Manual / Guide · 1965
Teletype BSP 570-224-100 TP177010 Selector Magnet Driver - Description and Principles of Operat 1965
Teletype / BSP 570-224-100, *TP177010 Selector Magnet Driver — Description and Principles of Operation*, Issue 1, Feb 1965
Manual / Guide · 1965
Teletype / BSP 570-224-100, *TP177010 Selector Magnet Driver — Description and Principles of Operation*, Issue 1, Feb 1965
(~) BELL SYSTEM PRACTICES Plant Series 10. 11. 12, CONTENTS GENERAL ..... DESCRIPTION .. FEATURES .... MOUNTING .... SECTION 570-224-100 Issue 1, February, 1965 AT&TCo Standard TP177010 SELECTOR MAGNET DRIVER DESCRIPTION AND PRINCIPLES OF OPERATION re ee GENERAL PRINCIPLES OF OPERATION ... GENERAL DESCRIPTION OF CIRCUITS ..... BIAS VARISTORS INPUT CIRCUIT. ovee eee eevee oe ee ee ee ee we ew ee SWITCHING CIRCUIT .......... OUTPUT CIRCUIT POWER SUPPLY SPACING CONDITION ........-.. MARKING CONDITION..... eee SPACE TO MARK MARK TO SPACE BLINDING..... TRANSITION.... TRANSITION ... ce ee ee we ew ew oe MONITORED SIGNALS ......... UNMONITORED SIGNALS........ PURPOSE OF 50 OHM RESISTOR .. VARIATIONS... oo oe © © © ew we co oe ew SELECTOR MAGNET CONNECTIONS... CONTENTS PAGE INPUT RESISTANCE ......-20 065 9 13, SPECIAL MODES OF OPERATION. . 9 1, GENERAL 1.01 The TPi77010 electronic selector-mag- net driver is a solid-state device which couples a signal line to a receiving teletype- writer and repeats the line Signals in a form that will effectively operate theteletypewriter's selector mechanism. It is designed specifically to drive one 28 selector magnet or one 15 selector holding magnet. When used in place of a line relay (such as the Western Electric 255A) to drive the specified selector magnets, it will provide improved operation, greater reliability and reduced maintenance. However, it is not intended as a general purpose driver for all types of selectors, nor as a replace- ment for relays in general. 2. DESCRIPTION (Figure 1) 2.01 The driver includes a de power supply and an etched circuit card which are mounted on a small chassis (Figure 1). The power supply consists of an isolation trans- former, a full-wave rectifier and single-capac- itor filter. Various electrical components in- cluding five transistors are mounted on the circuit card which is supported by three posts. Power and signal cables equipped with spade lugs provide for electrical connections to the driver. 3. FEATURES 3.01 With different strapping arrangements, the driver will accept neutral signals of either 0.020 or 0.060 ampere or polar signals of up to 0.030 ampere. The output is 0.060 am- pere regardless of input. Prepared for American Telephone and Telegraph Company by Teletype Corporation € 1961 by Teletype Corporation All rights reserved. Printed in U.S.A. Page 1 SECTION 570-224-100 FILTER CAPACITOR TABLE 1 TECHNICAL DATA DIMENSIONS (Overall) Length ........- Cw ee ee eee pe eee eee See eee ee eee wees 4-3/4 inches Width ...... Lene vec eee ee vce ee ee eee ee eee eens . 2-7/8 inches Height ........200e cee eee ees . Sec eee eee teeeeeeee 5-1/4 inches ELECTRICAL Power SOUrCe .. eee ee ete ee ees eee ec ee ee eee 117 v ac + 10% 50-60 cps Max Power Consumption... .. 0... eee eee eee eee ees rarer .. 12 watts Input SignalS .... ee ee ee es 0.020 a.cor G.060 a.z 10% neutral or up to 0.030a. polar Output SignalS .. 1... ee ee ee ee ees Ct ew we te et tees wee eee wee eee eee GO. 060 a. Signalling Speed . oe. ce ee ee eee ee ee eee eens sone Jp to 200 wpm Wiring Diagram ......-+---5 wee ewe ee cee eee ewes wee eee wee ae ~... 4445WD ENVIRONMENTAL REQUIREMENTS Operating Temperatures ..... eee eee eee eee ceeees 0° C (432° F) to +65° C (+149° F) at specified power and input requirements Storage Temperature ....-++- eee eee Ce ew et ee ee ee ee wee.) Max +85° C (+185° F) Page 2 3.02 The driver may be strapped so that, when used in conjunction with external contacts, the selector does not receive (is blinded to) certain incoming messages and does receive (is unblinded to) others. 