Half-duplex industrial communications system

A serial signal communications system includes a central station unit connected through a balanced impedance signal communication bus to one or more remote station units.

1. Technical Field 
This invention relates to signal communications systems, and more 
particularly to serial digital signal communications systems. 
2. Background Art 
Serial digital signal communications systems are well known. These systems 
vary in architecture, protocol, baud rate, etc., depending on the 
application and type of information to be exchanged. All serial systems, 
however, share a common susceptibility to induced noise which may affect 
signal transmission accuracy. In control system communications of the type 
in which a central control regulates the operation of one or more remote 
controlled devices, induced noise considerations are paramount. Although 
conventional noise protection, such as shielding of the systems' 
transmission lines, does provide a high degree of noise immunity, it is 
costly. In addition, where the serial communications system must be 
installed in existing structures the use of shielded transmission lines 
may be impractical. 
In providing serial communications between a central control station and 
one or more remote stations a number of overhead operations must be 
performed by the central control signal processor to regulate data flow 
and monitor data integrity. In an industrial environment (e.g. high noise 
environment) distortion of the transmitted signal by ambient noise may be 
the most important consideration, since induced noise may create erroneous 
data and, therefore, incorrect control commands. As a consequence the need 
for constant error checking to ensure data integrity may result in higher 
control system overhead, necessitating a larger signal processor than that 
required for the control system protocol. In other words, the control 
signal processor must be larger simply to service the demands of the 
communications network itself, apart from the actual control function. 
DISCLOSURE OF INVENTION 
One object of the present invention is to provide a serial data 
communications system for reducing the input/output (I/O) overhead between 
a central station signal processor and I/O signals remote from the central 
station. Another object of the present invention is to provide a 
communications system with high noise immunity for providing high 
integrity serial data communications in industrial applications. 
According to the present invention, a serial signal communications system 
includes a central station unit connected through a balanced impedance 
signal communication bus to one or more remote station units, the central 
station unit having I/O signal ports responsive to each remote station 
unit and responsive to the signal processor of a user control system, each 
remote station unit similarly having I/O signal ports responsive to the 
central station unit and to an associated remote controlled device of the 
user control system. In further accord with the present invention, the 
communications bus includes a signal data transmission line connected at 
each end to low pass filter termination networks and connected along its 
length to the I/O signal ports of the central and remote station units, 
the low pass filter termination networks providing a balanced impedance 
matching of the transmission line to the central and remote station unit 
I/O ports at the selected signal transmission frequency, and providing 
common mode rejection of signal frequencies above the selected signal 
transmission rate. In still further accord with the present invention, the 
data transmission line comprises unshielded, twisted pair signal lines 
connected at each end to one input of an associated termination network 
low pass filter, the other side of each filter connected to a signal 
ground center tap of the network, and the lines connected along their 
length to a differential I/O signal port provided on the central station 
unit and each remote unit, whereby transmission line signal information is 
transmitted and received differentially at the I/O signal ports of the 
central and remote station units. In still further accord with the present 
invention the communication bus further includes electrical power 
distribution lines, including high potential and low potential lines 
connected between the high potential and low potential outputs of an 
associated power supply and the corresponding electrical power inputs of 
the central and remote station units, the low potential line being 
connected to the signal ground center tap of the transmission line 
termination networks. In still further accord with the present invention, 
the central station unit provides synchronous, bidirectional communication 
with each remote station unit in a tristate signal format. 
In still further accord with the present invention, the central and remote 
stations each include an identical industrial control unit (ICU), the ICU 
capable of functioning alternately in a master mode for central station 
use and in a slave mode for remote station use, each ICU providing the I/O 
differential signal port interface with the signal communications bus and 
providing the I/O signal interface with the corresponding user system 
signal processor or remote controlled device. 
The present communications system is characterized by high noise immunity 
signal transmission despite the use of unshielded transmission lines. This 
is due to the system architecture which includes a balanced transmission 
line terminated at each end in narrow band, low pass filter networks which 
provide low signal frequency impedance matching and high signal frequency 
common mode rejection. The signal information on the transmission line is 
transmitted and received differentially at all station ICUs to further 
enhance noise rejection. In addition, the lack of transmission line 
shielding permits flexibility in system installation in existing 
structures, and an inherently lower system cost. 
