Universal PBX interface

A universal PBX-to-central office interface for standards conversion is described for supporting normally incompatible telephone equipment in PBX environments. The present invention is adapted for connection between the central office and a PBX along DID and DOD trunk lines. The present invention will capture incomping signaling and data in any one of three dialing modes (DTMF, MF and pulse dial) and convert this standard to whatever dialing mode is required by the equipment attached to the interface. The present invention will also do signaling conversion for peripheral equipment attached to PBX systems such as voice mail systems. The interface is capable of capturing extension signals transmitted from the central office to the PBX and saving them in memory for later downloading to a voice mail system to recover the extension of the dialed party after it has been forwarded to the voice mail system.

FIELD OF THE INVENTION 
The present invention relates to the field of PBX technology and in 
particular to PBX-to-central office interface systems for DID (direct 
inward dialing) and DOD (direct outward dialing) trunk lines to support 
signaling translation, voice mail systems and the like. 
DESCRIPTION OF THE PRIOR ART 
Three different signaling standards are available for use in PBX (private 
branch exchange) systems; dial pulse, DTMF (dual-tone multi-frequency) and 
MF (multi-frequency) signaling. A PBX user may subscribe to the telephone 
company and lease a number of DID (direct inward dialing) and DOD (direct 
outward dialing) lines for connection to a purchased or leased PBX system. 
The PBX system is generally tailored to recognize signaling with only one 
of the three available signaling modes. The leased lines must therefore be 
compatible with this mode of signaling (for example, DTMF, DID and DOD 
lines from DTMF signaling central office equipment compatible with a DTMF 
signaling PBX). It may be desirable, however, to connect an existing PBX 
system which recognizes one mode of signaling to a number of DID and DOD 
trunks which operate with another mode of signaling for reasons such as 
cost, availability, etc. In particular, a number of dial pulse PBX systems 
exist in older businesses which may, for various reasons, need to be 
connected to more modern DTMF signaling trunks. Another example may be the 
upgrading of portions of a PBX system using modern equipment which only 
uses DTMF signaling (such as voice mail systems) to older style PBX 
systems and DID/DOD lines, which only recognize dial pulse signaling. This 
type of incompatibility between the different modes of signaling prohibits 
many users from adapting different modes of equipment within the same 
system. 
A further example of this incompatibility problem is voice mail systems 
adapted to PBX systems. Most voice mail systems available today operate 
with DTMF signaling. When connected to an older style PBX system which 
operates on dial pulse signaling or to a central office which uses MF 
(multi-frequency) signaling, the incompatibility of the signaling modes 
may render the system inoperative. 
A further problem with voice mail systems and other telephone peripheral 
equipment adaptable to attachment to PBX systems is the identification of 
the telephone extension of the incoming call. In PBX environments, a 
common voice mail receiving station is typically connected to receive 
calls from the entire PBX system. In many applications, this connecting 
structure results in a bothersome problem for users of the voice mail 
answering service. This problem is caused by the manner in which the PBX 
system handles incoming calls. The central office downloads the extension 
of the called party on a DID trunk and the PBX system receives the 
extension number (transmitted using one of the three aforementioned 
dialing modes) to route the call. After the call has been routed to the 
appropriate extension, the extension telephone number of the incoming call 
is no longer needed and hence is lost. If, however, the called extension 
is busy or unavailable, the call is routed to the voice mail system. When 
the call is received by the voice mail system, the extension number 
originally dialed by the caller is no longer available and the voice mail 
service must ask the caller for the extension which was originally dialed 
so that the recorded messge may be later forwarded to the correct 
extension. This is a troublesome problem in the application of voice mail 
systems to PBX systems. 
SUMMARY OF THE INVENTION 
The present invention solves the aforementioned problems and other problems 
that will be recognized by those skilled in the art upon reading and 
understanding the present specification. The present invention is directed 
toward a universal PBX-to-central office interface for standards 
conversion for supporting normally incompatibile telephone equipment in 
PBX environments. For example, the present invention will adapt older 
style dial pulse PBX systems to more modern DTMF signaling DID and DOD 
trunks. The present invention will also capture incoming extension 
addresses on DID trunks transmitted in any dialing mode for later 
downloading to voice mail systems in whatever dialing mode is needed for 
reception. Those skilled in the art will readily recognize the many 
applications of the present invention for standards conversion in 
supporting of a wide variety of telephone equipment in PBX environments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the following detailed description of the preferred embodiment, 
reference is made to the accompanying drawings which form a part hereof, 
and in which is shown by way of illustration a specific embodiment in 
which the present invention may be practiced. This preferred embodiment is 
described in sufficient detail to enable those skilled in the art to 
practice the invention, and it is to be understood that other embodiments 
may be utilized and that structural or electrical changes may be made 
without departing from the spirit and scope of the present invention. The 
following detailed description is, therefore, not to be taken in a 
limiting sense, and the scope of the present invention is defined by 
appended claims. 
