Self-decoding FET-hybrid or integrated-circuit tree configuration uses minimum number of components and can be sequenced by clock or computer. Redundancy features can readily be incorporated into tree configuration; as tree grows in size and more sensors are included, percentage of parts that will affect given percentage of sensors steadily decreases

Easton, R. A.

Hilbert, E. E.

English

NASA Technical Reports Server

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July' 1973
	
B73-10289 
•	 NASA TECH BRIEF 
NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Data Multiplexer Using a Tree Switch 
.
The problem: 
To provide a telemetry commutator system which 
will be highly reliable in missions lasting more than 
12 years.
FET Switches 
Inputs 
The solution: 
A self-decoding FET-hybrid or integrated-circuit 
tree configuration that uses a minimum number of 
components and can be sequenced by a clock or by a 
computer.  
How it's done: 
Ordinarily, multiplexers are used in a block-type 
configuration with a driver for each commutator and 
a series of decoding gates. In the configuration devel-
oped for improved reliability, field-effect transistors 
(FET's) are connected in a self-decoding tree struc-
ture so that fewer drivers and no decoding gates are 
required; additionally, in the new configuration a 
single component failure will affect considerably 
fewer sensors. The clocked-tree configuration gener-
ates its own fixed sequence and requires only a clock 
for a driver. The diagram illustrates two decks of a 
typical high-reliability tree-type commutator. 
Redundancy features can readliy be incorporated 
into the self-decoding tree configuration; in fact, as 
the tree grows in size and more sensors are included, 
the percentage of parts that will affect a given per-
centage of sensors steadily decreases because only n 
drivers are required for 2n sensors and a fixed per-
centage of switches affects a fixed number of sensors. 
Therefore, if the number of sensors doubles, the per-
centage is halved.
A basic bipolar driver circuit, shown in the second 
diagram, includes all decoding and clocking gates 
necessary to decode parallel redundant control regis-
ters, and also has the low impedance output required 
for switching hundreds of FET's in a tree structure. 
The flip-flop outputs from the sensor address register 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19730000289 2020-03-17T06:54:58+00:00Z
are attached to Q and i7. While the clock input is low, 
the entire circuit is off. When the clock input is high 
(5V) one side of the driver switches to +V and the 
other side to —V, depending on the states of Q and i7. 
The switching rise and fall times are approximately 1 
jLs even for capacitative loads of several thousands 
of picofarads. The driver outputs 0 and 0 remain in 
the selected states as long as the clock pulse is high, 
and when the clock pulse ends, the high output falls 
to any selected Vi with a time constant determined 
by the capacitative load and resistor R1. Similarly, 
the low output rises to Vi. Thus, when the, tree is not 
being switched, all FET's can be off if Vi is made 
equal to —V. 
Reliability of the system is improved by isolating 
each FET from the driver lines by resistors (20 
kiohms); thus, FET failures resulting from a short 
between gate and source or gate and drain cannot 
affect driver control of the rest of the FET's and fast 
switching is maintained. Even higher reliability can 
be obtained by combining two sets of two units each 
of the basic circuit made redundant for protection 
against shorted output. 
Notes: 
1. The use of the tree commutator in a computer-ac-
cessed telemetry system is described in Tech Brief
B73-10290. 
2. Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP 73-10289 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,614,327). Inquiries concerning nonex-
clusive or exclusive license for its commercial devel-
opment should be addressed to: 
NASA Patent Counsel 
Mail Code 1 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Richard A. Easton 
and Edward E. Hilbert of 
Caltech/JPL 
under contract to 
NASA Pasadena Office 
(NPO-11333) 
. 
S 
. 
B73-10289	 Category 02


Data multiplexer using a tree switch

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