These and other objects, features, and advantages of the present invention 
will become more apparent in light of the following detailed description 
of a best mode embodiment thereof, as illustrated in the accompanying 
drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 is an architectural block diagram of the communications system 10 of 
the present invention, which provides signal communications between a user 
system central control 12 and one or more remote controlled devices 14, 
16. The user system central control and remote controlled devices are 
shown in phantom. 
The communications system includes a central (master) communications 
station 18 with input/output (I/O) lines 20 to the central control and I/O 
lines 22 to the signal communications bus 24. The bus is dual function. As 
described hereinafter with respect to FIG. 2, the bus includes a data 
transmission bus with balanced termination networks 26, 28 for providing 
time division multiplexed signal communication between the central station 
and one or more remote (slave) stations 30, 32, and it further includes a 
power bus for providing DC power to all stations. The DC power is supplied 
to the bus on lines 34 from power supply 36. The remote stations 30, 32 
are connected through lines 38, 40 to their associated remote devices 14, 
16, and through lines 42, 44 to the bus. 
FIG. 2 is a schematic diagram of the communications system 10. As shown the 
bus 24 includes wire pairs 46, 48. Both wire pairs are unshielded to 
facilitate installation in the user system's structural environment, and 
to reduce cost. The pair 46 with lines 46A, 46B, is the data transmission 
bus and is preferably a twisted wire pair. The pair 48 is the power bus 
with lines 48A, 48B connected to the low and high voltage potential 
outputs of the power supply 34. It provides DC power to the central and 
remote stations and their associated remote devices. The pair 48 may also 
be a twisted pair, but not necessarily so. 
Since each wire pair is unshielded and subject to both high and low 
frequency ambient noise levels the data transmission line pair 46 is 
connected at each end to impedance termination networks 26, 28. Each 
network includes low pass filters 50, 52 for network 26 and 54, 56 for the 
network 28. The filters are connected at a high signal side of each to 
respective ends of each of the transmission line wires 46A, 46B, and at a 
low signal side to the network center tap. The power distribution line 
wire 48B, connected to the low potential output of the supply 34, is also 
connected to the center tap 57, 58 of each termination network. The line 
48B is grounded at the center tap, i.e. the low side of each of the 
network filters, to provide the current return signal path for the line 
driver currents from the various stations on the data transmission line. 
As described hereinafter, the filters provide a low frequency impedance 
match and maximum high frequency common mode rejection within the selected 
data transmission rates. Grounding the filter low sides (i.e. the network 
center tap) provides the shortest noise return path; the highest amplitude 
noise levels being at the midpoint of the transmission line. As a result, 
the filters are selected to provide a narrow transmission line bandwidth 
and high common mode rejection of high frequency voltage transients 
induced by foreign noise sources, e.g. relay coils and induction motors. 
The central and remote stations include identical industrial communications 
unit (ICU) modules, each connected in the same manner to the communication 
bus transmission and power distribution lines. The ICU modules are capable 
of being programmed in either a master or slave mode; depending on the ICU 
module application in either the central or remote station. The central 
station master ICU module 60 receives the four wire bus inputs through 
lines 22, and provides I/O interface with the central control signal 
processor through lines 62-65 (included within lines 20, FIG. 1). 
Similarly, the remote station slave ICU modules 68, 70 receive the four 
wire bus inputs through lines 42, 44 and interface with their associated 
remote devices through lines 72-74 and 76-78 (included within lines 38, 40 
of FIG. 1). 
To provide the greatest noise immunity the unshielded transmission lines 
46A, 46B are preferably 18-24 AWG (0.20 to 0.86 mm.sup.2) twisted pair 
with an approximate 100 ohms characteristic impedance and an approximate 
capacitance of 18 pf/ft (60 pf/meter). The power distribution lines 48A, 
48B are a minimum 20 AWG wire size; the size depending on the power 
distribution line length with the minimum wire size associated with the 
maximum line length of 1,000 feet (305 meters). The power distribution 
line is used to supply current to only noninductive DC loads at stations, 
and the load current is limited to a maximum voltage drop of 2.0 VDC on 
the power line return. When inductive loads, large DC loads, or AC loads 
are controlled by the remote stations they are supplied by separate 
(local) power sources, and are electrically isolated from the remote 
station ICU. 