3.03 Strapping arrangements can be changed by removing and/or soldering bare wire between terminal posts on the circuit card. The posts are accessible so that the card need not be removed for this procedure. 3.04 Since its input is essentially resistive rather than inductive, the driver permits the inclusion of additional receiving units on a teletypewriter loop without introducing signal distortion. 4, MOUNTING 4.01 The driver can be mounted in any vacant position on the 28-type electrical serv- ice units and in the position provided for the line relay on the 15-type bases. Mounting hardware is provided with the driver. Note: The TP177015 modification kit is re- quired to mount the driver on 15-type equip- ment. 5. GENERAL PRINCIPLES OF OPERATION 5.01 This section explains the operation of the selector magnet driver. It is based on the schematic wiring diagram in the section entitled “TP177010 Selector Magnet Driver - Wiring Diagram." Basically the driver oper- ates as follows: Neutral or polar teletype- writer signals are applied to its input. These signals are repeated at its output in a form that will effectively operate the selector of a re- ceiving teletypewriter. Direct current for the driver's circuits is provided by an internal power supply. 5.02 Terms: The transistors as used in the driver either do not conduct or conduct into saturation. They do not operate at inter- mediate stages. Thus throughout this discussion they are referred to as being either "on" (conducting) or "off" (not conducting). 5.03 Signals: The driver is specifically de- signed to receive teletypewriter, start- stop signals. These signals are explained in detail in the appropriate sections. For this ex- planation, it is sufficient to say that each signal element (or pulse) may occur in either of two ISS 1, SECTION 570-224-100 conditions — marking or spacing. For neutral operation, current flows in the signal line cir- cuit during the marking condition, and no cur- rent flows during the spacing condition. For polar operation, current flows during both con- ditions, but the polarity is reversed to differen- tiate between marking and spacing. 6. GENERAL DESCRIPTION OF CIRCUITS BIAS VARISTORS 6.01 The elements referred to as "bias varis- tors" (CR1 through CR5) are used in the circuit to develop transistor bias voltages and low-voltage references that set the input switch- ing level. They consist of two diffused-junction, silicon diodes mounted in opposite directions side by side. They have a forward diode char- acteristic in either direction and thus their voltage drop varies slightly with the current. At 0.10 a.the drop is 0.8 v. Stabistors CR6, CR7 and CR13 have characteristics similar to varistors, but pass current in only one direc- tion. INPUT CIRCUIT 6.02 The input circuit receives the teletype- writer signals and determines when the driver shifts from marking to spacing. It in- cludes R1 through R38, bias varistors CRI1 through CR5, and transistor Q1. In neutral op- eration, it keeps the driver spacing or marking until the line current rises above or drops be- low the switching value. In polar operation it does the same until the current rises above or drops below zero. 6.03 Resistor R1 protects transistor Q1 from high current surges that might damage it. It samples the current flowing into Ql1's base, and, if the current exceeds a value of ap- proximately 0.100 a., the voltage developed across R1 and the base-emitter junction ex- ceeds the forward drop of CR1 and CR2, and most of the excess current is shunted around Q@1. Since the bias varistors are bidirectional, Q1 is protected for inputs of either polarity. an 6.04 For 0.020 ampere neutral operation, R2 is disconnected by removing the strap at terminals 1 and 2, and R3, CR3, CR4 and CR5 ars all left in the circuit. For 0.060 ampere neutral operation, R2, R3, CR38, CR4 and CR5 are all left in the circuit. For polar operation up . nA AN an) ns ee Ee eo. roar oo Sar TO