Carrier sense data highway system

A data transmission system includes a transmission medium which has a certain propagation delay time over its length. A number of data stations are successively coupled to the transmission medium for communicating with one another. Each of the data stations includes a transmitter for originating signals, each signal beginning with a carrier of a duration which is at least the propagation delay time of the transmission medium. Each data station also includes a receiver which receives other signals from other data stations and inhibits operation of the transmitter at the same data station when a carrier of another signal is received.

1. Field of the Invention 
This invention relates to the transmission and receipt of data between a 
number of stations successively arranged along a transmission medium such 
as a coaxial cable. 
2. Description of the Prior Art 
The use of coaxial cable or similar transmission medium (e.g., fiber optic 
light pipes) for conveying various messages between a number of user 
stations arranged along the medium is becoming widespread. Among the first 
industries to bring the coaxial cable into the home of residential users 
are the community Antenna Television (CATV) Systems, wherein a master 
television antenna array is located to receive standard broadcast 
television signals from points relatively far away, and amplifiers 
connected with the array distribute the received signals over a number of 
cables into the homes of users who otherwise would not be able to receive 
the signals with their own antennas. Present day cable television networks 
provide not only standard television broadcasts through a cable into the 
user's home, but also provide locally generated closed circuit programming 
and other information services from private studios. 
The above examples of cable communication are basically one-way systems in 
which each user has only passive capabilities, that is, one user cannot 
send information to any of the other user stations or to the master 
distributing station since the cable arrangement does not allow 
information to propagate away from each of the user stations. Recently, 
however, great gains in computer technology have made it possible to make 
a great deal of various information available at relatively low cost, so 
that it is not unusual for a private firm or corporation to purchase a 
computer which is tailored for its needs, and to make access to the 
computer available at a number of locations where potential users are 
usually found. Data stations having both passive, i.e. receiving 
capability, and active, transmitting capability are arranged at such 
locations and are each connected with the computer by a coaxial cable or 
like transmission medium. Each station thus can enter data into the 
computer, and thereafter receive the desired computational data. By 
connecting the cable in a closed loop arrangement with each of the data 
stations and the computer so that the stations and the computer are, in 
effect, wired " in series" with one another, it is then possible for each 
data station not only to transmit and receive data from the computer, but 
also to communicate with any desired one or a number of the other data 
stations in the loop. Examples of such loop-type communications systems 
appear in U.S. Pat. Nos. 3,752,932 issued Aug. 14, 1973, to J. B. Frisone; 
3,597,549, issued Aug. 3, 1971, to Farmer, et al; 3,732,543, issued May 8, 
1973, to Rocher, et al; and 3,904,829, issued Sept. 9, 1975, to Martin et 
al. 
A problem arises when two or more users, each at different data stations, 
desire to transmit data along the cable simultaneously or sufficiently 
close together in time, so that the beginning of one data message from one 
station collides with another data message originating from a different 
station along the cable. After the messages collide, the resultant data 
propagating along the cable will be incorrect and no valid data message 
can be received at a station further along the cable in the direction of 
propagation. 
The problem of data collision is not peculiar to a closed loop 
communications system, but is encountered with other arrangements such as 
an open end configuration as disclosed in U.S. Pat. No. 4,210,780, issued 
July 1, 1980, to Hopkins et al. This system includes an inbound 
unidirectional signal path, an outbound unidirectional signal path, and a 
unidirectional path coupler which joins the output of the inbound signal 
path to the input of the outbound signal path. Both of the inbound and 
outbound paths are coupled to a number of bus interface units each having 
associated subscriber devices distributed along the lengths of the inbound 
and outbound signal paths. Before any subscriber device transmits data 
into the inbound signal path, its bus interface unit senses for the 
presence of a data signal on the outbound signal path and, if no signal is 
detected, allows the data to be transmitted to be sent down the inbound 
signal path. This transmitted data then proceeds through the path coupler 
and propagates along the outbound signal path so that it can be sensed and 
received by any of the bus interface units of the entire system, including 
the bus interface unit of the transmitting station which then compares the 
data it receives from the outbound signal path with that transmitted by it 
down the inbound signal path. If this comparison is correct, no collision 
has occurred and the remainder of the data is transmitted. If the results 
of the comparison are negative, that is, if a collision has occurred and 
an invalid message has resulted, transmission is aborted and attempted 
once again after passage of a random time period. 