The communications system protocol is synchronous, half duplex, serial line 
format by which the master ICU of the central station communicates 
bidirectionally, with as many as 60 slave ICU remote station units. The 
serial line protocol is illustrated in FIG. 3, illustrations (a)-(c). The 
master ICU module transmits data to, and receives data from, each of the 
remote slave ICUs in successive transceive cycles 80 (illustration (a)). 
Each cycle includes a sync frame 82 followed by 128 information frames 
divided equally between a transmit interval 84 (master ICU transmits to 
slave ICUs) and a receive interval 86 (master ICU receives from slave 
ICUs). Each information frame is marked by a line clock pulse transmitted 
at the system clock frequency. The sync frame 82 provides master to slave 
ICU synchronization once per cycle. It includes two line clock intervals 
which, when added to the 128 information frame clock pulses, requires 130 
equally spaced line clock intervals for each transceive cycle. 
To provide the highest noise rejection the system frequency and baud rate 
is selected at the lowest frequency required to satisfy the particular 
control application; the bandwidth being limited to compensate for the 
unshielded transmission line. In an exemplary embodiment of the present 
communication system as used in an elevator control system, as disclosed 
in a copending application of the same assignee entitled "ELEVATOR CONTROL 
SYSTEM" U.S. Ser. No. 546,225 filed by Mendelson et al on even date 
herewith, the transceive cycle time is 104 milliseconds (ms) to provide an 
approximate 9.6 Hz transceive frequency (i.e. sample time frequency). For 
the total 130 clock pulses and a selected 104 ms cycle time the line clock 
frequency is 1250 Hz (clock period 800 microseconds). Illustration (b) 
shows the 130 clock pulses as including two sync frame clock pulses 
(S.sub.1, S.sub.2) and 128 information frame clocks divided equally 
between the transmit frame 84 (clock pulses 1 through 64) and receive 
frame 86 (clock pulses 65 through 128). The sync frame clock pulses are 
actually missing. The sync frame itself is defined as the "dead time" 
interval (which includes the missing clock pulses S.sub.1, S.sub.2) 
between the 128th clock pulse of a preceeding cycle and the 1st pulse of a 
present cycle. For the 104 ms cycle time the dead time is 2300 
microseconds. 
The sixty four information frames in the transmit and receive intervals 
service up to a maximum of sixty slave ICUs. The first group of four 
information frames in each interval 88, 90 (clock pulses 1-4 and 65-68) 
are reserved for special command information to all station ICUs, such as 
diagnostic/maintenance testing, or control of any optional features which 
may be incorporated in the remote controlled devices; the remaining sixty 
information frames are data frames. The master ICU transmits information 
to each slave ICU in a related transmit interval data frame and receives 
data from each slave ICU in a corresponding receive interval data frame. 
All remote station slave ICUs receive and store the commands of frames 1-4 
and 65-68 as internal commands related to their operation. These commands 
may include turn on and turn off of the slave ICUs (all or a selected 
number), or may command the slave ICUs to send specific data patterns in a 
diagnostic mode to allow integrity check by the central control. 
Each slave ICU has an assigned clock count address. The line clock pulses 
are counted and decoded by the slave ICUs following each sync frame to 
determine the presence of an assigned count address at which time the ICU 
reads or writes a data frame from or to the transmission line. The format 
for the information frames, both special command frames 88, 90 and data 
frames, are identical, as shown by information frame 92 in illustration 
(c). The frame time interval is divided into eight 100 microseconds 
states. The first state (0-100 microseconds) corresponds to the clock 
pulse interval 94 and must be a minimum of 50 microseconds wide to be 
valid. The second state 95 (100-200 microseconds) is a "dead time" 
interval which allows for response time tolerances and sample time delays 
between the frame clock pulse and the data bits. The next five states 
96-100 (200-700 microseconds) are five signal bit time intervals; the 
first four of which (96-99 correspond to the four data bits D.sub.1 
-D.sub.4. The bit time is equal to the state time, or 100 microseconds for 
the selected 104 ms transceive cycle time. The fifth bit is a special 
feature bit which may be received and transmitted by each of the slave 
ICUs. This fifth bit is used for special feature information which may 
include test routines, i.e. parity test. In the best mode embodiment the 
fifth bit is used to convey the special information in 36 of the 
available 64 information frames in each transmit and receive interval; 
specifically in information frames 5-40. The last state 101 is also a dead 
time interval prior to the beginning of the succeeding data frame. 