U.UV0IU dw, NS 1D UISCUNNTCLTEU Vy ASAIO V ANE WIS Page 3 aban nt tannrinata to anrA 9- MD DA anA ODE Sibdadp dat WWiliiitiaid 4 GU Sy Viiv, CR4, Givi Waid are shunted out by placing a strap acrosspoints 3 and 4; and R3 alone is left in the circuit. SWITCHING CIRCUIT 6.05 Transistors Q2, Q3 and associated com- ponents form a snap-action trigger cir- cuit. In the marking condition, Q2 is off and Q3 is on. The opposite exists for the spacing con- dition. The change from one State to the other occurs at the midpoint value of the input cur- rent and is very rapid regardless of the slope of the input waveform. OUTPUT CIRCUIT 6.06 The output circuit, which includes Q4, Q5 and associated components, controls the current in the selector magnet coils. During the space to mark transition, it places essen- tially the fuli supply potential across the coils and causes the current to rise rapidly to the operating value of 0.060 a. This rapid rise pro- vides quick selector armature pickup. Once the operating value is reached, the circuit adds resistance to maintain the operating current. During the mark to space transition, a con- trolled discharge circuit, which includes CR9 and Q2, rapidly dissipates the coil's energy without developing any high reverse voltage transients that might cause transistor damage. POWER SUPPLY 6.07 The power supply provides dc for the driver's circuits. ft includes an isolation transformer, a full-wave rectifier, and single capacitor filter. It operates from a 117 v, 50-60 cps, ac source, and provides a nominal dc voltage of -40 v. This value provides a safety factor against transistor breakdown under the most unfavorable circumstances of high tem- perature, high line voltage and maximum line signaling frequency. Note: The actual potential of the negative ove, side of the power supply rélative to ground depends on where the signal line is grounded. Relative to the local earth ground, it can have almost any value in the range of +120 v to -120 v. 7. SPACING CONDITION 1 In the spacing condition transistors Q1, Q3 and Q5 are off; Q2 and Q4 are on; in the selector magnet and no current flows coils. Page 4 7,02 no current flows in In neutral operation, no cur the signal line during spacing. Thus, the base of @1 is connected to the positive side of the power supply either through R3 alone (for 0.020 a. operation) or through R3 and R2 in awe paraliél (ior U. G8 a. operation). internal cur- rent through varistors CR3, CR4 and CR5 places a -2.4 v bias on the emitter of Q1 and keeps it off. In polar operation (R3 alone in circuit and CR3, CR4 and CR5 shunted), TP1 is positive and TP6 is negative during spacing. Thus the signal line through R1 drives the base of @1 positive and keeps it off. Th rtmal t L 7.03 The base of Q2 is driven negative with respect to its emitter through resistors R4 and R5 and resistors R7 and R8&. Thus Q2 is on, and the voltage drop from its emitterto col- lector is very small ——-less than 0.1 v. 7.04 The output of Q2 is applied to the base of Q3 through germanium diode CR11. The current flow through CR11 and R6 to the negative side of the power supply causes a maximum drop of 0.4 v across CR11. Thus the base of Q3 is at most -0.5 v with respect to the emitter of Q2. Resistor R12 permits suffi- cient current to flow through CR7 so that the latter's voltage drop is atleast0.6v. Q3's emit- ter is more negative than its base; thus Q3 is off, 7.05 CR10 prevents current from flowing from the collector of Q2 through the selector coils, CR8 and R4 to the negative side of the power supply. Therefore, the selector magnet is not energized in the spacing condi- tion. 7.06 The collector of Q2 is connected to the emitter of Q4 and to one side of CR13. The negative side of the power supply through RQ makes the base of Q4 negative in relation to its emitter and keeps it on. A 0.1 v drop across Q4 is applied to the base of Q5. Because of the voltage drop across CR13 caused by the cur- rent flow