The preferred embodiment of the present invention is an apparatus for 
connection between the DID or DOD trunk lines to the central office and a 
PBX system. This system is designed to operate as a standards conversion 
interface between the DID or DOD trunk lines (using either wink start or 
immediate start) and the PBX system. The present invention also supervises 
the line and stores the digits of the called extension on a DID trunk for 
later downloading to other telephone equipment attached to the PBX such as 
voice mail systems. 
For the purpose of a better understanding of the present invention, a brief 
description of the handshake protocol typically used on DID and DOD lines 
is included here. PBX systems using DID and DOD trunk lines allow 
disproportionate ratios of the incoming and outgoing lines so that, for 
example, a business may handle a large number of incoming calls while only 
needing a small number of output lines to handle outgoing calls. Thus, for 
example, the business may have 100 DID lines and only 10 DOD lines. The 
DID lines are unassigned until an incoming call to the PBX is routed by 
the central office. One of the plurality of DID lines is selected by the 
central office and the call is routed to the PBX for final connection. The 
same system is used on DOD lines. The trunk signaling between the central 
office and the PBX on the DID and DOD lines most often used is loop 
reverse battery, more commonly called wink start protocol. A less common 
form of signaling is immediate start, in which the extension digits are 
downloaded at a specific time after the central office seizes the trunk. 
Referring to FIG. 8, the prior art description of the wink start protocol 
is described for a DID trunk (identical to the protocol used on a DOD 
trunk). Central office seizes one of the DID trunk lines 736 by placing a 
DC load impedance across the trunk line. Since the PBX sources the talk 
battery voltage on a DID trunk (the central office sources talk battery on 
a DOD trunk), a current loop is created between the tip and ring wires of 
the trunk line. The current flowing through this loop is immediately 
sensed by the PBX. After an approximate 100millisecond wait period 738, 
the PBX sends an approximate 200-millisecond wink 740 in the form of 
reversing the battery voltage between the tip and ring wires of the trunk 
line. This signals the central office that the PBX is ready to receive the 
in-house extension for the incoming call. After sensing the 200 
millisecond wink, the central office waits for an additional 30 
milliseconds after the wink has completed 742. The central office then 
sends the digits for the in-house extension 744 and the PBX connects the 
trunk line to the appropriate extension 746. This completes the call. 
In the foregoing description of the wink start handshaking between the 
central office and the PBX, it can be seen that the extension of the 
called party is transmitted by the central office only once and the PBX 
connects the extension after receiving the digits. The vast majority of 
PBX's currently available on the market do not store the incoming digits, 
but rather use them directly to make the connection. The extension number 
of the party called is then immediately discarded by the PBX, since the 
design of these PBX's recognizes no further need to store these digits. 
For a better understanding of the application of the present invention to 
peripheral telephone equipment such as voice mail systems, a brief 
description of the operation of a voice mail system in a PBX environment 
is described here. When an incoming call is directed by the PBX to an 
extension, the extension is either answered or directed to a voice mail 
system for automatic answering. When the call is directed to a voice mail 
system, the voice mail system is unaware of the extension number of the 
called party to which the DID trunk was connected. Thus, when the voice 
mail system is invoked, the voice mail device must query the caller as to 
the extension number of the called party. This is a cumbersome method of 
identifying the called party for the voice mail system, since all of the 
incoming calls to the voice mail system come from unknown sources. 
Referring to FIG. 1, the preferred embodiment of the present invention is 
described. Reference number 100 generally refers to the universal 
PBX-to-central office interface. Although the preferred embodiment of the 
present invention is described in terms of a DID-to-PBX interface, those 
skilled in the art will readily recognize that the present invention is 
adaptable to performing the function of a PBX-to-DOD interface without any 
modifications. All handshaking and protocol dealing with a DOD connection 
from a PBX is identical to the handshake protocol described here. All that 
is required is that the interface be installed in the opposite direction 
than the DID installation direction. Thus it is manifestly intended that 
the present invention not be limited to inward dialing trunks only. 
Network separation relay circuit 101 is shown as a simple relay connection 
between the DID network and the PBX. As will be shown in detail below, the 
operation and structure of the network separation relay circuit 101 is 
more complicated than is shown in FIG. 1. However, for purposes of this 
discussion it functionally operates as shown. The network separation relay 
circuit 101 serves to disconnect the DID network from the PBX during 
handshaking and signaling. After the appropriate connections between the 
central office and the called extension are established, the network 
separation relay will close to allow voice communication between the 
caller and the called party. 