As shown in FIG. 3 the signal data format is tristate, i.e. bipolar. The 
transmission line provides a differential, three state signal transmission 
in which the signal, as measured between the transmission line wires 46A, 
46B, is in one of three states. The line 46A is the clock line input to 
the ICUs; the line 46B being the data line input. The three differential 
states are measured with respect to the difference potential between lines 
46A and 46B. When the signal magnitude on the line 46A is greater than the 
sum of the signal magnitude on the line 46B plus a threshold voltage 
(V.sub.th) 104 then the differential state is equal to a line clock pulse 
(94, illustration (c)). When the signal magnitude on the line 46B is 
greater than the sum of the line 46A magnitude plus the selected threshold 
voltage the differential state input is recognized as a logic one in 
signal bit times 96-100. If the line 46A-46B differential magnitude is 
less than the threshold value the differential state is recognized as a 
signal bit logic zero 102. 
The approximate data rate for the selected 104 ms cycle time is 10 KBAUD 
for the four data bits (D.sub.1 -D.sub.4) and special fifth (test) bit of 
each information frame. It should be understood, however, that the present 
system is not limited to either the illustrated baud rate or bit number. 
In the present communications system higher data rates and/or more 
information bits may be traded off against maximum line length and noise 
immunity requirements. 
Referring now to FIG. 4, in a schematic illustration of the ICU modules 60 
(68, 70), each module comprises an ICU 110 together with peripheral 
circuitry described hereinafter. The data transmission lines 46A, 46B are 
presented through input filters 112, 114 to the differential data inputs 
(L.sub.2, L.sub.1) of the ICU module. The filters are typically first 
order RC networks with a time constant dependent on system speed. At the 
cycle time and data rates selected for the exemplary elevator control 
system embodiment each filter time constant is on the order of 2.5 
microseconds (typically 5K ohms and 500 pico farads) which limits common 
mode voltage transients without degrading system data rate. Input power to 
the ICU is a regulated DC voltage (V.sub.DD) provided by voltage regulator 
115 from the power bus. The regulator is a known type preferably a three 
terminal LM78L08 or LM317L. 
The ICU input data at L.sub.1, L.sub.2 is received by differential 
comparators 116, 118 which provide in combination a differential line 
receiver. The data and line clock threshold voltages Vth (104, FIG. 3 
illustration (c)) which the receiver uses to sense a clock pulse at 
L.sub.2 with respect to the data input L.sub.1, and to sense the data bits 
at L.sub.1 with respect to L.sub.2 are differential voltages; the presence 
of a DC common mode voltage will not affect the threshold set point. 
Typically, the Vth threshold is equal to one-half the minimum voltage 
swing on the transmission line (or the minimum line current) (Io .sub.MIN) 
multiplied by the minimum line impedance (Z.sub.L); typically 0.5 to 0.6 
volts. The signal outputs of each comparator are logic zero whenever the 
L.sub.1 and L.sub.2 inputs are less than Vth. 
The output signals from each comparator are presented to digital filters 
120, 122. For the selected system data rates the filters are preferably 
four bit digital filters with a sample rate of 8.9 microseconds. The 
filters use a best three out of four sample averaging algorithm before 
allowing the filter output to change states. The signal output from the 
line clock (L.sub.2) filter 120 is presented simultaneously to address 
select/recognize logic circuitry 124, control unit 126, and through output 
buffer 128 to the serial data clock output from the ICU (SCLK) which may 
be used for peripheral equipment. The signal data from the L.sub.1 filter 
122 is presented on line 129 to a serial data input of I/O shift register 
130; a multifunction five bit shift register with dual serial-to-parallel 
modes. The serial data received on line 129 is parallel formatted by the 
register and presented to the ICU data bus 132 which interconnects the I/O 
register 130 with: a command register 134, an output register 136, and the 
fifth bit I/O logic circuitry 138. The fifth bit logic circuitry is used 
to transmit and receive fifth bit information to the associated remote 
devices, as necessary. The I/O shift register 130 is interconnected with 
the fifth bit I/O logic circuitry through control lines 139, 140, and 
receives command information from the control unit 126 on lines 142. The 
control unit also provides command information to: the ICU data 
transmitter circuitry 143 (which includes AND gate 144 and line driver 
146), to the fifth bit I/O logic circuitry 138, to the ICU master/slave 
logic circuitry 148, and to the ICU output register 136. 
