Apparatus for multiplexing switch state signals comprises a plurality of switches and parallel weighted resistors connected in series between circuit ground and a node at a utilization device. The resistors are weighted as a function of a power of the same base, such as the power of the base 2, for coding the multiplexed switch state signals. A constant current source connected between the node and circuit ground drives current over a single cable conductor through the resistor. Each switch may be independently closed to change the switch state voltage signals multiplexed to the node. An analog-to-digital converter connected between the node and circuit ground demultiplexes the switch state signals received at the node and provides a switch state signal at each analog-to-digital output corresponding to the state of the switches at the moment. A potentiometer may replace a resistor and bypass switch combination in a position where the potentiometer has a maximum value of the lowest power of the base in order to multiplex a true analog voltage signal while switch state signals are unambiguously coded and multiplexed. The potentiometer in the least significant position permits the analog value to be in the range from 0 to a maximum corresponding to the least significant position of the switch state encoding. The invention may be used in redundancy systems by duplicating the invention with corresponding switches in each duplication ganged to open and close simultaneously upon operation of a pushbutton switch

Hannaford, Blake

NASA Technical Reports Server

United States Patent [191 [ i l l  Patent Number: 4,884,070 
Hannaford [451 Date of Patent: Nov. 28, 1989 
[54] METHOD AND APPARATUS FOR 
MULTIPLEXING SWITCH SIGNALS 
[75] Inventor: Blake Hannaford, South Pasadena, 
Calif. 
[73] Assignee: California Institute of Technology, 
Pasadena, Calif. 
[21] Appl. NO.: 176,871 
[22] Filed: Apr. 4, 1988 
[51] Int. Cl.4 ............................................... H04Q 9/00 
[52] U.S. Cl. .......................... 340/825.78; 340/870.38; 
341/26 
[58] Field of Search ...................... 340/825.77, 825.78, 
340/825.38, 870.38; 341/22, 26, 20; 178/18, 20 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,906,369 9/1975 Pitman et al. ......................... 341/20 
4,118,700 10/1978 Lenihan .............................. 340/524 
4,458,238 7/1984 Learn .................................... 341/20 
4,580,138 4/1986 Morrison ........................ 340B25.78 
OTHER PUBLICATIONS 
Belove, C., Handbook of Modern Electronics and Electri- 
cal Engineering, pp. 924-926, John Wiley & Sons, New 
York, 1986. 
Fink, D. G. and D. Christiansen, Electronic Engineers 
Handbook pp. 8-71-8-74, McGraw Hill, New York, 
1982. 
Landee, 8. W., D. C. Davis, A. P. Albrecht, and L. J. 
Giacoletto, Electronics Designer's Handbook pp. 20-7- 
1-20-74, McGraw Hill, New York, 1977. 
Rhyne, V. T., Fundamentals of Digital Systems Design, 
p. 504, Prentice Hall, New York, 1973. 
Sheingold, D. H., Analog-Digital Conversion Handbook, 
pp. 11-36-11-39, Analog Devices, Inc., Norwood, 
Mass., 1972. 
Bekey, G. A. and W. J. Karplus, Hybrid Computation, 
pp. 27-28, John Wiley & Sons, New York, 1968. 
MICROCOMPUTER 
IO" 
Susskind, A. K., Analog-Digital Conversion Techniques, 
pp. 5-46-5-47, MIT Press & John Wiley & Sons, 1962. 
Huskey, H. D. and G. A. Kom, Computer Handbook 
pp. 18-44-1845, McGraw Hill, New York, 1962. 
Primary Examiner-Donald J. Yusko 
Assistant Examiner-Edwin C. Holloway, I11 
Attorney, Agent, or Firm-Freilich, Hornbaker, Rosen & 
Femandez 
1571 ABSTRACT 
Apparatus for multiplexing switch state signals com- 
prises a plurality of switches and parallel weighted 
resistors connected in series between circuit ground and 
a node at a utilization device. The resistors are weighted 
as a function of a power of the same base, such as the 
power of the base 2, for coding the multiplexed switch 
state signals. A constant current source connected be- 
tween the node and circuit ground drives current over 
a single cable conductor through the resistor. Each 
switch may be independently closed to change the 
switch state voltage signals multiplexed to the node. An 
analog-to-digital converter connected between the 
node and circuit ground demultiplexes the switch state 
signals received at the node and provides a switch state 
signal at each analog-to-digital output corresponding to 
the state of the switches at the moment. A potentiome- 
ter may replace a resistor and bypass switch combina- 
tion in a position where the potentiometer has a maxi- 
mum value of the lowest power of the base in order to 
multiplex a true analog voltage signal while switch state 
signals are unambiguously coded and multiplexed. The 
potentiometer in the least significant position permits 
the analog value to be in the range from 0 to a maximum 
corresponding to the least significant position of the 
switch state encoding. The invention may be used in 
redundancy systems by duplicating the invention with 
corresponding switches in each duplication ganged to 
open and close simultaneously upon operation of a 
pushbutton switch. 