A relay interface 110 allows microcontroller 108 to control the relay 
control circuits of the network separation relay circuit 101, the polarity 
reversal relay circuit 102, the termination impedance circuit 103 and the 
line interface circuit 114. Each of the afore-said circuits of FIG. 1 
contain relay contacts which under microprocessor control will perform the 
functions of the interface 100. 
Polarity reversal relay circuit 102 allows the interface 100 to send a 
polarity reversal wink to the DID line to inform the central office that 
the interface recognizes that the central office has seized the DID trunk 
and is requesting to send the digits of the dialed extension. The high 
impedance monitor circuit 104 serves to isolate the DID line from the 
receiver circuits 105, 106 and 107 of interface 100. The dial pulse wink 
and line seizure detector circuit 105 serves to receive dialed digits 
transmitted in the pulse dial mode. The detector circuit 105 also 
recognizes when the central office seizes the line and when the wink 
protocol has been properly invoked by the PBX. 
The DTMF receiver 106 and MF receiver 107 serve to receive the transmitted 
digits of the dialed extension in either dual tone multi-frequency or 
multi-frequency modes, respectively. Circuits 105, 106 and 107 are 
connected to microcontroller circuit 108 and are controlled directly 
therefrom. The digits received from circuits 105, 106 or 107 are 
transmitted to micro-controller circuit 108 in the form of binary digits 
and are stored in internal memory of the microcontroller circuit 108. The 
binary digits of the dialed extension are then converted by circuits 111, 
112 or 113 to the dialing standard used by the PBX or used by the 
telephone equipment peripherals attached to the PBX, such as voice mail 
systems (which almost exclusively use DTMF signaling). It is possible that 
the PBX and the telephone peripherals connected to the PBX operate on 
different dialing standards and hence microcontroller circuit 108 will 
transmit the information according to the standards recognized by the 
receiving device. 
Dial pulse sender 111 will transmit the dialed digits to the PBX system or 
the telephone peripherals using the dial pulse standard if so required. 
The DTMF sender circuit 112 and the MF sender circuit 113 will likewise 
transmit the dialed digits in the form of either DTMF or MF tones. Line 
interface circuit 114 serves to allow circuits 111, 112 and 113 to 
transmit information codes onto the tip and ring wires of the PBX line 
through the appropriate impedance matching interface circuits. The line 
interface circuit 114 also contains relays controlled by the relay 
interface circuit 110 to connect or disconnect the line interface circuit 
during operation of interface 100. 
When the network separation relay circuit 101 opens the line between the 
DID network and the PBX, polarity reversal relay circuit 102 allows the 
minus 48 volts DC talk battery voltage to be applied to the DID line so 
that the central office maintains the line active by keeping the line 
terminated. In a similar fashion, termination impedance circuit 103 must 
be kept across the tip and ring wires of the line to the PBX so that the 
PBX does not drop the line. Thus when network separation relay circuit 101 
is open, the interface 100 appears to the central office to be a PBX 
sourcing battery voltage and the interface 100 appears to be a central 
office to the PBX by maintaining a termination impedance across the line. 
The microcontroller circuit 108 then operates the various associated 
circuits of interface 100 to perform the handshaking between the central 
office and the PBX so that, in effect, the central office sees the 
interface 100 as a PBX and the PBX sees the interface 100 as a central 
office. 
The microcontroller circuit 108 of the present invention, along with the 
associated circuits shown in FIG. 1, is capable of supporting peripheral 
telephone equipment attached to the PBX, in addition to performing its 
interface functions. For example, one primary function that the interface 
100 of the present invention is capable of performing is storing the 
digits of the dialed extension in the memory of the microcontroller 
circuit 108 for later re-transmitting or downloading to a voice mail 
system after the call has been routed from the dialed extension to the 
voice mail extension for answering. As was discussed above, when a call is 
routed from a dialed extension to the voice mail system, the voice mail 
system is unaware of the intended extension of the incoming call. Thus, if 
the voice mail system requests that the interface 100 re-transmit the 
digits of the dialed extension in DTMF mode, the voice mail system can 
recover the dialed extension without requesting that the caller provide 
this information. 
FIG. 2 is a detailed electrical schematic diagram showing the 
implementation details of the network separation relay circuit 101, the 
polarity reversal relay circuit 102, the minus 48 volts DC battery voltage 
circuit 109, the termination impedance circuit 103 and the line interface 
circuit 114. Referring to FIG. 2, the tip and ring connections to the DID 
network of the central office and the tip and ring connections to the PBX 
are over-voltage protected by surge protectors 201 and 203, respectively. 