through CR13 and R13, the emitter of Oa at laagat N FR we wanen nocatitra than tha we iS at i€ast v.s0 V more negative than toe emitter of Q4. Thus there is a 0.45 volt back bias across Q5 which remains off, 8. MARKING CONDITION 8.01 In the marking condition, Q1, Q3 and Q4 are on; Q2 and Q5 are off; and a current of 0.060 a. flows in the selector magnet. ana ne 2) ee ee Pen alan Maoaentinn thi 0.vUe iii VOLT TICULT aL and poG.ar operation, the marking signal places anegative potential on the base of Q1 and keeps iton. Current flows from its collector to the negative side of the power Supply through Rd and R4 and through R8 and R7. Current also flows through CR6, CR7 and R12 to the negative side of the supply. A drop of at least 0.65 v across CR6 is applied to the emitter of Q2. The base of Q2, which is con- nected directly to the collector of Q1, is about -0.1 v with respect to the emitter of Ql. There- fore, Q2 has a back bias of 0.55 v and is off, 8.03 Since Q2 is cut off, it does not back bias Q3 as it did in the spacing condition(Par- agraph 7.04). The negative side of the supply through R6 puts a negative potential on the base of Q3 and keeps Q3 on. Current flows from the collector of Q3 through CR8 and R4 tothe nega- tive side of the supply, and through CR10 and the selector coils to the junction of Q4's emit- ter and CR13. Since the base of Q3 is only about -0.4 v with respect to its emitter, the drop of almost 5 v across Q3, CR10, and the selector coils back biases diode CR11. Q2 is unaffected Since it is off and its collector is négative with respect to its base. 8.04 Q4 is on and Q5 is off as described in Paragraph 7.06. 9. SPACE TO MARK TRANSITION 9.01 In neutral operation, varistors CR3, CR4 and CR5 place -2.4 y onQ1 keeping it turned off during spacing Paragraph 7.02). As the signal line changes from spacing to marking, current begins to flow in the input line and a negative potential is developed across the input resistor(s) (R3, or R3 and R2). Q1 remains off until this current exceeds its mid- point value of 0.010 a.for 0.020 aor 0.030 afor 0.060 a. operation. This midcurrent value is detected when a voltage Slightly greater than -2.4 v is developed at the base of Q1 turning it on, 9.02 In polar operation R3 alone is in circuit and CR38, CR4 and CR5 are shunted, As soon as the current moves slightly beyond zero in the marking direction, it places a negative voltage on the base of Q1 and turns it on. 9.03 Q3 turns on, and Q2 turns off under the conditions described in Paragraphs 8.02 and 8.03 above. Q2 and Q3 form a trigger cir- cuit which utilizes positive feedback. The col- lector of Q2 is connected to the base of Q3, and the collector of Q3 is connected to the base of Q2, Q2 through CR11 switches Q3, and Q3 then feeds back to @2 through CR& controlling the ISS 1, SECTION 570-224-100 resistor network of R4 and R5. As the signal changes from spacing to marking, Q1 turns on and causes Q2 to begin to turn off. Q2 in turn causes Q3 to begin to turn on. The collector of Q3 drives the junction of R4 and R5 positive. Less base current is supplied to Q2 whichturns off even more. This trigger action removes any point of uncertainty and prevents the driver from being damaged by locking up on an inter- mediate point or going into oscillation as the input switching level is crossed. 9.04 Initially, the induced voltage of the mag- net coil opposes the current flow from the collector of Q3 through the coils to the neg- ative side of the supply. This prevents current from flowing through CR13 and Q4 (which was on during spacing). The collector of Q4 and the base of Q5 move toward the negative supply po- tential. Q5 is off and its collector is at the neg- ative supply potential. Since its base and col- lector are negative, the emitter of Q5 also moves toward the negative supply potential. 