Schemes for avoiding continued collisions of data in the closed series loop 
communications systems are known. For example, in the '932 Frisone patent, 
above, a message which originates from any one of a number of remote 
stations in the loop begins with a zero bit. Reception of this zero bit by 
any of the remote stations further down the loop in the direction of 
propagation operates to inhibit the receiving station from originating its 
own message, and causes the receiving station to allow the data from the 
originating station to pass unaltered. Transmission priority for the 
several remote stations therefore is determined in accordance with the 
location of each station along the loop. Further, in the '549 Farmer et al 
patent, above, priority among a number of data stations in a closed loop 
is determined by the first station on the loop to alter the last bit of an 
end-of-message code, which code is circulated about the loop, from a 
binary 1 to a zero. Subsequent stations thus are alerted that a station 
further up the loop desires to originate a message, and the subsequent 
stations then are inhibited from originating their own messages. However, 
none of the foregoing collision avoidance schemes operates to prevent the 
transmission of an invalid or incomplete message. Specifically, the bus 
interface unit of the '780 Hopkins et al patent operates to abort 
transmission by its associated subscriber device after a collision has 
been detected, rather than enable the subscriber device to complete a 
valid data transmission after it has gained access to the inbound signal 
path. Also, with the systems of the Frisone and Farmer et al patents, a 
data message originated by a station further down the loop transmission 
line in the direction of propagation is always subject to having its data 
message interrupted by a station closer to the beginning of the line, so 
that a priority according to position along the loop is established. In 
other words, only the first station along the loop can originate and 
complete a valid data message once it has gained across the loop, with any 
degree of certainty. 
SUMMARY OF THE INVENTION 
The present invention overcomes the above shortcomings in the prior art by 
providing a data transmission system including a transmission medium for 
propagating information along a desired path, the transmission medium 
having a certain propagation delay time over its entire length, and a 
number of data stations successively coupled to the transmission medium 
for communicating with one another. Each of the data stations includes 
transmitting means for originating signals wherein each signal begins with 
a carrier of a duration at least equal to the propagation delay time of 
the transmission medium, and receiving means for receiving other signals 
which originate from other data stations and for inhibiting the 
transmitting means at the same data station from originating a signal in 
response to reception of any of the other signals. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its use, reference 
should be had to the accompanying drawing and descriptive matter in which 
there are illustrated and described preferred embodiments of the invention 
.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1, a carrier sense data highway system according to the invention 
includes a send line 10, a receive line 12 and a head end 14 which serves 
to couple the output 16 of send line 10 to the input 18 of the receive 
line 12. Send line 10 and receive line 12 each can be conventional coaxial 
cable, or equivalent transmission medium which is capable of propagating 
information including data signals in a given direction. The head end 14 
includes a conventional head end amplifier A.sub.H or equivalent coupling 
device. A number of data stations, Station 1, Station 2, Station 3 . . . 
Station N are coupled successively along the send line 10 and the receive 
line 12, and each station is arranged to originate a signal which 
propagates along the send line 10 in the direction toward the head end 14. 
The message thereafter is coupled by head end 14 to the receive line 12 
where it can be sensed and received by each of the data stations along the 
entire length of the send line 10 and receive line 12. Additional data 
stations (not shown) can be added, successively, by extending send line 10 
and receive line 12 to each additional station from the right side of 
Station N as viewed in FIG. 1. 
One or more send line amplifiers A.sub.S and one or more receive line 
amplifiers A.sub.R can be located along the send and receive lines 10,12 
as may be required to maintain sufficient signal strength, as is well 
known in the art. 
In the following discussion, it will be assumed that the send line 10 and 
the receive line 12 are conventional coaxial transmission lines of the 
same length. It will also be assumed that the system operates with a radio 
frequency (RF) carrier signal which is modulated in accordance with the 
data information to be exchanged along the data stations, and that the 
send line 10 and the receive line 12 each exhibit a propagation delay time 
.tau. over their entire lengths. The propagation delay time through the 
head end 14 will be considered to be negligible. Of course, the present 
invention is not limited by these assumptions, and may be embodied as well 
in systems using other transmission media as long as the propagation delay 
times along various parts of the chosen transmission medium are known. 