A plurality of relay contacts K1A-K6A and K1B-K6B are shown with the 
interface in an idle, active state. In other words, the relay contacts are 
positioned such that the interface is sourcing battery voltage to the tip 
and ring wires of the DID trunk in a ready-to-receive state while the 
connection to the PBX is held open, indicating no call in progress. Minus 
48VDC talk battery voltage is sourced through battery source chip 202 
which in the preferred embodiment is part number LB1011AB, available from 
AT&T. This is a commonly available integrated circuit which is used in the 
telephone industry as an electronic battery feed circuit which supplies DC 
currents to a telephone line with minimal loading on the AC signals. This 
battery feed chip 202 is connected according to manufacturer's 
specifications and supplies a -48VDC to the telephone line from a -54VDC 
source. Those skilled in the art will readily recognize how to generate 
the appropriate the -54VDC source. Those skilled in the art will also 
readily recognize that other techniques may be used for sourcing the 
-48VDC talk battery to the telephone line in an equivalent fashion to the 
technique shown here. 
The -48VDC supply from battery source chip 202 is supplied to the ring 
connection through relay contacts K6B and K4B. The common return or ground 
path for the battery source voltage is through relay contacts K6A and K4A 
to the tip connection of the DID network line. The interface 100 receives 
information from the DID network by means of connections B and C which are 
shown to be connected to the appropriate signaling, transmitting and 
reception circuits found in FIGS. 3 through 6. In the present idle-ready 
state configuration of the relays of FIG. 2, the B and C connections 
monitor the tip and ring connections of the DID line through relay 
contacts K1A and K1B. Relay contacts K2A, K2B and K3A, K3B are shown open, 
indicating that the connection between the tip and ring lines of the DID 
trunk are open, or not connected, to the tip and ring lines of the PBX 
line. 
Relay connections K1A, K1B and K2A, K2B are controlled such that their 
positions are always the opposite of one another. In other words, when 
relay contacts K1A, K1B are closed, relay contacts K2A and K2B are open. 
In this fashion, the monitor contacts B and C connected to the circuitry 
of FIGS. 3 through 6 are always connected to either monitor the tip and 
ring wires of the DID trunk or the tip and ring wires of the PBX line, but 
never both or neither. The details of the control of the K1 and K2 relay 
control circuits is described in conjunction with FIG. 6, below. 
In a similar fashion, the relay contacts K6A, K6B and K5A, K5B are always 
positioned opposite one another. These relay contacts serve to create the 
reverse battery voltage condition or wink signal on the DID trunk. In the 
relay positions shown in FIG. 2, normal battery voltage polarity is 
sourced between the tip and ring wires of the DID trunk through normally 
closed relay contacts K6A and K6B. If, however, the interface 100 needs to 
create a wink signal on the DID trunk, relay contacts K5A and K5B will 
close simultaneous with the opening of relay contacts K6A and K6B. Thus, 
the polarity of the -48VDC talk battery would be reversed when the relay 
contacts are reversed. The control circuit for relays K5 and K6 are 
discussed below in conjunction with FIG. 6. 
After all signaling, monitoring and other supervising operations of the 
interface 100 have been completed, and the telephone call is cut through 
for voice communications, relay contacts K3A and K3B will close to 
directly connect the tip and ring lines of the DID trunk with the tip and 
ring lines of the PBX line and the PBX will assume supervision (such as 
sourcing the talk battery voltage). 
Referring to the right of FIG. 2, relay contacts K8A and K8B serve to 
connect a 330 ohm termination impedance between the tip and ring wires of 
the PBX line. This termination impedance is necessary to indicate an 
incoming call, to hold the PBX line active while the connection between 
the DID trunk and the PBX is held open and to perform pulse dial 
signaling. The PBX line is held active with a 330 ohm impedance to make 
the interface appear to be the central office from the PBX frame of 
reference. In a similar fashion, the -48VDC battery source supplied by the 
interface to the DID trunk makes the interface appear to be a PBX from the 
central office frame of reference. 
Relay contacts K7A and K7B serve to connect the impedance matching 
transformer T1 to the tip and ring wires of the PBX line. Through this 
transformer T1, the DTMF and MF signaling and data can be transmitted to 
the PBX through connection point A. Connection point A is shown connected 
in FIG. 4. The primary of transformer T1 is over-voltage protected with 
Zener diodes to protect from back EMF. The secondary of transformer T1 is 
designed to present a 900 ohm impedance to the PBX line. Those skilled in 
the art will readily recognize that transformer T1 could optionally be 
designed to have a tap connected through a switch to one of the tip and 
ring leads so that the line matching impedance of transformer T1 could be 
field selectable to be, for example, 600 or 900 ohms matching impedance. 
