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UNIVAC I Mercury Delay Line Memory

Manufacturer UNIVAC
Identification,ID -
Date of first manufacture-
Number produced -
Estimated price or cost-
location in museum -
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Univac I mercury delay line memory

Link to UNIVAC I delay line memory - by Ron Mak



For Use With Univac I Central Computer

Remington Rand Univac

NEW YORK 10, N. Y.


For Univac I Central Computer Group

Physical Description

physical overview of Univac 1 note mercury memory tanks INSIDE the computer. The UNIVAC 1 computer was a little room.


1-76. The principal internal storage in the Univac I system is the 1000-word acoustic delay-line memory, consisting of 100 10-word mercury registers. Twelve additional 10-word registers function as intermediate storage for input and output; six more are spares. With modified circuitry, seven more channels control the temperature of seven mercury tanks, and one more channel is used for the 10-word Y- register.

1-77. The total of 126 mercury channels is contained in the seven mercury tanks mounted on the backs of sections MT, MV, MX, NT, NV, NX, and GV. Each tank is divided into 18 channels.

1-78. Physically, each of the 10-word register circuits is made up of three sections:

  1. The acoustic delay, consisting of a channel in a column of mercury, with receiving and transmitting crystals mounted at opposite ends.

  2. An intermediate-frequency (i-f) chassis, electrically connected to the receiving crystal, and containing amplifiers, a detector, and a compensating delay. The i-f chasses are mounted on the shell of the mercury tank which they serve.

  3. A recirculation chassis, containing a cathode follower, a pulse former and retimer, a modulator, which drives the transmitting crystal, and input, clear, and memory-switch gates. These chasses are mounted in the sections adjacent to the mercury tanks.

1-79. All mercury channels except the 10-word Y-register channel are identical, as are the recirculation amplifiers and recirculation chasses of all 10-word memory registers and the six spares. The recirculation chasses of the input register and output register are slightly modified to enable use of control signals different from those used in the main memory. The 10word Y-register mercury channel is shorter than the others, and the recirculation chassis is different, since this register is completely independent of the main memory controls. The temperature-control chasses have the following modifications:

  1. In the amplifier (i-f) chassis the compensating delay is removed (from V7) , and a dummy plug with dummy connections is substituted.

  2. The bay-mounted chassis (chassis 2 in each memory section) is not a recirculation chassis. The temperature- controlling signal enters the mercury column from the cycling unit each word time. At the bay-mounted chassis, this signal is used to adjust the current through the heating coil to maintain constant temperature in the tank. Each temperature- control channel uses an entire bay-mounted chassis.

1-80. The interconnections among the three groups of circuitry in a standard channel are shown in figure 1-18.

1-81. MEMORY TANKS. A memory tank consists of two concentric cylinders. The inner tank is made of stainless steel and contains the column of mercury that is used in common by all the channels in the tank. The inner tank is 223/4 inches long and 33/4 inches in diameter (figure 1-19).

1-82. Crystal-mounting plates are placed on the ends of the inner tank. Eighteen transducing crystals are mounted in each plate. One face of each crystal is in contact with the mercury. The crystals are aligned so that each receiving crystal receives acoustic waves from the corresponding, transmitting crystal at the other end of the tank. To minimize crosstalk between channels in the common mercury column, chrome steel tubes are mounted between corresponding transmitting and receiving crystals. These tubes act as waveguides.

1-83. Heating coils are wound around the outside of the inner tank. The spare between the inner and outer tanks is filled with insulating material.

1-84. The outer cylinder of the mercury tank is approximately 35 inches long and 81/2 inches in diameter. On this shell (figure 1-20) are placed mounting brackets for the i-f chasses, contact boards for the i-f chasses, input and output terminals, and r-f filters for the heater circuits. Electrical connection to the mercury tank is made by two cables, which terminate in 21- pin male connectors. As seen from inside the computer, the connector on the right (JP-2) carries automatic gain control (AGC) monitor lines, and the one on the left (JP-1) carries the power leads. On the opposite end of the tank from the contact board is an overheat neon which lights if do is cut off because of overheating of the tank on which it is mounted. On a removable end-plate at the same end is a four-terminal barrier strip for the overheat and standby power lines. Under the end- plate are two adjusting screws for the microswitch stops and the overheat neon.

1-85. Each long mercury tank has two heating systems, each of which uses coils wrapped around the inner tank:

(1) Standby a-c heat; high power used to bring tank to approximate operating temperature, coarsely controlled by the contraction and expansion of the bellows which opens and closes the standby microswitch.

(2) The d-c heat; low power used to maintain operating temperature, accurately controlled by an electronic system.

1-86. The a-c standby heating system makes use of a 230-ohm coil powered with 230 volts from phase 1, lines 8 and 9.