9.05 A short time later, the voltage drop across the coils decreases, and current begins to flow in the coils, increasing nearly linearly with time. The negative terminal of the coils begins to move toward the positive supply potential. The emitters of Q4 and Q5 begin to go positive, Since the negative potential on the collector of Q4 is applied to the base of Q5, the latter turns on as its emitter goes positive and @Q4 turns off, 9.06 Since R11 is small and Q5 is on, essen- tially the full supply potential is placed across the coils. The coils' current, limited only by R11 and the small resistance of the coil, increases rapidly. (It aims at point much higher than the desired 0.060 a. operating current.) Thus the operating current is reached very quickly, and effects a fast pickup of the selector magnet armature. 9.07 During the Spacing condition, Q4 was on and its base was no more than -0.6 v with respect to its emitter. As described above, during the space to mark transition, the current flow through Q4 drops to almost zero, and its emitter goes toward the negative Supply poten- tial. Capacitor Cl holds Q4's base at the con- ducting potential which is positive with respect to the emitter, and @4 turns off. Q5 is still on. The base potential of Q4 drops exponentially toward the negative supply as Cl discharges thane Loma aan Vupas LW Page 5 SECTION 570-224-100 Signals may be monitored at terminal 5 NY = oe a iat) fas) 1) 4 1 Watt Make Before Break oo od | & oi e oO c- = 10% Contacts eo Strap Removed ee ~— Dds Added Circuitry is shown in solid lines; that which is part of Driver is shown in broken lines. - Blinding Arrangement to Permit Signai Monitoring Added Circuitry is shown in solid lines; that which is part of Driver is shown in broken lines. Figure 3 - Simple Blinding Arrangement To Driver Output Figure 4 - Selector Coils Connected in Parallel Page 6 9.08 Normally the voltage divider network of R7 and R8 places a potential of +10 v with respect to the negative supply on the anode of CR12 which is back biased. However, during the space to mark transition period, the base potential of Q4 drops exponentially toward the negative supply. When it gets just below the di- vider's voltage, CR12 becomes forward biased and clamps the base to this value. 9.09 As the current begins to flow through the coils, Q4's emitter drops from the negative supply potential until it is more posi- tive than its base, and Q4 turns on. Its collector potential becomes nearly the same as its emit- ter and is applied to the base of Q5 which turns off, CR12 is again back biased, and the current to Q4's base is supplied through R9 as before. The current flow from the coil to the negative supply is now through @4 and R10 which limits it to the operating value of 0.060 a. R9 and R13 are essentially in parallel with R10 and help to datayrminea tha aurrantta valuia Govermine wie Currents Vaud, 9.10 The voltage divider network of R7 and R8 determines the current value at which Q5 turns off and Q4 turns on. The closer the potential on the anode of CR12 approaches the negative supply, the sooner @5 turns off. Cur- rent through the divider network always flows to the positive side of the supply either through the collector of Q1 or the base of Q2. CR12 re- mains backbiased except for the brief period described above during the space to mark tran- sition. 10. MARK TO SPACE TRANSITION 10.01 As the signal changes from marking to spacing, Q1 and Q3 turn off and Q2 turns on under the conditions described in Paragraph 7.02 through 7.05. The operation of the trigger circuit of Q2 and Q3 is the opposite of that de- scribed in Paragraph 9.03. Q4 remains on and Q5 remains off as described in Paragraph 7.06. 10.02 When Q3 turns off, it no longer supplies current to the selector coils. The coils resist a drop in their current by developing a negative voltage transient at their normally positive end, This transient is passed by CR10, blocked by CR8 and blocked by CR9 until it ex- ceeds the negative supply potential at which time CR9 conducts. CR9 insures that the volt- age rating of Q3 is not exceeded by clamping it at about -40 v. 10.03 @Q2 holds the normally negative end of the coil at near positive supply poten- tial When QS turns off. A consiani poiential of ISS 1, SECTION 570-224-100 about 35 v is thus placed across the coils. The rate at which the current through the coil de- creases is constant. 