FIG. 3 shows detailed electrical schematic diagrams of a portion of the 
high impedance monitor circuit 104, and the dial pulse, wink and line 
seizure detect circuit 105. Operational amplifier 301 in the preferred 
embodiment is connected in differential mode operation with the inverting 
input connected to point B (tip) of FIG. 2 and with the non-inverting 
input connected to point C (ring) also of FIG. 2. The high input impedance 
of operational amplifier 301 serves to present a very high impedance to 
the tip and ring wires of the DID trunk. Operational amplifier 301 is in 
the preferred embodiment part number LM1558 available from National 
Semiconductor and other vendors. Zener diodes connected around operational 
amplifier 301 serve as over-voltage limiting. External resistors to 
operational amplifier 301 serve to select a gain at approximately 
one-fifth, thus attenuating the incoming signals. 
The output of operational amplifier 301 is connected to the non-inverting 
input of operational amplifier 303. This amplifier is also in the 
preferred embodiment part number LM1558 available from National 
Semiconductor. Operational amplifier 303 is also over-voltage protected 
with Zener diodes. External resistors select a gain of this amplifier to 
be approximately 3.3. This amplifier is designed to detect voltage 
reversals on the tip and ring wires of the DID trunk to detect the wink 
signal. The output of operational amplifier 303 is buffered through a 
tri-state digital buffer 305 which in the preferred embodiment is part 
number 74HC244 available from Texas Instruments and other vendors. Buffers 
305, 306 and 307 are found in the same package and thus the tri-state 
outputs are commonly connected to and controlled by port P3.7 to control 
the data from the aforesaid buffers to the I/O ports of microcontroller 
chip 501 of FIG. 5 (discussed below). Thus, the output of operational 
amplifier 303 buffered through the digital CMOS gate 305 creates a digital 
wink detect signal sensed by microcontroller chip 501 through port P0.5. 
The output of operational amplifier 301 also drives the non-inverting input 
to comparitor 302. Comparitor 302 is in the preferred embodiment part 
number LM311 available from National Semiconductor and other vendors. This 
operational amplifier serves as a threshold detect with the inverting 
input connected to a voltage divider network to select the threshold of 
the comparitor 302. The output of comparitor 302, pulled up by a pull up 
resistor, drives digital CMOS tri-state buffer 306 and serves to detect 
voltage thresholds between the tip and ring wires of the DID trunk to 
indicate dialed pulse signaling and off-hook conditions. This signal is 
input to port P0.6 on microcontroller chip 501 and indicates the status of 
the DID trunk. 
The output of operational amplifier 303 also drives the non-inverting input 
of comparitor 304 which in the preferred embodiment is also part number 
LM311 available from National Semiconductor. The inverting input of 
comparitor 304 is connected to a voltage divider network to select the 
threshold of the comparitor 304. The output of comparitor 304 is pulled up 
with a pull up resistor and drives digital CMOS tri-state buffer gate 307 
to present the PBX disconnect signal to port P0.7 of microcontroller 501. 
All tri-state buffer gates 305, 306 and 307 of FIG. 3 are commonly 
controlled from port 3.7 so that the microcontroller 501 can activate 
these signals when the microcontroller pulls the status of the DID trunk. 
The output of comparitor 304 also drives point E which connects to an 
optional status indicator shown in FIG. 6. 
FIG. 4 shows the detailed electrical implementation of a portion of the 
high impedance monitor circuit 104, the DTMF receiver circuit 106, the 
DTMF sender circuit 112, the MF receiver circuit 107, the MF sender 
circuit 113 and a portion of line interface circuit 114. The DTMF receiver 
circuit 106 and the DTMF sender circuit 112 are commonly implemented in 
integrated circuit 401 which in the preferred embodiment is a DTMF 
transceiver part number MT8880 available from Mitel Corporation. This 
integrated circuit is shown capacitively coupled between points B (tip) 
and C (ring) of FIG. 2. DTMF transceiver 401 is connected according to 
manufacturer's specifications and serves to decode received DTMF signals 
into a four bit binary code presented on output data lines D0-D3. The DTMF 
transceiver 401 also serves to receive four bit binary codes on lines 
D0-D3 and produce the DTMF tones on the TONE output. Integrated circuit 
401 is connected to microcontroller chip 501 of FIG. 5 to the appropriate 
I/O ports selected for data and control. Data lines D0-D3 are connected to 
ports P1.1-P1.3 with the control lines connected to ports P2.1, P3.2-P3.5. 