4 Claims, 2 Drawing Sheets 
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'atent NOV. 28,1989 
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MICROCOMPUTER -------- 
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Sheet 1 of 2 4,884,070 
1 10 
HAN D HELD CONTROL DEVICE 
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A I D  
14 
12 
FIG. I 
FIG. 3 
U.S. Patent NOV. 28,1989 Sheet 2 of 2 4,884,070 
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COMPUTER A ----- 
----A 
COMPUTER B ----
FIG. 2 
4,884,070 
1 2 
are not important. The only design requirement is that 
each switch state must be able to be identified (uniquely 
decoded into its state) by the demultiplexer (analog-to- 
digital) converter at the apparatus to be controlled. 
BRIEF DESCRIPTION OF THE DRAWING 
METHOD AND APPARATUS FOR 
MULTIPLEXING SWITCH SIGNALS 
ORIGIN OF THE INVENTION 5 
The invention described herein was made in the per- 
formance of work under a NASA contract, and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the Contractor has elected to retain title. 
TECHNICAL FIELD 
This invention relates to multiplex telegraphy, and 
more particularly to method and apparatus for multi- 
FIG. 1 illustrates a preferred embodiment of the pres- 
FIG. 2 illustrates application of the present invention 
10 to fault tolerant switch signal multiplexing to redundant 
FIG. 3 illustrates a variation of the invention illus- 
ent hention. 
control system in apparatus to be controlled. 
trated in FIG. 1. 
DETAILED DESCRIPTION OF THE 
INVENTION 
plexing switch state signals from ahandheld control 
device over a single conductor, such as for multiplexing l5 
control switch states to a robot over a cable. 
The invention shown in FIG. 1 is comprised of a 
handheld control device (box or handgrip) 10 which 
In many applications, such as telerobotics, a handheld contains resistors R1, RZ and R3(weighted 200, 100 and 
device is used to control many different functions of 2o 50 ohms) in series between ground and a single conduc- 
electromechakd aPPaatUs. A hand-held Control de- tor coaxial cable 11. Switches SI, Sz and S3 are con- 
vice @OX O r  handgrip) is often Provided With Push nected in parallel with the respective weighted resistors 
buttons and/or switches to be actuated by an operator. R1, Rz and R3. The center conductor of the cable 11 is 
For example, moving a toggle switch or moving a Push connected to a node 12 at a circuit comprised of a con- 
button switch, and the l i e ,  controls special functions of 25 stant current driver (CCD) 13 and an analog-to-digital 
a robot such as to grasp Or release, lock Or release Cur- ( A D )  converter 14 which is part of a microcomputer 
rent position, and emergency stop Or remme. Thus, the system, or any digital computer system, programmed to 
handheld controller has a multiplicity' of control func- decode the switch state information at the output of the 
tions which are transmitted to the controlled apparatus converter 14. In that manner, the A/D converter 
straints, it is often desirable to reduce the number of coaxial conductors in this cable to a minimum. 
BACKGROUND ART 
Over a Because Of space* weight, and Other 30 &multiplexes the multiplexed signal received over the 
It should be noted that there are no active elements in 
STATEMENT OF THE INVENTION the circuit at the control device which fulfills the 
switch-state multiplexing function, and that the digital- 
to-analog conversion voltage reference provided by the vide a single conductor for transmitting a signal con- 
veying information as to the state (open or closed) of a current driver is connected to the node 12 at the mi- 
of which may be crocomputer system located on or in the apparatus to be 
simultaneously actuated from one position to a second controlled, 
position. In operation, the state of each of the three switches is 
A further objective is to provide for demultiple,g separately actuated to the closed or open position, such 
the signal received Over a single conductor in order to as by squeezing or releasing Pushbutton microswitches 
provide the information as to which of the plurality of m ~ ~ & d  on a grip with any one of the three fingers 
switches have been actuated to the second position. between the thumb and little finger (or pushing the 
inven- 45 microswitches mounted on one side Of a handheld box 
tion are achieved by providing a digital-to-analog con- with the same three fingers). The constant current 
verter in a handheld control device, and providing driver 13 provides a fixed current I through the 
switches to enter binary control information, such as weighted resistor Ri, R2, and R3 for a maximum output 
stop/resume, forward/backward. The output of the voltage V=IRo, where Ro is the sum of Ri, R2, R3. 
