design

The 20" strips of tape were arranged in rectangular frames that each held 10 strips, with 10 blocks of data recorded on each. The frames were arranged 5 wide and 50 deep in the storage bin. There were five read/write heads mounted on a bar, separated by the width of the frame. (The stringing of the tape frames was done by contracted labor that including mentally retarded individuals.)

Accessing a particular block of data required three linear motions:

moving the read/write head bar forward or back to one of the 50 positions for the correct frame

moving the read/write head bar left or right to one of the 10 positions for the correct tape in a frame

pulling the frame up to one of the 10 positions for the correct data block on the tape

The linear motion in each of the three directions was accomplished by a set of hydraulic cylinders in series, each driven to one end or the other, with articulation lengths that were powers of two. By using binary coding, any required extension position could be reached. For the two 1-of-10 movements it took 4 cylinders in series (1-2-4-8 lengths), and for the 1-of-50 movement to choose the frame it took 6 cylinders (1-2-4-8-16-32 lengths)

After positioning in all three dimensions, each of the five heads was positioned over a data block that could be read by "stroking" the head along the tape for a distance of about 3/4". Each block represented the data from one punched card: a header, 80 7-bit characters, and a single parity character to check the data.

Accompanying this document are two versions of a 4-minute video showing the Potter tape storage device in action. The first is a digitized version of the original Kodachrome film shot by George Comstock. The second is an edited version that includes explanatory subtitles.

Design parameters

characters per block 80

characters per inch 128 block length, with header and parity 0.75 inch

interblock gap 0.75 inch

blocks per tape strip 10 recorded length of tape strip 15 inches

total length of tape strip 20 inches tapes per rectangular frame 10

frames per bin 250 (5 wide x 50 deep)

total storage, in characters 250 x 10 x 10 x 80 = 2,000,000

Speed of operation

pneumatic/hydraulic selection time 1.5 seconds

read/write head activation time 3 msec read/write head stroking speed 60 inches/second

read/write time for one data block 25 msec maximum data block selection time 3 seconds

minimum data block selection time 28 msec

Conclusion

Although there is no data about reliability and error rates in the field, the three units delivered to Univac worked.

Because the design was mechanically complex, it did not become a standard Potter product, or the model for future removable random-access data storage. But it pointed the way for later removable disk drives, the first of which was IBM's 1311 in 1962.

Potter nonetheless earned patent royalties from NCR for their CRAM (Card Random Access Memory) storage system, and from RCA for their RACE (Random Access Card Equipment) storage system used with Bendix G-20 computers, both of which used similar mechanical configurations for magnetic storage of punched card images.

Early Potter patents on mass storage memory

Number Filed Issued Title Notes

2,620,389 Sept. 1, 1948

Dec. 2, 1952

"Three-Dimensional Selector and Memory Device"

Frames of steel wires in rectangular array

2,674,728 April 26, 1949

April 6, 1954

"Three-Dimensional Memory Device"

Frames of steel wires in rectangular array; patent

continuation-in-part

2,650,830 May 19, 1949

Sept 1, 1953

"Electronic Memory Device" Frames of steel wires in cylindrical array

2,862,389 Nov. 24, 1951

Dec. 2, 1958

"Access Mechanism" Binary-coded movement articulator using rotating rings

2,620,380

5 ' 6 lwo dlmonslom. E•ch ol U'leoe 110\cnold• In ona producing mn.anoUcally recorded lnfornuttlon, tho ot a group ·or ten cepresontabte by n. digit from comb1natlon of, & areat pturaUty ot substantlo.Uy o to O eo tho.t rorty solenoids ropffilentablc by a equn.l le-oath stratgbt magnetto recording .paths, tour digit number from 0000 to 9990 pormll.• nny moan,; for supp0r1Jng gJ'OUP• of said P<>ths In nnt ono Cit 10.000 wll'es grouped on 100 trnmes andlOO o eheet.-Uko arrays, means ror supp0rt.lna' O.· pJu-wtres ton frl!no. mllty of snld arrays In J)&mllel sandwtch fonn•·