Any given data station includes a data transmitter X.sub.n where n=1 to N, 
which transmitter is coupled to the send line 10 by a conventional CATV 
directional coupler so that the RF signals originating from any of the 
transmitters X.sub.n each propagate in the direction toward the head end 
14. Each data station also includes a dual receiving system R.sub.nA, 
R.sub.nB, where n=1 to N. One receiver portion R.sub.nA is also coupled to 
the send line 10 through a conventional CATV directional coupler and 
operates to establish a priority of data among the various data stations 
in accordance with their relative positions along the send line 10. 
Specifically, in the event the receiver portion R.sub.nA detects an RF 
carrier signal which originates from a transmitter X.sub.m, where m&gt;n, the 
receiver R.sub.nA provides a priority sense signal PS to its associated 
transmitter X.sub.n. Sense signal PS serves to inhibit transmitter X.sub.n 
from continuing with any signal transmission which may have commenced 
prior to detection of the carrier by receiver portion R.sub.nA. 
Each receiver portion R.sub.nB of the receiving system at a given data 
station is coupled to the receive line 12 also by way of a conventional 
CATV directional coupler. Receiver portion R.sub.nB operates to receive 
and demodulate or decode all RF signals which propagate along the receive 
line 12 away from the head end 14, and to allow display devices (not 
shown) or other processing means located at the data station to respond to 
data information contained in the signals. Further, each receiver portion 
R.sub.nB provides a carrier sense signal CS to its associated transmitter 
X.sub.n in the event any signals are present on the receive line 12. 
Signal CS operates to prevent the transmitter X.sub.n at a given data 
station n from commencing to transmit a signal which includes data (DATA 
n) supplied to the transmitter X.sub.n at the data station n. 
Operation of the data highway system of FIG. 1 will now be described. 
Assuming, for example, that at data station 2, neither sense signal PS nor 
CS is produced by the receiver portions R.sub.2A, R.sub.2B to inhibit the 
transmitter X.sub.2 from transmitting, transmitter X.sub.2 begins 
transmitting an unmodulated RF carrier signal for a time period of at 
least 2.tau. where, as noted above, .tau. is the propagation delay time of 
either the entire send line 10, or the entire receive line 12. After the 
time period 2.tau. has elapsed, data station 2 then transmits a data 
message or packet which is preceded by a station address code in 
accordance with conventional data transmission formats. 
Receiver portion R.sub.nB at each of the data stations will then detect the 
presence of the carrier or the data message originating from data station 
2 by way of its directional coupling onto the receive line 12, and will 
inhibit its associated transmitter X.sub.n from commencing its own data 
transmission except, of course, for transmitter X.sub.2 which began 
transmitting before receiver portion R.sub.2B detected any carrier or data 
message on the receive line 12. 
Thus far, the only possible way for two data messages or packets, each 
originating from a different data station, to collide is if they originate 
at times which are within the time period .tau. of one another, so that 
neither one of the originating stations has yet received the other 
station's transmission. This possibility of a data collision is 
forestalled by receiver portions R.sub.nA which establish a priority of 
transmission in accordance with the relative positions of the data 
stations along the send line 10. 
As an example, if receiver portion R.sub.2B of data station 2 detects that 
the receive line 12 is free, and transmitter X.sub.2 at the same data 
station has been provided with data (DATA 2) for transmission, transmitter 
X.sub.2 will first transmit an unmodulated RF carrier for a time period of 
2.tau., this carrier being coupled into the send line 10 to propagate in 
the direction toward the head end 14. Assuming next that data station 3 
begins to originate data from its transmitter X.sub.3 at the same time 
that the transmitter X.sub.2 began transmitting, the signal from 
transmitter X.sub.3 will reach receiver portion R.sub.2A of data station 2 
before the time period 2.tau. has elapsed, since the propagation delay 
time of the entire send line 10 is only .tau.. Receiver portion R.sub.2A 
will then produce a priority sense signal PS which serves to inhibit the 
transmitter X.sub.2 from continuing to originate its own carrier, and the 
transmission from transmitter X.sub.3 will continue to propagate down the 
send line 10 through data station 2 (including transmitter X.sub.2) in the 
direction toward head end 14. The transmission from data station 3 then 
passes through head end amplifier A.sub.H and on through all the receiver 
portions R.sub.nB at each of the data stations by way of the receive line 
12. Since the signal originating from data station 3 includes an address 
or destination code, those data stations which are among those designated 
in this code to receive the data within the signal will then decode the 
signal, and display or otherwise process the data obtained. Importantly, 
since no collision of data messages within the transmitted signals can 
ever occur with the system of FIG. 1, the data messages received and 
decoded at any of the data stations are always valid and complete. 