External capacitors and resistors are selected according to gain and 
frequency response characteristics as suggested by the manufacturer. 
The output of DTMF transceiver chip 401 is capacitively coupled to a 
summing point connected to the inverting input of operational amplifier 
403. This amplifier in the preferred embodiment is part number LM1558 
available from National Semiconductor and other vendors. This amplifier 
serves primarily as a 1:1 gain buffer to sum the tone outputs of DTMF 
transceiver chip 401 and MF transmitter chip 404 (described below). The 
output of operational amplifier 403 drives point A of the line interface 
portion in FIG. 2. Thus, the tone outputs of DTMF transceiver chip 401 or 
MF transmitter chip 404 drive the PBX line. 
Also connected between points B (tip) and C (ring) in FIG. 2 is operational 
amplifier 406 capacitively coupled and connected in differential mode. The 
gain of this operational amplifier is unity to buffer the incoming tone 
signals for MF receiver chip 402. Operational amplifier 406 is also 
over-voltage limited by Zener diodes. 
MF receiver chip 402 in the preferred embodiment is part number SSI207 
available from Silicon Systems, Inc. This commonly available MF 
transceiver chip is designed to decode multi-frequency tones into 4-bit 
binary codes. The binary codes corresponding to the decoded tones are 
placed on output lines D0-D5 and presented to I/O ports P1.0-P1.5, 
respectively, of microcontroller chip 501. As those skilled in the art 
will readily recognize, the data bus outputs of chips 401, 402 and 405 are 
tri-state controlled for time-multiplexed connection to the I/O ports of 
microcontroller chip 501. The microcontroller chip 501 uses control lines 
on the various chips to control the tri-state selection such that only one 
chip is driving the common I/O ports at a given time. 
Control lines for MF receiver chip 402 are also connected to various I/O 
ports of microcontroller chip 501 as shown in FIG. 4. Unlike the shared 
data bus lines, the control lines for chips 401, 402, 404, 405, etc. must 
be attached to dedicated I/O ports, since the sharing of control lines is 
difficult to time multiplex and would require additional hardware. 
DTMF transceiver chip 401, MF receiver chip 402 and MF transmitter chip 404 
all operate from a 3.58 MHz crystal. The designers of these chips 
recognize that more than one of these chips may be connected in a single 
circuit and hence to save board space and cost, allow the chips to be 
daisy chained and controlled from a single crystal. Thus, chip 402 is 
shown connected to a 3.58 MHz (an inexpensive NTSC color subcarrier 
frequency crystal) and the CLK pins of chips 401, 402 and 404 are commonly 
connected through point D. Thus, all three of the aforementioned chips 
share this single 3.58 MHz crystal. 
MF transmitter chip 404 receives four bit binary codes through data ports 
D0-D3 from the microcontroller chip to produce the multi-frequency tones 
out of the TONE line. The data lines are connected to ports P1.4-P1.7 of 
microcontroller chip 501 through a tri-state buffer chip 405. This chip in 
the preferred embodiment is part number 74HC373, a CMOS tri-state latch 
circuit available from Texas Instruments and other vendors. The clocking 
and enable line of chip 405 is connected to port P2.7 and the chip select 
or chip enable line of MF transmitter 404 is connected to microcontroller 
port P2.3. The output of MF transmitter chip 404 is capacitively coupled 
to the summing point connected to the inverting input of operational 
amplifier 403. This summing point sums the tone outputs of DTMF 
transmitter chip 401 and MF transmitter chip 404 for transmitting the tone 
codes to the PBX system. 
FIG. 5 shows the microcontroller chip 501 used in the preferred embodiment 
of the present invention. This chip is an Intel 8751 single chip 8 bit 
microcomputer with on-board EPROM memory and RAM scratch pad memory. The 
architecture of this microcontroller includes four 7 bit I/O ports which 
are wired in the preferred embodiment to control the operation of the 
interface 100. In the preferred embodiment of the present invention shown, 
many of the ports can perform dual functions by time multiplexing the 
operation of the ports, as in the technique described above for the 
tri-state data bus. For example, ports P1.0-P1.7 could be time multiplexed 
to be connected to a plurality of DIP switches using tri-state controllers 
and programming the microcontroller chip 501 to read the DIP switch 
positions using a single dedicated I/O port for tri-state control. In this 
fashion, the interface 100 may be field programmable to control optional 
operational functions of the interface such as wink start or immediate 
start, the number of extension digits transmitted, pulse, DTMF or MF 
operation of the CO, pulse DTMF or MF operation of the PBX, etc. 