digital-to-analog converter is connected to a single con- 50 closing any one of the switches SI, sz and s3 decreases 
ductor which extends into electromechanical apparatus the output voltage by IRi, IR2 and respectively, 
to be controlled, where an 'analog-to-digital converter with the minimum reference voltage at the node 12 
demultiplexes the information as to the position of the being zero. 
switches in the control device so that the apparatus will If the resistors are weighted in powers of the same 
respond to the switch position of each switch. 55 value, the switch state configuration multiplexed on the 
In this digital-to-analog converter, the circuit em- single conductor cable can be uniquely inferred from 
ployed applies a voltage reference and senses the analog the total voltage at the node 12 which is a linear func- 
output at the same circuit node. This is accomplished by tion of the weighted resistors by-passed in the series 
weighted resistors connected in series between the cir- circuit of the digital-to-analog converter by selectively 
cuit node and circuit ground, and a separate switch 60 closing one or more of the switches. 
connected in parallel with each resistor. In that manner, In some applications, the switch signal multiplexing 
the circuit can be used to multiplex switch signals over method may be extended to fault tolerant redundant 
the single conductor without active components in the systems, such as fly-by-wire aircraft control systems, as 
handheld control device. an additional capability for the systems. In such sys- 
Because the manual input to the digital-to-analog 65 tems, control integrity may be guaranteed by parallel 
converter is a pattern of switch states (open or closed), microcomputer systems reading multiple sensors (ana- 
and not a binary representation of a value, traditional log-to digital converters) and driving multiple actua- 
concerns in digital-to-analog design, such as linearity, tors. The switch signal multiplexing method saves 
Accordingly, it is an object of the invention to pro- 35 
of switches, one or 
as a robot. 
These and other objects and advantages 
4,884,070 
3 4 
weight by allowing several manually operated switches Low order bits of Pi field may or may not be used 
in a control device to be connected to a control com- depending on accuracy of R1 and Rz, the current 
purer via a wire.  hi^ in itself should result in source, and the internal resistance of the current source. 
count. However, in control systems, such as have been described and illustrated herein, it is recog- nized that modifications and variations may readily flight control of commercial aircraft, safe operations in the art. For example, although 
failure. switches, it would be possible to utilize electronic 
The weight g a b  and reduced complexity benefits of switches which are normally off and are turned on by 
the invention also apply to such redundant fault tolerant lo grounding a control terminal with pushbuttons. Conse- 
systems. FIG. 2 shows how such a system would be quently, it is intended that the claims be interpreted to 
implemented with three level redundancy. For each cover such modifications and variations. 
operator switch Si, S2 and S3, three sets of normally I claim: 
open contacts are mechanically coupled to the mecha- 1- A system for multiplexing switch State signals from 
ism. one multiplexing is formed for each 15 a device to a utilization system over a single conductor, 
contact position on each of the three switches as shown comprising a series resistor network at said device, said 
in FIG. Each individual circuit is driven network having n weighted resistors in series in which the resistances of said resistors are functions of different by a separate current Source with a separate power powers of the same base, each resistor except one hav- 
computer* The 20 ing a switch in parallel therewith for bypassing it when 
separate computers already exist in the fadt-tolerant the switch is closed, a source of constant current with 
control system. OdY the separate Current SOurceS are internal resistance connected by said single conductor 
part of the invention. Thus, three sets of operator con- between circuit ground and one terminal of said series 
trolled switches are connected to a triply redundant resistor network, a connection between a second termi- 
control system with a three conductor cable. A compa- nal of said series resistor network opposite said one 
rable system without switch multiplexing requires 18, 25 terminal and said circuit ground, and means connected 
12, or 10 wires, depending on ground wire require- to a node between said single conductor and said utiliza- 
ments. tion device for converting the analog voltage at said 
In FIG. 1, the ground wire is provided by the outer device to a digital representation of the state of said 
switches in parallel with said resistors, said one resistor conductor Of a be pro- 30 without a switch parallel therewith comprising a poten- 
significant reliability gains through reduced component particular embodiments Of the invention 
to those must be guaranteed in spite of any sin& component mechanical switches are shown, which may be micro- 
and fed to a separate 
It could as 
vided by a second conductor in the Same Or separate tiometer connected in series with other ones of said n insulating sheath* second technique for providing weighted resistors, said potentiometer being adjustable 
the ground wire to each set of weighted resistors and between a maximum resistance that is a function of the 
switches is illustrated in FIG. 2. lowest power of the same base and a minimum resis- 
2 (as indicated by the same reference numerals used in 2. Apparatus as defined in claim 1 wherein said de- 
the two figures) contemplates only multiplexing the vice is a handheld controller for said utilization device. 