In the preferred embodiment the control unit is sequential; providing a 
series of ordered, chronological commands within each information frame. 
The control algorithm commands are marked by a control unit clock having a 
higher frequency than the line frequency clock. The exact control unit 
frequency is selectable; depending upon the number of sequence steps 
involved. Typically, the control unit provides a sequence of 17 command 
instructions, and the control clock pulses are provided from the 
oscillator and clock divider circuitry 152 on lines 154 at a frequency on 
the order of 20 KHz for the selected 104 ms cycle time. 
In the operation of the ICU the received line clock pulses on line 121 are 
counted by address circuitry 124 and compared with the ICU's assigned 
address, as programmed by the multibit address circuitry input (J.sub.1 
-J.sub.Q). The address input is either fixed (for a slave ICU) or dynamic 
(for master ICU). The master/slave status is set by logic circuitry 148; 
for a master ICU in the central station (18, FIG. 1) the SLV input 150 is 
set at logic zero and for a slave ICU the SLV input is a logic one. For 
the master ICU of the central station the J.sub.1 -J.sub.Q inputs are 
connected through address lines 156 to the signal processor of the central 
control (12, FIG. 1) to allow the central control to change the master ICU 
address to allow it to access specific information (data) frames during 
I/O transfer. In a slave ICU the address inputs may be fixed encoded 
through connection of individual inputs to signal ground or VDD to provide 
the selected binary address. 
By keeping track of the input address count the ICU address recognition 
logic 124 differentiates between ICU read and write cycles; these two 
cycles are reversed in master and slave ICUs with respect to line clock 
count. The slave ICUs read command information from the master in the 
first four information frames of master transmit interval (84, FIG. 3), 
following the sync frame (82, FIG. 3). A fifth data bit (100, FIG. 3(c) is 
transmitted from the master ICU to the slave ICUs during each information 
frame of the master transmit interval (84, FIG. 3). The slave ICUs may 
multiplex the fifth data bit of information frames 4-40 to the output of 
bits I/O logic circuitry 138 (T.sub.T, FIG. 4) under command. All slave 
ICUs read master data in the master transmit interval information frames 
5-64. The master reads each of the slave ICU data outputs in the master 
receive interval (86, FIG. 3) on line clocks 68-128; no data is written by 
the slaves in addresses 65-68, which are the master ICU read frames for 
addresses 1-4. In event that the address recognition circuitry detects an 
addressing error, e.g. more than 128 line clock pulses, the ICU 
transmitter 143 is disabled and a "loss of sync" is signaled on ICU 
(LSYNC) output 158. This occurs in both master and slave ICUs and the loss 
of sync signal state persists until a new sync frame is detected. 
The command data in the first four information frames is read by the ICU 
command register from the ICU data bus 132. A slave ICU receives the 
command data from the transmission line 46 through the serial input of 
register 130; the master ICU receives the command data at parallel inputs 
I.sub.l -I.sub.p 160 of the register 130, from lines 162 and the user 
system central control. The function of each of the command bits, the four 
data bits and special fifth bit, are established based on the user system 
requirements. However, at least one bit (the most significant bit) is used 
for parity indications. 
Aside from command frame inputs the ICU detects its assigned address from 
the clock count and latches the data frame from line 129 onto data bus 
132. The slave ICU transmitted data (master receive interval 86, FIG. 3) 
is the data received by the slave ICU at inputs 160 from the associated 
remote devices (14, 16, FIG. 1) on lines 162. The data is latched into 
bits 1-4 of I/O register 130 during the master transmit interval 84, FIG. 