Those skilled in art will readily recognize that the preferred embodiment 
of the present invention may utilize a wide variety of alternate control 
structures without departing from the scope and spirit of the present 
invention. For example, a wide variety of microcontrollers or 
microprocessors, whether they be single chip or multi chip 
implementations, may be substituted for the microcontroller chip of the 
preferred embodiment. In a like fashion, hard-wired control or 
microprogram control could be substituted. In addition to this, the 
control functions of the present invention could be implemented by 
ROM-based control or PLA-base control or other software or hardware 
control structures. 
The microcontroller chip 501 indirectly controls the control circuits of 
relays K1-K8 of the preferred embodiment of the present invention. The 
relays used in the present invention are solid state relays with optically 
isolated control lines and in the preferred embodiment are part number 
LH1065 available from AT&T and commonly used in the telephone industry. 
These solid state relays offer nearly zero cross-talk between the control 
lines and the relay contacts while offering very low contact impedance. 
Those skilled in the art will readily recognize that electro-mechanical 
and other types of relays could be used throughout the present invention. 
The relays chosen in the preferred embodiment of the present invention are 
designed to handle an extremely high number of closings without 
degradation in performance. The solid state relay control circuits K1-K8 
shown in FIG. 6 are the equivalent of the relay coils in 
electro-mechanical relays. For example, relay control circuit K1 controls 
relay contacts K1A and K1B of FIG. 2. Each relay control circuit is wired 
in series with a current limiting resistor and an optional LED indicator. 
These indicators can be mounted on the front panel of the interface 100 to 
indicate the status of the interface. 
The relay control circuits are controlled from the microcontroller ports 
P1.0-P1.4, the signals of which are buffered through an octal D-type latch 
integrated circuit 601 which in the preferred embodiment is part number 
74HC373 available from Texas Instruments and other vendors. Each latch 
within integrated circuit 601 is commonly controlled from the enable line 
attached to a dedicated control port P2.2. The outputs of the latch 
circuits 601 are used to indirectly control the relay control circuits 
K1-K8 through digital inverting buffers. These inverting buffer circuits 
are used to obtain the correct logic level for driving the relay control 
circuits. 
As was described above, the relay contact points for relay K1 are designed 
to operate the opposite of the relay contacts of K2. Thus, when the relay 
contacts K1A and K1B are closed, the relay contacts for K2A and K2B are 
open. Relay K1 in its normally closed position indicates that the 
interface is monitoring the central office from the DID trunk. When the 
relay contacts for normally open relay K2 are closed, the interface is 
monitoring the line from the PBX. Thus, the configuration of the inverting 
buffers connected to the Q0 output of D-type latch chip 601 indicates that 
the relay control circuits K1 and K2 operate mutually exclusive of one 
another. 
In a like fashion, relay control circuits K3 and K4 operate mutually 
exclusive of one another. Thus, when the normally closed relay contacts 
K4A and K4B are closed, the DID trunk to the central office has the talk 
battery sourced from the interface. When the normally open relay contacts 
K3A and K3B are closed, the talk battery voltage on the DID trunk is 
sourced from the PBX (and the PBX is connected to the trunk). Once again, 
optional LED indicators are attached in series with the K3, K4 relay 
control circuits to indicate their position. 
Relay contacts K6A, K6B also operate mutually exclusive of relay contacts 
K5A, K5B. These relay contacts indicate the polarity of the -48VDC battery 
voltage source to the DID trunk. When relay control circuit K6 is in its 
normally closed position, the normal polarity of the -48VDC battery 
voltage is applied to the DID trunk. When the normally open relay control 
circuit K5 is closed, the polarity of the battery voltage applied to the 
DID trunk is reversed in the form of a wink transmit signal. Optional LED 
indicators are also wired in series with the K6 and K5 control circuits to 
indicate status. 
Relay control circuit K8 controls the 330 ohm DC termination placed between 
the tip and ring wires of the PBX line. Relay control circuit K8 is 
normally open, but when closed places the termination across the line. 
Relay K8 can be rapidly opened and closed to perform pulse dialing. Relay 
control circuit K7 serves to connect the impedance matching transformer T1 
between the tip and ring wires of the PBX line. When the normally open 
relay contacts K7A, K7B are closed, the interface can transmit tone 
signals to the PBX. 
FIG. 7 is a high-level control flow chart of the software control program 
sequence contained in the EPROM program memory of microcontroller chip 501 
for the interface 100. Those skilled in the art will readily recognize the 
translation of this control sequence into the appropriate microprocessor 
control commands necessary to operate the interface 100. 