In sum, the system of FIG. 1 provides a number of distinct advantages over 
existing data networks, among which are the following: 
1. The system acts as a master/master democracy until a carrier collision 
(as opposed to a collision of data messages) occurs. 
2. Even if a carrier collision occurs, a valid message will always be 
transmitted and received without incurring a random waiting period. 
3. Only a time period 2.tau. must elapse before a data station can begin 
sending its data message or packet (headed by any desired destination 
code). 
4. Conventional, CATV directional couplers allow most of the cable 
connections to be of a passive nature and, thus, more reliable. 
5. Transmission media other than coaxial cable, e.g. fiberoptics, can be 
employed and will allow the system to operate in the same manner. 
6. Conventional frequency shifting schemes or multiplexing may be employed 
at each of the data stations and the head end 14 so that a one-cable 
system is possible. 
7. Frequency shifting schemes may be employed to provide transmission 
priority among the data stations which is not dependent on the position of 
any one station relative to the other stations along the send line 10. 
8. The system can use conventional, low cost CATV components which greatly 
simplify the electronic circuit implementation of the system arrangement. 
FIG. 2 shows a second embodiment of a carrier sense data highway system 
according to the present invention, including random priority. 
Basically, the system of FIG. 2 includes a number of data stations . . . 
Station N-1, Station N, Station N+1 . . . each of which are coupled to two 
data highway lines or cables T.sub.L and T.sub.R. Cables I.sub.L and 
T.sub.R may each be a conventional coaxial cable or other transmission 
medium. 
Each data station has a transmitter-receiver pair or a transceiver 
allocated to each of the cables T.sub.L, T.sub.R, and information 
originating from each of the data stations propagates only in the 
direction toward the left along cable T.sub.L, and in the direction toward 
the right along cable T.sub.R, as viewed in FIG. 2. 
For example, at data Station N, transmitter X.sub.NA and receiver R.sub.NA 
are each coupled through a conventional directional coupler (not shown) to 
the cable T.sub.L so that, when DATA N is supplied to the transmitter 
X.sub.NA, it will transmit a signal containing DATA N along cable T.sub.L 
toward Station N-1 and all data stations beyond. 
Further, data Station N includes another transmitter X.sub.NB and an 
associated receiver R.sub.NB both of which are directionally coupled to 
cable T.sub.R so that a signal transmittted by transmitter X.sub.NB 
propagates along cable T.sub.R only in the direction toward data Station 
N+1, and all data stations beyond Station N+1 which are coupled to cable 
T.sub.R. The receiver R.sub.NB responds only to signals propagating along 
the cable T.sub.R from data stations toward the left of Station N, as 
viewed in FIG. 2. 
At data Station N, receiver R.sub.NA provides a carrier sense signal CSR 
which serves to inhibit both of the transmitters X.sub.NA, X.sub.NB from 
originating a signal from data Station N, in the event a signal which is 
propagating along the cable T.sub.L and which originates from a data 
station to the right of Station N is sensed by the receiver RNA. Likewise, 
receiver R.sub.NB provides a carrier sense signal CSL which inhibits both 
of the transmitters X.sub.NA, X.sub.NB from originating a signal from data 
Station N in the event a signal which is propagating along the cable 
T.sub.R and originates from a data station to the left of Station N is 
sensed by the receiver R.sub.NB. 
For purposes of the following discussion, it will be assumed that a finite 
number of data stations are successively coupled to the cables T.sub.R, 
T.sub.L as shown in FIG. 2, and that the greatest propagation delay time 
between two data stations along either one of the cables T.sub.R, T.sub.L 
is .tau.. 
Preferably, each complete signal begins with a carrier of a time period of 
at least .tau., followed by a data message or packet which corresponds to 
the data (DATA N) supplied to the transmitters X.sub.NA, X.sub.NB. 