1110 selector shown in ptg, a has ten aciectDr ~on, mea.n.s tor reproducing reoordlna:& carried by keya n. SS, 19. 60, 63, 6•. 61, 6°6, G1 nnd 61 carrying sa.ld P<>tha lnoludlna a maenctlc reproducer, desfanauon" 1, 2, 3, 4, 5, fl, -r. 8, .9 a.nd O. E1;1ch mea.ft3for seieouna: a prodotcrmlncd o.rrny. mean.s seieetor koy closes n ctrcult to lte tour Col'rospond .. to for •lecUng a pre(letermlned path Ln said ae .. tng aolenolds. \VbJcb one ot the tour aolenold6 lected 11.rmy and m(!a.ns lor rolo.tJ.veJy moving ts actuat..cd t.s controUed by n seotor swlt.oh 98. l51lld reproducer and aald selected path a loncr " Th.lJ .sector .&wito.b Is sdvru1ccd rt qunrt-ttr turn by stmight Une to reproduce the rocordlng canied rntchct motor 01- 98-99 eAch tune a. selector by no.Id .selected path. key ts pr~sod actJvattng tho next _group or sole- ts 2. rn a coded pulse ln!ormaLton atortnr. select,. rtotd.s. 1nJ and reproduc.lng system, means tor 111upportJng

1'raclng A typictil clroutt, &Mumo kC)• tll (nwn~ IJO.ld !mmea In parallel ao.ndwich tormatlon, tho ber 1> ts prea&ed closing contacts a:•-&5. 'l'bla combtna.tton of, a plunilltyot recLoniwlGor trame.a. COD1J>lotts n otrouJt over wJrGi t9 lo one 8h:lo of all steel wlro ca.rryl~ nu'l.g-netJcoJJy reoorded. coded the number I 001euolds, 10, 12, lt and 80 corre- eo pUl'!cintormatfonmountect oncachotsB.td!rt..rnc1.1 apondtn.ir to the number 1 ,.:olonu1da tl, 21. 31 etc. to Iotm tt. t1:reat. plumUty of parnUel wlre ftLra.nds of Flp . 1 ond 5. 'rho return clrcutt-a or the8e ln nnt shcet-lllcc l l'Oups, and tuca.ns to1· rep1·oduc-oote:nolds 1 t, 13', 15 o.nd ti f go to t.bo sector swJtcJ1 Ing said Jnfot1nntlon lncludlnlt n. mQ-8Dt!tlt l'e· .. at 81, •• . 91 t\nd 98 re.:ivectJ.vely. Sector 91 Is of producer, menus tor eeJoctlng a. p1·cdctemnin~ couauctlng mnterial and ctoees theoJrouJt betwaen ets one or s:a.td Ira.moo, means fur s6lectlng f\ prcdc .. point.a 9Z and 98 thereby conrpletlni: the oircuJt o! Larmtnod strnnd on said selected fro.mo 11.nd mffns aoJenold. 10 through rAtohet c:o0U 99, lcod 100, ener· for movlng &llld reproducer and sa1d predeter .. gl$lng batlAley 102. o.illl lead 1oi to oontnct n . mined strand relatively nlonf " otrnlght lino to Menco. when key 63 f8 prll83ed, solenoid to pulla reproduce recording cnrrled by aald selectoCt t>Rth. ln and tho ratcnet motor Qdvanae:> ihe con· 30 a. In "' nyetem ror &tor1n¥, ftclo~llnli •nd ro-duc.tng sector 9t to the nexc'<:tuilrter po..qtton in 11 producln ir maanetlcally recorded coded tntorma. .. counter-clockwllJe dJreoMon. If koy Ga hnd ,been tion, the corublnatlon or at 1ciast. scvcrn.l thousand pressed inBtead of koy e,3, 1Jolcnold 83 In the eec- ractlllnotu· mngnatlc record trAoks, menna ror oud •1·oup would hnve been o~tuo.ted. WWle only aupportlng Sl'Jd tro.okn In c:.oplO.Wll' 1·olatlon.shlp tho eight tcl&Ya correspandJna to the fl.rat two keys 3$ U> form a pluro.Uty or &beets o! tracks, moon& tor nro shown tt wUl be undor1tood t.ha.t thlrty .. two supporting a.C le:.urt. twenty.five ot said aheot.i: moro solenoid.$ nre reaul.red ln the syJtcm. The In closely 'spnoed nrrny to aub.at.n.ntlll.Uy occupy n way ln which the solttnOld.'J o_poru.t;e eelccttnn ptna s:lmpJe aeomotrlcal thrc~ dlmen&ional t ll'f\Ce, nn thtOUlh wlro te, ll, 18 eto. ls shown tn Fig. B. ntcctromngnctic reproducer, moons ror relatively

ln the second M!Ct.41' aw'ltch tho scoond rela)' ln 40 movlng an.v rrtvan tro.ck nnd said reproducer to ea.oh EU'O\lP ls e.ctuated upon prtt.!ISln" n elector oooperatJveJy p0$1UOu s~ld t.ro.ck and &aid re-k.et and 50 on untn the tour .&electing rein.ya tor producer, &nd means t or further re l.atlv,ly mo\1ng determ.Jnlng tho sclectlon of & po.rUoular wire said track and tufd ~producer a.long ft. strnlgbt have opci·n~d. lino to rep!'oduoe the i·eeordJng c::arrled by anld

