US Navy Teletype Equipment and Emanations Security

Motivation - Early in the development of military teletype communications, engineers discovered that the make/break of 60ma current loops created small signals that could be propagated via electromagnetic radiation, or conducted along signal and power lines. This meant that crypto systems could be vulnerable if they also inadvertently emitted the plain text along with the desired encrypted text. This discovery led to efforts to reduce the undesired signals through shielding, filtering, and signal strength reduction. This program became known as TEMPEST. Emanations Security is another name for this effort.

Red & Black - It is critical that plain text and encrypted text should never be associated with each other. RED refers to signals that represent classified plain text (which need to be hidden from public view). BLACK refers to unclassified plain text or encrypted text (which may be intercepted during transmission). RED and BLACK signals, equipment, and operating areas are kept as separate as possible to minimize the possibility of RED signals appearing in BLACK transmissions. Extensive guidelines for signal routing, grounding, etc. should be followed.

RED teletype signal patch panels are used on the plain text side of encryption equipment and BLACK/GRAY patch panels on the encrypted side. If a piece of teletype equipment can switched between the two, a locking switch and indicator lamp are used to minimize mistakes.

SB-4035/UGQ Patch Panel
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SB-4034/UG Patch Panel
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SA-734/SG Switch
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ID-866/SG Indicator
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  id866-1412-03.JPG (186267 bytes)

Low Level Signaling - To reduce unwanted emissions, the military adopted +/- 6vdc signaling (MIL-STD-188B) to replace the earlier 60ma current loop (High Level) signaling. This low level signaling greatly reduced emanations security problems. Existing High Level equipment could be converted to Low Level by the addition of Electrical Service Units which converted the signals. New Low Level equivalents for most older models were also produced - these had more extensive shielding and filtering as well as the Electrical Service Units. See The 1977 Index for a full listing of high-level and low-level equipments. See NAVSHIPS 0967-173-7030 and 0967-173-7040 for maintenance details on low-level equipments. Also see 0967-391-6010 "Low Level Technical Control Equipment". Also see more info below.

Teletype Corp. Equipment - The terms RFI,  TEMPEST, and CRITICOM show up in Teletype publications. Here are my guesses as to meaning -

RFI Suppression - In early publications, RFI (Radio Frequency Interference) suppression meant the addition of spark suppression filters to signal and power lines in order to minimize interference to radio receivers. Later publications, like 311B/RF, use RFI to mean +/- 6vdc Low Level signaling in addition to shielding and filtering (TEMPEST). 311B/RF says "RFI suppression as applied to teletypewriter equipment is accomplished by means of shielding and wave shaping a low level electrical telegraph signal throughout the equipment. The installations vary with each set, but produce the same results of insuring signal line privacy."

TEMPEST - shielding and +/- 6vdc Low Level signaling. A typical Teletype Corp. spec would say "Includes MIL-STD-188B six volt polar interface and Teletype's TEMPEST feature."

CRITICOM - CRITICOM or CRITICOMM refers to very high priority intelligence communications (see also original NSA document) Both High-Level and Low-Level sets are listed as "CRITICOMM type". The 270B manual for CRITICOM sets does not mention the word "CRITICOM", but the sets are 7.00 unit, polar signal, with stunt box contacts for recognizing message Z signals related to CRITIC messages (ZYH, ZEM, ZYI, ZYE, "X CRITIC", and message start/end ZCZC and NNNN). CRITICOM equipments appear to be unrelated to Emanations Security.

NIK - Normal Input Keying was used with some crypto equipment -  A TSEC/KW-7 interface drawing says the marking contact current is 80ua. So NIK is evidently a 50-80ua loop current. See this document for modifying High Level teletype equipment for NIK.

I need info on AN/UGC-20B vs AN/UGC-77 - UGC-20B has 70uA @ 1.5vdc keyboard output? Is this NIK?
MK-1158/UG modification kit provides +/-6v low level output from UGC-20B. Does it include 334439 ESU?

Typical TEMPEST Model 28 Teletype Equipment
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28 Compact RO
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28 Rack-mount KSR
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28 KTR
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28 Rack-mount RO
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An external Electrical Service Assembly will contain a Selector Magnet Driver (SMD)  for converting +/-6v external signaling to 60ma to drive the selector magnets of a typing unit or reperforator via a double shielded cable. The Electrical Service Assembly will contain a Low Level Keyer (LLK) for converting 28 keyboard or TD spacing contact closures (or 250ua photoelectric signals from a Compact KSR) to +/-6v external signaling. The Electrical Service Assembly will also contain Clutch Magnet Drivers (CMD) as needed for keyboard and TD sets.

ESA Electronic Service Assembly
(outer shield)
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ESA Electronic Service Assembly
(inner shield)
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ESA Electronic Service Assembly
(card slots)
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ESA Electronic Service Assembly
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The RFI signal generator contact box is composed of two metallic boxes. An inner box completely enclosed by an outer box. The two boxes are mechanically fastened together with fiber hardware and insulating material to electrically isolate each box from the other.

The contact assembly is provided with gold-plated contacts for low voltage operation. The assembly is enclosed within and electrically insulated from the inner box. The filter card assembly is mounted to and above the contact assembly. The filter is a network of three resistors and a capacitor mounted on a circuit board. When used in conjunction with associated shielded cables, power supplies, and keyer the filter provides a low-level interface and RFI suppression.