1-87. Current through the ac standby heating coil is controlled mechanically by the expansion and contrac. tion of the mercury in the tank. A port through the front crystal- mounting plate allows the mercury to flow into an expansion chamber. This chamber senses volumetric changes in the mercury as the temperature varies. As the mercury expands, it works a bellows which moves two microswitches against set- screw stops. One microswitch controls the a-c standby heating power, and the other is an emergency overheat cutoff. When the expansion of the mercury indicates approximate operating temperature, the microswitch contacts open and cut off the a-c heat. Should the tank cool and the mercury contract sufficiently, the contacts close and apply power to the coil again. If the standby microswitch fails to shut off ac, the tank continues to heat and the mercury continues to expand. The bellows then operates the overheat switch. The overheat switch cuts off a-c heat to all tanks, cuts off d-c power to the computer, and lights the indicator neon on the overheated tank. The tanks should be inspected immediately, because after the tanks have cooled the overheat switch closes again and the neon goes out.

1-88. A 3500-ohm coil provides d-c heat. This is the fine temperature-control coil. The current through the coil is adjusted by the temperature-control channel, which measures the transit time of a pulse through the mercury.

1-89. The pulse is sent through the delay and then matched against the sloping wavefront of a standard timing pulse. The position of the delayed pulse on the standard pulse determines whether the heat should be on or off. Just enough power is supplied to the heating coil to balance the heat dissipated from the tank.

1-90. MEMORY RECIRCULATION (I-F) AMPLIFIERS. The i-f amplifiers are mounted directly on the mercury tanks (figure 1-20). There are 18 chasses mounted radially around each tank. They are numbered counterclockwise from "three o'clock" as seen from inside the computer. Chassis 14 of each tank except tank GV is a spare and is on the bottom. (Tank GV has no spare chassis.) With the exception of chassis 18, the others are used as amplifiers in the recirculation path of one of the information channels. Channel 18 is the temperature- control channel.

1-91. The i-f chassis is built on the standard channel and has standard mounts. One half of the chassis contains the amplifiers; under this half of the chassis is a shield. The other end of the chassis contains external component boards, the compensating-delay stick and a 14-contact chassis terminal- board. Tube positions are numbered Vl through V7 from the amplifier end of the chassis.

1-92. The input point of the chassis is a spring-leaf contact inside the shielded section just ahead of the Vl position. The shielded section rests directly on the shell of the mercury tank, and the spring-leaf contacts a special coaxial output stub from the mercury channel. The signal goes through three stages of amplifications in Vl, V2, and V3, which are controlled by V6, the AGC tube. Tube V4 is also an amplifier. The tuning slugs associated with Vl, V2, V3, and V4 are factory-adjusted and require special equipment for setting. Tube V5 is a broad- band video amplifier.

1-93. In the V7 position of the i-f amplifier is a plug-in compensating delay unit. Because of uneven heat distribution through the mercury, various channels have different delay characteristics. Compensating delays equalize this difference. The delay units are color-coded with a dot on the top of the delay stick. Usually, these sticks are placed in the chassis as shown in figure 1-21. Regardless of this layout, however, whenever a chassis is replaced, the delay stick from the old chassis or one of the same color should be used.

1-94. The output of the i-f amplifier chassis is taken from the 14-contact terminal board, which makes direct contact with a female-contact terminal strip on the shell, from which the signal lines run to the bay end of the tank. Terminal 7 on the board is the memory-output terminal; terminal 11 is the AGC-monitor output.

Figure 1-22. Recirculation Chassis

1-95. The line from pin 7 of the terminal board on the shell of the tank carries the memory output as far as a standoff post on the bay end of the tank. On the top of this post is a pin. A jumper wire from the bay fits over the pin. The jumper wire is soldered at the other end to a terminal on the backboard of the bay and connects the output line to the bay-mounted recirculation chassis.

1-96. All of the AGC lines from the contact boards on each tank are bound into a cable and connected to the AGC- monitor system by way of connector JP-2.

1-97. RECIRCULATION CHASSES. The recirculation chasses of the memory are standard Univac I system chassis (figure 1-22). They are located in chassis positions 3 to 10 of sections GV, NT, NV, NX, MT, MV, and MX. Each chassis contains two identical circuits, and serves two memory locations. Each halfchassis has an address number, differing by 100 from the other half. The only exceptions are locations M3X to M8X and N5V to N8V. In both cases, the input and output registers share recirculation chasses. For example, channel 1 of r0 and channel 1 of rI share chassis M8X. In all memory sections, chasses 1, 11, and 12 contain miscellaneous circuitry, such as output whiffletrees, continuous wave buffers, and local drivers. In all sections, chassis 2 serves memory channel 18, the temperature-control channel.

1-98. On a recirculation chassis, tubes V 1 and V14 are identical cathode followers; tubes V13, V12, and V11 make up one pulse former and retimer; tubes V2, V3, and V4 make up the identical circuit. Tubes V5 and V10 are the output modulator tubes. Coaxial cables from terminals T63 and T79 respectively supply the continuous-wave signal from the cw buffer-drivers to these two modulator tubes. Tubes V6 and V9 are normally conducting amplifiers; tubes V7 and V8 are input- output control amplifiers.