3. The fifth data bit (if present) is loaded from the fifth bit I/O logic 
circuitry 138. At the slave ICU transmit address state register 130 shifts 
the five bit information frame serially through line 131 to AND gate 144, 
which controls the line driver 146 of the ICU transmitter 143. The control 
unit 126 provides a gate enable signal on lines 142 such that a data logic 
one turns line driver 146 on and a logic zero turns it off. The ICU data 
is transmitted through ICU XMT output 164 and steering diode 166 to the 
transmission line 46B. The diode 166 allows the ICU to "source" current to 
the transmission line during transmission of logic one bits, but prevents 
any "sinking" of current when the transmission line is more positive than 
the XMT output. This prevents ICU latch-up. 
ICU data transmission is single ended with respect to ground, e.g. line 
46B. Therefore, the termination networks are an integral part of the ICU 
transmitter, providing a ground return for the sourced logic one currents 
on the line. The ICU line receiver (comparators 116, 118) is differential, 
such that the receivers provide the common mode noise rejection. 
Latching of the data on at inputs 160, shifting transmission line data from 
filter 122 to I/O register 130, and shifting either data from register 130 
to the data bus 132, is provided by sequence instructions from the control 
unit 126. Similarly the control unit allows output register 136 to use I/O 
register 130 in a serial in/parallel out mode to shift transmission line 
data from line 129 to the ICU data bus 132. Four data bits are parallel 
loaded from the bus into the output latch of the output register as ICU 
outputs O.sub.1 -O.sub.p (LSB to MSB). The fifth data bit is presented 
through I/O logic circuitry 138 to the T.sub.T output 167 of the master 
ICU in any of the master receive interval information frames, and to the 
T.sub.T output of the slave ICUs, on command, in frames 4-40 of the master 
transmit interval. The ICU output bits (O.sub.1 -O.sub.p) are presented 
through line drivers 168-170 to I/O lines 64, (73, 77) to the associated 
user system equipment (e.g. central control or remote devices). 
The slave ICU peripheral elements include crystal (XTL) 172 connected 
between the XTL input 174 to ICU oscillator 152, and line 176 to the 
regulated V.sub.DD voltage. The crystal provides a typical 3.58 MHZ signal 
to the oscillator. The master ICU XTL input 174 is connected to the line 
clock driver output of the I/O interface with the central control 
circuitry. 
The comparators 116, 118 of the ICU line receiver are negatively biased by 
a DC voltage signal at the V.sub.EE input 178. The bias is provided on 
line 180 from V.sub.EE charge pump 182. The charge pump, which is also 
connected to a XTL/2 signal frequency from the ICU oscillator 152 at BIAS 
output 184, includes series capacitor 186 connected through a pair of 
oppositely polled, parallel diodes 187, 188 to opposite sides of a second 
capacitor 189. The diode 188 and capacitor 189 are connected to signal 
ground. The capacitors, approximately 0.01 microfarads each, in 
combination with the diodes invert and rectify the 1.78 MHz BIAS output to 
produce an approximate -6.0 VDC at 1.0 milliamp to each comparator. This 
negative bias increases the comparator's negative common mode range to 
nominally center the comparator inputs and provide optimal common mode 
range to the differential input signals on the transmission lines 46A, 
46B. 
In a serial data communication system of the present type there are three 
major sources of signal-to-noise (SNR) degradation. These include (i) 
signal attenuation due to the length of the transmission line (1,000 ft, 
305 meters maximum), (ii) signal reflections due to mismatched impedances 
(both characteristic line impedance and I/O impedance of ICU), and (iii) 
common mode noise due to lack of transmission line shielding. In the 
present system the twisted pair transmission line is preferably stranded, 
to allow ease of installation and lower parasitic capacitance. For 18 AWG 
to 24 AWG wire sizes the characteristic impedance varies from 90 to 120 
ohms per 1,000 feet. With 10K baud data rates the signal attenuation is on 
the order of 0.25 db/100 feet, or 2.5 db for a 1,000 foot transmission 
line. A logic one signal of 2.5 V transmitted from one end of a 1,000 foot 
line arrives at the other end at 1.87 volts, neglecting signal 
reflections. The effect is negligible, and with proper selection of the 
V.sub.th threshold levels the line attenuation effect is invisible to data 
transmission. 