A double shielded cable assembly is provided to electrically connect the contact box to a three-pin electrical receptacle. The shielded cable is composed of three electrical conductors encircled by braided inner and outer shields. Two of the three internal wires are electrically insulated and transfer the telegraphic signals to associated equipment. The remaining wire is bare and electrically connected to the inner contact box, inner braid shield, and cable receptacle. The inner and outer braided shields are electrically separated from each other and the wires by flexible solid dielectric. The inner braid is electrically connected to the inner contact box and the outer braid is electrically connected to the outer contact box. The cable assembly provides RFI suppression when used with associated RFI equipment.

The 303142 LLK (Low Level Keyer), when used in conjunction with the 321268 filter card assembly, is intended for use with the 323644 and 323645 signal generator (one contact) assemblies. This LLK is adaptable to various types of 28 type equipment when used with the applicable ESA and is designed to operate from one set of contacts. Two signal generator outputs (filter card outputs), however, may be paralleled to drive one signal line from either of two signal generators.


The signal generating mechanism utilizes photoelectric cells instead of a contact mechanism to generate a signal. A lamp assembly provides the necessary light source to electrically activate the cells. A mechanical shutter assembly, linked with the keyboard codebars and located between the photocells and lamp assembly, provides windows to either allow light from the light assembly to pass and activate the cells (mark) or block the light and not activate the cells (space). The photocells will generate a parallel electrical signal of approximately 300 microamperes (ua). The generated signal travels along a shielded cable to a photoelectric distributor. The photoelectric distributor serializes the signal, and by means of shielded cables, routes it to the input of a polar line keyer.

The 323130 LLK (Low Level Keyer) is for use in photoelectric systems (such as 28/32 keyboard) requiring a low-level interface and extreme RFI suppression. 

The 323130 keyer takes a 250 ua (min) photocell signal from the distributor and by means of passive and active filtering, shapes the output. It is used in conjunction with a 333069 CMD (Clutch Magnet Driver).

In the marking state (photocell illuminated), Q5 is turned off causing the bases of Q1 and Q2 to go positive through the passive shaping network made up of R2, Cl and R4. With the bases of Q1 and Q2 positive, Q1 will turn on turning Q4 off and Q2 will turn off turning Q3 on. Capacitor C2, resistor R6, R9, and capacitor C 3 further shape the wave by providing feedback and phase shift thereby controlling the rate at which the active filter Ql, Q2, Q3, Q4 will switch.

In the spacing state (photocell dark), Q5 is turned on providing a negative signal to the bases of Q1 and Q2. The switching occurs as above except, transistors that are off turn on and those that are on turn off.

During the transition from on to off and off to on, one of the output transistors of the active filter is always conducting. This will provide a smooth transition from plus volts through 0 volts to minus volts and back again. The rate of switching being controlled by the feedback and phase shift of C2, R6 , R9 and C3.

Diode CR1 compensates for the non-symmetry of the first stage. Resistors R10 and R5 and capacitors C6 and C7 provide for the proper output impedance and some additional shaping.

keyboard lamps, shutters, & photocells
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distributor lamps and photocells
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distributor and clutch on gearshift shaft
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distributor slots for serialization
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The RFI selector mechanism mounts on the upper right side frame of the typing unit or the main frame of the reperforator. The selector consists of a special three-pin electrical receptacle, double shielded cable and metallic container.

The three-pin electrical receptacle insures a secure and shielded electrical connection to other associated apparatus,. The double shielded cable electrically connects the three-pin electrical receptacle to the selector magnets. The shielded cable is composed of three electrical conductors encircled by braided inner and outer shields. The inner and outer braided shields are electrically separated from each other and the three electrical conductors by flexible solid dielectric. The metallic container functions as a shielded enclosure for the selector magnet assembly.

Enclosed within the metallic container are the selector magnet coils, coil mounting bracket and selector armature. Each selector magnet coil contains an electrostatic shield which surrounds the coil windings. The selector coil mounting bracket provides mounting facilities for the coils, armature, and biasing spring. The receptacle, shielded cable, metallic container, and selector coils provide RFI suppression when used with associated RFI equipment.


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The input current to the 323810 selector magnet driver (SMD) is a low level +6 volt for a marking state, and a -6 volt for a spacing state.

The output current of the SMD is 60 mA +/-10% during the marking state. The output is zero during the spacing state. Overall receiving margins of properly adjusted 28 type selectors driven by this SMD (polar rectangular wave input) should exceed 70 points at either input. The SMD operates at bit rates up to 75 baud.

The SMD assumes the marking state with positive input voltages not greater than 0.5 volt and the spacing state with negative voltages not greater than 0.5 volt. The marking and spacing switching levels are adjustable within 10% of each other. This requirement applies to either input. Each input of the SMD has a minimum input resistance of 50,000 ohms. The maximum input capacitance of either input is 2500 picofarads.

The SMD provides two inputs and makes possible reception from either one of two separate transmitters (single input operation) while the input line from the other transmitter is open. A spacing signal at either input will provide a spacing output. The SMD provides a marking output when both inputs are open. Both inputs cannot be in the marking condition simultaneously without producing a garbled output.

AN/UGC-77 Information Page