1-99. Several components in the modulator stage are mounted in a nonstandard manner. These parts are capacitors that form an r-f-bypass network for the modulator. At operating frequencies like 11.25 megacycles, it is advisable to keep leads as short as possible. Consequently, the components are mounted between connecting points on the base of the tube instead of being put on the mounting boards. The parts so mounted are identified by the initial letter, the tube number, and one of the pins to which they are connected. Thus R10-6 is a resistor connected to pin 6 of V10.

1-100. Information from the i-f amplifier enters the chassis on backboard terminals T26 and T53. Terminals T26 and T53 are connected directly to cathode followers V 1 and V 14. A jumper wire with pinconnector is also connected to terminals T53 and T26. These jumpers are video monitor lines. They plug into pin jacks on the video monitor relay boxes mounted on the framework next to the backboard.

1-101. Connection from the modulators to the memory tank is made by means of short lengths of flexible coaxial cable connected to backboard terminals T5 and T21. This cable is terminated in a phono-pin connector, which mates with the coaxial stub on the memory tank.

1-102. Timing pulses for the pulse formers and retimers, and the continuous-wave signal for the modulator, are supplied by local driver's and cw bufferdrivers located in the memory sections. These signals are distributed to the chassis backboards over a rigid coaxial line. The inner conductor of this line is the sheathed inner conductor from a standard coaxial cable. A piece of aluminum pipe mounted on standoff posts, with a hole cut in its side over every chassis location, takes the place of the outer conductor. The inner conductor passes in and out of the pipe through the holes. Memory- section backboard-layout drawings and distribution charts supplied in the drawing file give details concerning the connections for these signals.

Notes concerning the above manual:

  1. There were seven (7) mercury memories, not six as shown in diagram "physical overview of Univac 1"
    "1-77. The total of 126 mercury channels is contained in the seven mercury tanks mounted on the backs of sections MT, MV, MX, NT, NV, NX, and GV. Each tank is divided into 18 channels.
  2. On figure 1-18, there is an input "CW" to the modulator.
    • This was 11.25 Megahertz as per figure 4.2 .
    • The clockpulse was 2.25 Megahertz as per figure 4.1 .
    • So - a data bit time was 5 cycles of the 11.25 Megahertz carrier
    • Each of the 12 decimal character per word had odd parity.
    • This would allow a peak detecting AGC (Automatic Gain Control) of each channel to work properly, and a "1" bit could have a known level different from a "0" bit.
  3. 100 channels of the 126 total channels were used for data.
    As per 1-90 above:
    • "Channel 18 [of each tank] is the temperature- control channel. "
    • "Chassis 14 of each tank except tank GV is a spare and is on the bottom. (Tank GV has no spare chassis.) "
    As per 1-76 above
    • "Twelve additional 10-word registers function as intermediate storage for input and output; "
    • "one more channel is used for the 10-word Y- register. "
    That seems to be
    126(total 10 word channels)
    - 7(temperature control)
    - 6(spare channels)
    - 12(input/output)
    - 1(10 word Y register)
    = 100 10 word data channels giving 1000 words of program and data space
    each word is 11 decimal digits plus sign and could contain 2 instructions
    each decimal digit had 6 data bits and one odd parity bit
  4. Ballistic Research Labs REPORT NO. 971 1955 says that the maximum latency is 400 microseconds,
  5. with a clock rate of 2.5 megahertz, (mentioned above) that is 1000 clocks. This seems to indicate that there were 1000 bits in sequence per channel.
  6. There is probably 1 bit in transit in the electronic chassis (see figure 1-18) where there is the
    IF amplifier
    Detector, Video amplifier
    Plug-in delay
    and the Recirculation Chassis with
    GC which looks like a clocked flipflop
    so in fact there might be only 999 bits in the tank, and one bit in the electronics ;-)
  7. Computer Structures Readings and Examples says that there were 12 seven bit characters (6 bits plus odd parity) per word, and one 7 bit character time between words, used for switching. That is 91 bit times (clock times) per word or 910 clock times per ten word channel. Yes, I know - the numbers don't all exactly agree. A probable cause is counting the sign character as part of the number (the numbers were sign, magnitude, excess three coding).

Special features
Reliability from Grant Saviers
The UNIVAC I had around 5200 tubes, most common type 25L6. I have written about the various purchasing, testing, selection, burn in and maintenance procedures before. Also, Eckert & Mauchly were very careful in the design re reliability and error detection.

The end result at the CASE U-I was a machine that had maybe a couple of tube failures per month. This was during the time I was there and the machine was turned on once a day for a third shift operation by C&O Railroad. The Hg memory tank thermostats were on 24x365. So on an operating basis, probably a 300 to 500 hour MTBF. BRL has some stats for other installations.

I would also bet that there were tubes in that machine when it was retired that were original from the factory. See the BSTJ for tubes designed for undersea cable service.


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Updated October 2009