11. BLINDING 11.01 A receiving teletypewriter is "blinded" when it does not respond to the line signals, ie it does not print or perforate the information being transmitted on the line. To effect blinding, the selector is kept steadily marking regardless of the line signals. Blinding can be accomplished in a number of ways de- pending on the specific application, but may be considered in two categories based on whether or not it is necessary to monitor the signal line while the equipment is blinded. MONITORED SIGNALS 11.02 If it is necessary to monitor the line Signals, the driver can be blinded as enltlectaor nf O11 ja AlaepanmnasrtodA a0 UL Ga 2D GLOCONNECLG icsllows. The col from the base of Q2 by removing the strap be- tween terminals 5 and 6. Terminal 6 is then connected (by relay contacts, etc) tothe positive side of the power supply at terminal 4 through a 50 ohm resistor (see Figure 2), Because of the drop across CR3, CR4, CR5, CR6 and CR7, a back bias of about 3.2 v is placed on Q@2. Thus Q2 is off, Q3 is on, and the driver is locked in the marking condition regardless of the line signals (see Paragraph 8). By auxiliary circuits not part of the driver, the line signals can still be monitored at terminal 5. Normal operation of the driver can be restored by externally re- connecting terminals 95 and 6 and disconnecting the 50 ohm resistor between terminal 6 and the positive side of the power supply. 11.03 Blinding can be accomplished either manually or automatically. For exam- ple, the stunt box of a page printer (which would be receiving the line signals through its own selector) could be arranged so that its contacts would blind the driver of a reperforator at the receipt of a certain character sequence, and would unblind it at the receipt of another se- quence. Thus the reperforator could be made to ignore certain traffic. UNMONITORED SIGNALS 11.04 If it is not necessary to monitor the line signals, a simpler method of blind- ing may be used. Terminal 6 is connected to the power supply (signal line) at terminal 4 through a 50 ohm resistor and the strap between 5 and 6 is noi removed (see Figure 3), Page 7 SECTION 570-224-100 11.65 This approach is feasible providing that the reduced input resistance will not disturb external circuits that operate into the driver. PURPOSE OF 30 OHM RESISTOR 11.06 The 50 ohm, i-watt resistor connected between terminals 6 and 4 prevents damage to Q1 in the event the signal line cur- Yellow Input Leads From 4 Drivers 12. VARIATIONS SELECTOR MAGNET CONNECTIONS 12.01 The circuits of the driver are designed for connecting the two selector coils in parallel to reduce required de voltage and to 1 lan ----4 ] Fora Reo pr +4 ™, NN N coos N 5 —--— oReT P---P EON oe _ oS e Lo eT a a ne -|-- —_Re y ¢ 7 4 —- oo Re pa Green Input Leads From 4 Drivers —— a. Isolation with series Resistors. Yellow Input Leads F From 4 Drivers) CR ( re--4 ry -~-----/2 4 hee Ld - i Re_ r ™~. ‘. 1 CR roa n4 \, S TT TTP Pa Ret rn TL ‘ v7 aT! 1 R iia CR p---4 ~-c7 need a aie Re*™ rrr re orm va (a ya = lit if = 7 —__ CR ~---|-F- tRet} --+ 1 Retl'3 7 YL Green Input Leads From 4 Drivers J b Isolation with Diodes R,R',R"&R" - Current Limiting Resistors Re, Re! , Re", Re''! = Drivers' Equivalent Input Resistance. CR, CR‘, CR" & CR''! = Isolation Diodes S - Keying Contacts Circuitry that is part of Driver is shown in AAd-lis: broken lines. Amaaii in solid lines. Page 8 ional Circuitry is snown el Operation of Four Drivers reduce the overall self-induced voltage when the coils' current is increasing or decreasing. Operation with the coils in series will not harm the driver, but will reduce signal margins. It is not recommended. A typical parallel connection is shown in Figure 4. INPUT RESISTANCE 12.02 For the signal line, the input resistance of the driver varies in accordance with the polarity and magnitude of the input current. For example, the input resistors of 124 and 249 ohms are in parallel for 0.060 a.operation and have an effective measured value of 83 ohms. This is the input resistance for the signal line as long as the line current is below the switch- ing level, ie the level where Qi turns on (0.030 a.). 