Beginning at command box 701, the central office seizes the trunk by 
placing a termination impedance across the DID trunk, indicating that a 
call is to be forwarded to the PBX. At command box 702, the interface 
detects the current through the tip and ring wires caused by the 
termination impedance and sends loop reverse-battery-wink signal to the 
central office, indicating that the interface is ready to receive the 
extension of the called party. This wink protocol used by the interface is 
identical to the wink protocol used by a PBX, thus the central office 
recognizes this protocol as though it were attached to a PBX system. The 
central office will detect the wink and after an approximate 30 
millisecond wait period after termination of the wink, the central office 
sends the digits corresponding to the extension of the dialed party down 
the DID trunk, as shown in command box 704. The interface system receives 
these digits (in either dial pulse, DTMF or MF mode), translates the 
digits into binary and stores the digits in the microcontroller memory, as 
shown in command box 705. After the microcontroller receives and stores 
the extension of the called party, the interface seizes the PBX line by 
placing a termination impedance between the tip and ring wires. PBX 
recognizes the current through the loop caused by the termination 
impedance and sends a loop reverse-battery-wink as though it were 
handshaking with the central office, as shown in command box 707. The 
interface recognizes this handshake protocol and operates as a central 
office by transmitting the digits of the dialed extension in the mode 
recognized by the PBX (dial pulse, DTMF or MF), as shown in command box 
708. 
If the present invention is operating in a special mode to support 
telephone equipment peripherals of the PBX, such as a voice mail system, 
the control flow would be a bit different at this point. For example, if 
the present invention was operating to support a voice mail system, as 
shown in command box 710, control flow would pass to box 711 where the 
interface would wait for a request from the voice mail system to download 
the digits a second time. For example, at command box 711, the DTMF "A" 
tone could be used to indicate a request to send from the voice mail 
system. The "A" through "D" tones of the DTMF repetoire are not used by 
the central office switching equipment and thus would not interfere with 
any ongoing controller signaling. If the request to send tone were 
transmitted by the voice mail system, decision box 712 would pass control 
to command box 717 where the interface would once again download the 
digits in DTMF of the dialed extension to inform the interface the 
identity of the called party to which the trunk was attempting to connect. 
The detailed sequence of control commands to open and close the relays of 
the interface to download the digits in command box 717 are similar to the 
detailed sequence of events to download the digits in command box 708. To 
download the digits, relays K4 and K6 close their respective contact 
points to source -48VDC talk battery voltage to the DID trunk, 
simultaneous with the closing of the contacts associated with relay 
control circuits K7 and K8 to place a termination resistance between the 
tip and ring lines of the PBX line and attach the impedance matching 
transformer T1 to the PBX line to transmit data. Along with the 
aforementioned relay closings and openings, the DID trunk line is 
disconnected from the PBX line by the opening of the K1 relay contacts and 
K3 relay contacts. The interface is now positioned to transmit the 
extension of the called party to the voice mail system. 
The voice mail box receives the digits of the dialed extension in command 
box 717 of FIG. 7 and may optionally transmit an acknowledgement tone back 
to the interface when the digits have been properly received. Since the 
relay contacts associated with relay control circuit K2 are closed so that 
the interface may monitor the line from the PBX, the interface will 
receive the optional acknowledgement signal from the voice mail box and 
cut through the telephone line between the central office and the PBX for 
talking by (1) removing the interface battery voltage, (2) removing the 
line termination impedance, (3) removing the impedance matching 
transformer T1 for the transmit interface, and (4) closing relay contacts 
K3A, K3B to cut through for talking the DID trunk to the PBX line. 
Voice communication is then carried on between the caller and the voice 
mail system until the voice mail box disconnects at command box 719 in 
FIG. 7. The control flow at this point merges with the control flow from 
decision box 712 and decision box 710 at command box 713. The PBX will 
then disconnect from the central office when either party disconnects the 
call (whether it be the voice mail system, the caller or the called 
party). The interface monitors the line for the disconnect at command box 
714 and the interface releases the central office by putting the relays 
back into their idle position, in the positions shown in FIG. 2. In this 
position, the interface sources talk battery to the central office and an 
open line is maintained to the PBX. At command box 716, the interface 
resets its internal memory and transfers control of the control program 
back to command box 701 to monitor for the central office seizing the 
trunk once again. 
While the present invention has been described in connection with the 
preferred embodiment thereof, it will be understood that many 
modifications will be readily apparent to those of ordinary skill in the 
art, and this application is intended to cover any adaptations or 
variations thereof. Therefore, it is manifestly intended that this 
invention be limited only by the claims and equivalents thereof.