By transmitting toward the right on cable T.sub.R, and toward the left on 
cable T.sub.L, only the propagation delay time .tau. of either cable, 
rather than 2.tau., need be considered for purposes of collision 
detection. Also, by virtue of the directional characteristic of the 
coupling of the data station transmitters and receivers to the cables 
T.sub.R, T.sub.L, a particular data station will not receive its own 
transmission. Accordingly, any signal, e.g., an RF carrier, received at a 
particular data station while that station is transmitting indicates a 
collision. 
An example of the operation of the system of FIG. 2 now follows: 
DATA N is supplied to the transmitters X.sub.NA, X.sub.NB of data Station N 
for transmission to all of the other data stations coupled to the cables 
T.sub.R, T.sub.L. The receivers R.sub.NB, R.sub.NA of data Station N first 
sense the cables to determine whether or not any of the other data 
stations are transmitting. That is, receiver R.sub.NB senses cable T.sub.R 
for transmissions originating from stations toward the left of data 
Station N, and receiver R.sub.NA senses the cable T.sub.L for 
transmissions originating from data stations toward the right of Station 
N, as viewed in FIG. 2. If either receiver senses a signal on its 
associated cable, then the transmitters X.sub.NA, X.sub.NB of Station N 
are inhibited by either of the signals CSL or CSR from originating a 
signal containing DATA N. If neither carrier sense signal is produced, 
then data Station N transmits its signal to the data stations toward its 
left by transmitter X.sub.NA, and to the remaining data stations toward 
the right of Station N by transmitter X.sub.NB. As mentioned above, data 
Station N will not receive the signal it originates owing to the 
directional coupling arrangement employed. A complete initial transmission 
of a carrier of time period .tau., prior to transmission of a data message 
or packet corresponding to DATA N, will ensure that data Station N can 
continue to originate its message without collision. 
Because the propagation delay time which must be considered for purposes of 
collision detection is only that of either of the cables T.sub.R, T.sub.L, 
and not the sum of the propagation delay times for each cable, the system 
of FIG. 2 can be used advantageously for process control and computer 
communication over distances greater than 1000 feet and up to about 15,000 
feet, assuming the use of conventional coaxial cable. In addition to 
offering relatively short delay times for long cable runs, the system of 
FIG. 2 provides simple collision detection, and allows for random priority 
among the data stations coupled to the cables T.sub.R, T.sub.L. Of course, 
the system of FIG. 2 can also be configured to provide priority by 
position, if desired. 
FIG. 3 shows a third embodiment of a carrier sense data highway system 
according to the present invention, with which it is possible to provide 
random priority or other contention schemes to overcome collisions of 
data. 
The system of FIG. 3 also includes a number of data stations . . . Station 
N-1, Station N, Station N+1 . . . each coupled to a single line or cable 
T.sub.RL. Cable T.sub.RL may be a conventional coaxial cable or other 
transmission medium. 
Each data station comprises two transmitter-receiver pairs and a number of 
couplers arranged so that information originating from each station 
propagates in the direction toward the left along cable T.sub.RL at a 
frequency f.sub.1, and in the direction toward the right along cable 
T.sub.RL at a frequency f.sub.2, relative to the originating station. 
For example, at data Station N, a pair of directional couplers C.sub.N1A 
and C.sub.N1B are inserted serially along the cable T.sub.RL so that the 
output ports O of the two couplers are connected to one another. 
Accordingly, signals propagating in the direction from the input to the 
output port of the coupler C.sub.N1A, at frequency f.sub.2, will be 
sampled out of the coupler tap, as shown. Also, signals entering the 
coupler tap at frequency f.sub.1 will be coupled into the cable T.sub.RL 
and propagate out from the input port of the coupler C.sub.N1A in 
accordance with well-known characteristics of directional couplers. Of 
course, other signals at frequency f.sub.1 entering the output port O of 
the coupler C.sub.N1A will travel substantially unattenuated through the 
coupler and out of the input port to Station N-1 and all stations beyond. 
Also, signals entering the tap of coupler C.sub.N1B at frequency f.sub.2 
are coupled to the cable T.sub.RL to travel in the direction out from the 
input port of coupler C.sub.N1B, and signals at frequency f.sub.2 entering 
the output port of coupler C.sub.N1B will propagate out from the input 
port of that coupler toward Station N+1 and all stations beyond. 