12.03 When Q1 does turn on, however, the voltage across the 83 ohms is held at approximately 2.9 v by the clamping action of Ql, CR3, CR4 and CR5 even though the current is increasing. Thus the resistance is decreas- ing: eg at 0.060 a.the actual effective value is 48 ohms, or about one-half of the measured value of 83 (2.9 v/0.060a. = 48 ohms). The cor- responding effective resistance for 0.020 a. operation is 145 ohms. 12.04 This variation in input resistance can lead to discrepancies between the com- puted and actually required values of signal line series resistance. For example, assume a ten- station teletypewriter loop, 0.060 a. operation, 120 v line battery, and ignore the resistance of the signal line itself. Basing the values onohm- meter or bridge measurement, the total resist- ance in the circuit would thus be 83 x 10 or 830 ohms. The total required resistance would be 120/0.060 or 2000 ohms. The battery station would thus require 2000-830 or 1170 ohms of series resistance. However, in light of Para- graph 12.03, actually the effective value of the input resistance at the marking current value is approximately one-half of the measured val- ue, or 480 ohms. If 1170 ohms, as computed above, is used at the battery station, the total effective line resistance will be 1170 + 480 or 1650 ohms, and the resulting line current will be 120/1650 or 0.073 a. Thus the series resist- ance actually required at the battery station to ISS 1, SECTION 570-224-100 give 0.060 a. of line current is 2000-480 or 1520 ohms. 12.05 If line resistance is checked with line current of reversed polarity as inpolar operation, a Similar discrepancy appears. Since Q1 does not conduct when the polarity is re- versed, the input resistance seems to be high- er. Consequently, it is difficult to obtain equal current for marking and spacing on polar cir- cuits, unless each is set independently of the other. 12.06 With the driver strapped for polar op- eration, the computed input resistance for marking is R1 + (0.4 v)/0.030a.or 10 + 13.3 = 23.3 ohms. (0.4 v is the Q1 emitter to base drop.) For spacing, the computed input resist- ance is 1.6 v, the drop across CR1 and CR2, divided by 0.030 a. or 53 ohms. 13. SPECIAL MODES OF OPERATION 13.01 The drivers are designed to operate in the conventional manner with their in- puts in series. Paralleling the inputs of several drivers is not recommended, because the first to go marking holds the input voltage at its threshold value and prevents it from rising higher to turn on the other drivers. However, several modifications of the input circuit can be made to meet special requirements. 13.02 Parallel operation is often used to per- mit several circuits to be operated from a single keyer, although series operation can usually accomplish this end. When condi- tions make parallel operation desirable — as in hub circuits — additional circuitry mustbepro- vided to isolate the drivers from each other. Resistors can be used for this purpose as shown in Figure 5a, or diodes as shown in Fig- ure 5b. 13.03 The input terminals can be shorted to make the driver go spacing. CR1 and CR2 are placed in parallel with CR3, CR4 and CR5. Since CRi and CR2 have a lower voltage drop, all the current flows through them to the emitter of Q1 instead of through CR3, CR4 and CR5. The resultant voltage drop through CR1 and CR2 applies a negative bias to the emitter of Q1 which keeps Q1 shut off. Page 9 9 Pages