Fig. 9 shows • reprwintnl.lon of t)<J>lcal lnlor- .a In.de. mo.Won recor-ded on n shor t !.!eeUon of one ot the 4. In a. &)•stem tor atortug, selecting o.nd re .. memory wt.re.a. Numbers from O to 9 ond all let.-- produelnr moa:nettoo.Uy recorded coded tncorma.· ters of tho alphabet ma.y be represented by a Uon. t.he oombl_natton ot. a ereo.t plurality or nwnber from 1 to 36. 1"he representntlon showa ree·tWnear mRgnctto record tracks. moans !or fltst n ncraUvc prtrntng putae A. e. sroup of throe no aupp0rtJn« r:11td tr11ok8 Jn 11 ttahtly packed thl'C!O poslUve pul.ses B, tour po.tltttve putsee C :;paced dlmenslonu.l af'l'&Y, a mo.anoUc reproducer !or re .. lrom B and an endinJ.f pul5e D. Thl.I 1·eco1·d rev· produclnR intormntlon recorded uPon said tra.cke. rtisont-s the number 34 whlol\ may be coded a.g the. m eana !or rcJ&tlvcly moving 8ald reproducer and 24.th letter In the alphabet x . Ono wJre may a prattetcrm.lned tmck along o .Prcdctarmlned co.rry SO to 100 01· more groups of pu.Lses ooded lla po.t.h, means tor relatively moving "8Jd reproduee.r to do.•tgnato 50 to 100 num.bera and/or Jette1·a n.nd i>aJd predetorn:Uned I.rack aton&' a 3econd pre--spelling out certain desired ln.Corm•tlon. dot.ermined path lntersectltllf the lll'&t •aid patb

Ftg, 10 i hows tn bloek dtaga·am ft complete cl.cc.. at right a.na:tca. and mcana tor retatl\•CHY movinJ tronic:: t-hree. dtmcwilonat memory Fi)'1Jtiem. Sit· aold reproducer oud &nid. predetermined. track teotor I OS selects the wlra c11u·Tyin1 tho lntormo.· fl() along" third prcd.otcrmJued path interscctlng the tlon ro<tUlred. SolcctA:lr 10& Is connootod to tho oald. tiecond path at right angles Bnd contlnutna t-breo dtmezalona.1 tnemory untt 101 by su1tablo Jn a 1Jtraltht line along the las~ said path to olrcult.s tot. The publ08 h'om the &elected \Vlra rep1'0duce recorded lnrormaUon carried by &aid In tho memory u_nlt m~Y ~ utlllzed Jn an.v auJ.t.. p1·et1etcrmtncd trnok. able mam1er as by feedlng an ol~trontc decade 65 5. In a. system tor storblft, aelecting and re-counter 110 over &lgnaJ lend 108 and 109. S ult· 111'0ductng mognetlc•llY recorded cOde<l lntorma· M>l e decodlD..e" or othel' ulJlt:zatlon means 11 2 la tlon, the comblnatton ot, a aroat pturo.ttty o:f coupled to counter f IO over clrctllt& t It . reotUinear mngnetlc rocord trncks. meons tor

Wll1lo only a alngle embod,me-nb or tho in·esent aupJJorttns sa1d tracks 1n • t111htty packed three Invention hes boen sbown D..lld d~rlbed. nu~ny iO dhn"nston"t a rr&y, a rnagnc!tle reproducer for i·e· moc.t1ncatlons ·"'•tu be appttrect to U\0-'e skiUed in producing tntormat.lon reco·rdod uuon said t-rncks, tho nrt wtthJn the aplrlt. and $COPC of the invcn- means for rotatl\•cly mov-lng 1Aid rol)l'(ld.ucer and. tlon WJ set torth in tho o.ppeoded elntms. a predetermined trllcJt ulong a p1•ctle&crm1ncd

Wbn.t 28 claln1od Is: path, means to1· relattvoty moving sa.lcl repro-1. In a ant.am tor atorJna. selecttna and re- 'la ducer and said predetermined trn.cJr olons a. sec-