Station N also includes directional couplers C.sub.N2A and C.sub.N2B each 
with their input ports connected to corresponding taps of the couplers 
C.sub.N1A and C.sub.N1B, as shown. A transmitter X.sub.Nf.sbsb.1 which 
provides signals at frequency f.sub.1 is coupled to the directional tap of 
coupler C.sub.N2A so that the signal from transmitter X.sub.Nf.sbsb.1 
propagates out from the input port of coupler C.sub.N2A to be coupled into 
the cable T.sub.RL by way of coupler C.sub.N1A. Another transmitter 
X.sub.Nf.sbsb.2, which provides signals at frequency f.sub.2, is connected 
to the directional tap of coupler C.sub.N2B so that the signals from 
transmitter X.sub.Nf.sbsb.2 propagate out from the input port of coupler 
C.sub.N2B to be coupled into the cable T.sub.RL by the coupler C.sub.N1B. 
Data which is to originate from Station N, DATA N, is supplied to both of 
the transmitters X.sub.Nf.sbsb.1, X.sub.Nf.sbsb.2 so that, assuming the 
transmitters X.sub.Nf.sbsb.1, X.sub.Nf.sbsb.2 are not inhibited, a signal 
at frequency f.sub.1 will originate from Station N out from the input port 
of coupler C.sub.N1A, and a signal at frequency f.sub.2 will propagate out 
from input port of coupler C.sub.N1B, both of these signals containing 
DATA N. 
A receiver R.sub.Nf.sbsb.2, which is adequately filtered to respond 
substantially only to signals at frequency f.sub.2, is connected to the 
output port of coupler C.sub.N2A to receive data contained in signals at 
frequency f.sub.2 which are sampled from the cable T.sub.RL by coupler 
C.sub.N1A and pass through coupler C.sub.N2A. Such signals will originate 
only from Station N-1 or other stations further to the left of Station N, 
as viewed in FIG. 3, since signals at frequency f.sub.2 originating from 
Station N or any other station toward the right of Station N will not 
appear at the directional tap of coupler C.sub.N1A, according to 
well-known characteristics of directional couplers. Receiver 
R.sub.Nf.sbsb.2 responds by providing a carrier sense signal CSL which 
operates to inhibit the station transmitters X.sub.Nf.sbsb.1, 
X.sub.Nf.sbsb.2 from originating a signal containing DATA N from Station N 
at either frequency f.sub.1, f.sub.2. 
Another receiver R.sub.Nf.sbsb.1, which is adequately filtered to respond 
substantially only to data signals at the carrier frequency f.sub.1 is 
connected to the output port of coupler C.sub.N2B so that signals 
originating from stations to the right of Station N at frequency f.sub.1 
will be provided to receiver R.sub.Nf.sbsb.1 through coupler C.sub.N2B 
after these signals are sampled from T.sub.RL by coupler C.sub.N1B. It 
will be understood that signals originating from Station N at frequency 
f.sub.1 or from any other stations to the left of Station N at frequency 
f.sub.1 will not appear at the directional tap of coupler C.sub.N1B for 
detection by receiver R.sub.Nf.sbsb.1. Receiver R.sub.Nf.sbsb.1 thus 
provides a carrier sense signal CSR which also operates to inhibit the 
station transmitters X.sub.Nf.sbsb.1, X.sub.Nf.sbsb.2 from originating 
signals containing DATA N from Station N, inasmuch as the presence of a 
signal at frequency f.sub.1 at the output port of coupler C.sub.N2B 
indicates that another station to the right of Station N is originating 
the signal. 
The embodiment of FIG. 3 has the advantage that only a single cable 
T.sub.RL is required to extend between all of the data stations, rather 
than the two cables T.sub.R, T.sub.L in the embodiment of FIG. 2, while 
still reducing the amount of time it takes to transmit a signal to all 
stations as compared to the embodiment of FIG. 1. However, some 
limitations on the frequency spectrum which can be used for propagating 
signals from the various stations is imposed, since the station receivers 
must discriminate between signals at the frequencies f.sub.1 and f.sub.2. 
Otherwise, the basic operation of the system of FIG. 3 is similar to that 
of the system of FIG. 2. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the inventive principles, it 
will be understood that the invention may be embodied otherwise without 
departing from such principles.