Signal reflections due to laod mismatch between remote ICU stations (or due 
to the remote station line taps from the transmission line) are dampened 
by the termination networks at each end of the transmission line. The 
termination network impedance is set equal to the transmission line 
characteristic impedance Zo (determined by wire size). The transmission 
line load impedance Z.sub.L is the parallel combination of the termination 
network impedance and the I/O impedances of all ICUs connected to the 
line. The ICUs are preferably CMOS (Complementary Metal Oxide 
Semiconductor) IC devices. The ICU input impedance (ICU line receiver) is 
on the order of 100K ohms and the ICU output impedance (the tristate line 
driver, 146, FIG. 4) is on the order of 5M ohms. For a maximum of 60 
remote stations the equivalent I/O impedance is approximately 800 ohms. 
This value, in parallel with the termination network impedance (equal to 
the characteristic impedance Zo of the line, e.g. approximately 100 ohms, 
results in a load impedance of approximately 88.4 ohms. The line 
reflection coefficient is .rho..sub.v =(Z.sub.L -Z.sub.O)/(Z.sub.L 
+Z.sub.O), where Z.sub.L is the load impedance (88.4 ohms) and Z.sub.O is 
the characteristic line impedance (100 ohms), is 0.062v/v. A 2.5 V pulse 
started at one end of the line would result in 0.16 volts being reflected 
back to the source ICU and 2.34 volts would reach the termination. The 
reflection coefficient is negligible and it too may be accounted for by 
selection of the ICU threshold voltages (V.sub.TH). 
Signal reflections due to the remote station taps off of the transmission 
line are also negligible, since: (i) the characteristic impedance of the 
station tap is the same as the main transmission line, (ii) the twisted 
pair cable is not untwisted for cable lengths greater than one-quarter 
wavelength, and (iii) the tap length are typically three orders of 
magnitude less than one-quarter wave length. 
Common mode voltage noise sources could be signals propagating close to the 
transmission line, such as the power distribution line, or other control 
voltage signals associated with the user system. They may also be 
fluoresent lights or electric motors. For 60 Hz noise sources the 
transmission line termination networks and input RC filters on each ICU 
line receiver limit the common mode voltages to approximately one 
millivolt. For higher frequencies noise sources, such as motors or relays 
which produce a broad band spectrum of noise as high as five to ten MHz, a 
significantly higher common mode noise level appears at the ICU inputs. 
For example a 10 MHz capacitively coupled noise, the common mode voltage 
signal could be as high as 290 V for a 300 V noise amplitude, if line 
attenuation is neglected. In reality the magnitude of these high frequency 
common mode signals is considerably smaller due to second order effects 
caused by nonlinear processes in the noise source itself. In addition, 
transmission line signal attenuations significantly reduce the high 
frequency common mode signals, such that their common mode amplitude is 
limited a few volts as opposed to hundreds of volts. Finally, the 
differential comparators used in the ICU line receiver have common mode 
rejection ratios (CMRR) on the order of 60 db so that the perceived common 
mode signal from the high frequency sources would be reduced to the 
millivolt level. 
As indicated hereinbefore the central station master ICU and remote station 
slave ICUs are each tapped off of the transmission line 46 through quarter 
wave length, or less, connections, thereby limiting signal reflections 
back to the transmission line. In the preferred embodiment the central 
station with master ICU is shown connected at one end of the transmission 
line; the remote station ICUs distributed along the transmission line 
length between the master ICU and the far termination network. In this 
embodiment the close proximity of the master ICU to a line termination 
network allows the master ICU to be connected through a simple quarter 
wave tap. It should be understood that the master ICU need not be 
connected only at an end of the transmission line. However, if connected 
at other than an end of the transmission line the master ICU itself would 
require a termination network in addition to those connected at each end 
of the transmission line. 
Although the invention has been shown and described with respect to a best 
mode embodiment thereof, it should be understood by those skilled in the 
art that the foregoing and various other changes, omissions and additions 
in the form and detail thereof may be made therein without departing from 
the spirit and scope of this invention.