switch states. It would be possible to multiplex an ana- 3. A system for multiplexing switch state signals from 
log signal in place of one switch state, if the least sigifi- a device to a utilization system over a single conductor, 
cantly weighted resistor R~ is used to transmit the ana- 4o comprising a series resistor network at said device, said 
log value by substituting for a fixed resistor, a potenti- network having n weighted resistors in series in which 
ometer which can be set by a thumb wheel, for example. the resistances of said resistors are functions of different powers of the same base, each resistor having a switch This is possible because the most significant digits out of in parallel therewith for bypassing it when the switch is 
the switches si and SZ, while the least Significant digit 45 tance connected by said single conductor between 
position of the Converter determines an andog Value cuit ground and one terminal of said series resistor net- 
being transmitted in a range from zero to I(R3mm) as work, a connection between a second terminal of said 
the resktor R3 is increased in resistance from zero to a series resistor network opposite said one terminal and 
maximum value of resistance. said circuit ground, and means connected to a node 
The control computer reads the analog value by sim- 50 between said single conductor and said utilization de- 
ply selecting the low order bits for the value of the vice for converting the analog voltage at said device to 
analog quantity. For instance, as shown in FIG. 3, a digital representation of the state of said switches in 
switch SI may be closed to specify forward speed con- parallel with said resistors, and further including redun- 
trol, switch sz is left open. The desired speed set dancy in said utilization system for receiving multi- in the potentiometer p1 is then multiplexed with the 55 plexed switch states at separate nodes between separate 
constant current drivers and circuit ground, and redun- state of the switch Si. For reverse speed, the switch S2 dancy of series resistor networks at said device, each 
may be with the potentiometer p1 set at Some network having separate switches connected in parallel 
operating switches of respective ones of said resistors in potentiometer before closing switch 2. 
6o series to open or close in unison, and a plurality of sepa- 
rate conductors each connecting a separate one of said 
nodes with a separate one of said series resistor net- 
works, thereby providing redundancy in said system for 
multiplexing switch states from said device to said utili- 
4. Apparatus as defined in claim 3 wherein said de- 
vice is a handheld controller for said utilization device. 
The description of FIG. 1, which s applicable to FIG. 35 tance equal to zero. 
the didal-to-analog converter unambiguously code for closed, a Source of constant current with internal res& 
independent 'peed The Operator simply resets the with respective Ones of said resistors in series, means for 
EXAMPLE 
If the computer analog-to-digital converter has 12 
bits of resolution to represent 0 to lOV, a unit quantiza- 
tion level equals: 
If I=28.6MA, 
65 zation system. 
The computer recognizes an analog-to-digital con- 
verted word as follows: * * * * *  
UNITED STATES PATENT AND TRADEMARK OFFICE 
CERTIFICATE O F  CORRECTION 
PATENT NO. : 4,884,070 
DATED : Novgnber 28, 1989 
Page 1 of 2 
INVENTORtS)  : B l a k e  Hannaford 
is hereby corrected as shown below: 
Column 3, linea 60-68, delete 
It i s  certified that error appears in  the above-identified patent and that said Letters Patent 
"EXAMPLE 
If the  computer a n a l o g - t o - d i g i t a l  c o n v e r t e r  has 12  b i t s  of r e s o l u -  
t i o n  t o  r e p r e s e n t  0 t o  lOV, a u n i t  q u a n t i z a t i o n  l e v e l  e q u a l s :  
i f  1=28.6MA, 
The computer r e c o g n i z e s  an a n a l o g - t o - d i g i t a l  conve r t ed  word as 
follows: 
and substitute 
--Example: 
If  t he  computer a n a l o g - t o - d i g i t a l  c o n v e r t e r  has 1 2  b i t s  of r e s o l u t i o n  t o  
represent 0 t o  lOV, a unit q u a n t i z a t i o n  l e v e l  equa l s :  
R3=0-50n, R -1 O O a ,  R l  =200n 
2-. 
, IOV 10 - = - = 1.22Mv 
1 2  8192 . 
2 
UNITED STATES PATENT AND TRADEMARK OFFICE 
CERTIFICATE OF CORRECTION 
PATENT NO. : 4 ,884 ,070  
DATED : November 28, 1989 
INVENTOR(S) : Blake Hannaford 
Page 2 of  2 
it is certified that error appears in the above-identified patent and that said Letters Patent is hereby 
c o m t e d  as shown bdw: 
.05 x 28.6 
1.22MV 
p o t e n t i o m e t e r  range = = 1171 c o u n t s  
The computer r e c o g n i z e s  an h a l o g - t o - d i g i t a l  c o n v e r t e d  word as follows: 
TI  r ange  = 0+2’1-1=0+2047-- 
Signed and Sealed this 
Seven t een t h Day of Seyt ember, 1991 


Method and apparatus for multiplexing switch signals

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