A MULTIPLEXED PHONE SYSTEM FOR SMALL MINES 

James R. Means, Jro 

Pittsburgh Research Center 

Bureau of Mines 

U.S. Department of the Interior 

Pittsburgh, PA 15236 

George Bowser 

Catalyst Research Corp. 

Baltimore, MD 21209 

ABSTRACT 

A new phone system was developed that provides eight full duplex 

corncnur~icdtions channels over a sing1 e twi sted shielded phone 1 ine, with no 

cer~tral switching station or line amp1 ifiers. This system greatly sirnpl ifies 

i nstal 1 ation and maintenance over that r e q u i r e d with conventional switching 

systems . 

The system is based on :nicroprocessor technology and digital frequency 

synthesis. Frequency division multiplexing is used over a bandwidth of 

dpproximately 300 kHz, and a baseband voice channel (compatible with conventional pager phones) is provided as a backup to assure co~n~llunication if the 

multiplex features fdi 1 . Multiplex operations are control led over a 30-kHz 

digital channel. The systeln is designed to operate with up to 16 km (10 

vi 1 es) of cab1 e plant and a maximuin phone separation of 8 km (5 mi 1 es) . 

The digital techniques permit a lirnited amount of monitoring and control 

cdpabil i ty to each phone. This consists of two pair of contact inputs or 

outputs, to be used as desired. These "flags" may be read or controlled by a 

di spatcher's phone. 

INTRODUCTION 

Good coin~r~unication is necessary for effective mine ~nanagernent, without 

which a lack of coordindtion in the movement of personnel, ore, and supplies 

will inevitably result in periodic bottlenecks that cause lost production 

time. Additionally, valuable time may be lost in obtaining help for an 

injured miner or in correcting an unsafe mine condition if a good cornmunications system is not present. 

Unfortunately, this has not always been an easy task. The mine 

envi ronment can be extremely harsh on electronic equipment. Envi ronmental 

stress causes corrosion and physical abuse can cause catastrophic equi prnent 

fail ure. Consequently, many mines have found that any system more cornpl icated 

than a simple pager phone is unsuitable. 
Recent advances in techno1 ogy are I-ernovi ng these envi ronrnental restraints. Large-scdle integrated circuits are now rep1 acing racks of discrete 

circuitry, and they are inherently cheaper and more re1 iable. Consequently, 

it is now possible to introduce more sophisticated technology into mines while 

improving reliability. 

This technology has begun to find application in mine telephones and 

several ~nultichannel phone systems have been developed. These systems include 

both mu1 tiwi re and niul ti pl ex transmi ssion sc.hemes. Both types have performed 

we1 1 and met some communications needs of mines. However, there are some 

drawbacks to previ ously devel oped equi pment . 

Multiwire phone systems by nature require at least one wire pair per 

communication channel . This presents no particular technical difficulties, 

but often rnines balk at installing any system requiring a wiring practice 

unfami 1 iar to mine personnel. Consequently, mu1 tiwi re systems have not yet 

found widespread appl ication in the mining industry. 

Mu1 tiplex systems have eliminated rnost of these cable problems, but at 

the cost of added equipment complexity. To date, there is no commercial ly 

produced mu1 tiplex system for use in rnines that can function over a single 

wire pair on a nine-wide basis without in-Vine repeaters or central control. 

This extra equipment causes reduced re1 iability, and failure of an in-line 

repeater can destroy the integrity of the enti re communication system. 

To overcome these problems, the Bureau of Mines has entered into a joint 

venture wi th Catalyst Kesearch Corp. l to develop an eight-channel mu1 tip1 ex 

phone system capable of communicating over a ~ninirnum distance of 5 miles (8 

kin) with no in-line repeaters or central control. In addition to multiplex 

features, each phone contai ns a resident pager phone to provide a1 1 -page 

capability and act as a emergency backup. 

MULTIPLEX SCHEMES 

The first major decision in designing the multiplex phone system was in 

deciding whether to use time division mu1 tiplexing (TDM) or frequency division 

mu1 ti pl exi ng (FDM) . Such factors as cost, slystem re1 i abi 1 i ty , and technical 

constrai nts were considered. When a1 1 factors were weighed, FDM was chosen. 

A brief discussion of the factors pertinent to this decision follows. 

TDM cornmunication channel s must be sampled at a minimum rate of two times 

the bandwidth of the channel. This information is then digitized and 

combined with samples from all the other channels and transmitted to a 

receiver where it is decoded and reassembled into analog form. This method 

can utilize digital circuitry and is thus able to efficiently exploit recent 

Qureau of Mines Contract H0395000, Mu1 ti pl exed Telephone System for Small 

Mines, Catalyst Kesearch Corp. Avai 1 able for consul tation at Pittsburgh 

Research Center, Bureau of Mines, Pittsburgh, PA. 

kischa, Schwartz. Information Transmission, Modulation, and Noise. McGrawHill Book Co., New York, Second Edition, 1970, p. 119. 
advances in di y ital techno1 ogy. However, TDM does present difficulties in 

data transinission if distances exceed 1.6 km (1 mile) or so. 

An eight-channel full duplex system requires approximately 50 kHz 

bandwidth. In a TUN system this means a minimum sampling rate of lOOK samples 

per second. If samples are digi ti zed to an accuracy of 6.25% a minirnum of 

four bits are needed per sample. This results in a baud rdte of 400 kHz and 

the addition of a guard band "between bits raises this rdte to 800 kHz. To 

trar~srnit dn 800-kHz digital signal would require a cable with a capacity in 

excess of 2 MHz. The dddition of start bits, stop bits, address, etc., would 

incredse this requi rernent even further. It is almost impossible to attain 

this type of transmission capacity at a distance of 5 miles (8 km) without the 

aid of repeaters. Consequently the TDM option was eliminated as a viable 

choice. 

The second multiplexing scheme is FDM. In this scherne each comniunication 

channel is translated to a separate portion of the spectrum. Infor~nation in 

tilesc channels may be either FM or AM modulated, and bandwidth of about 10 kHz 

can easily accornmodate a voice grade channel . Eight full duplex channel s 

would require a bandwidth as small as 160 kHz. 

FDM is relatively straightforward and is frequently used in radio 

uncation. Troubleshooting is slightly easier than with a TDM system, but 

equiplnent does require the use of a number of analog circuits. However, 

the cu~~iplexi tyof this circuitry may be reduced by proper channel selection at 

the cost of sorne increase in bandwidth. Thus, FDM was chosen for the 

rnul ti pl exed phone system. 

r4ucti of the cornplexity in FDM systems results frorn the need to filter out 

si ynal s and noi se at frequencies outside the receive bandwidth. Therefore, a 

SChe~ils bvas devel oped to reduce fi 1 ter requi rernents for the rnul ti pl exed phone 

systerr~. This consisted of separating channels by 20 kHz instead of 10 kHz and 

it1 raising ttie channel frequencies to a point where the entire eight full 

duplex channels would be contained in a single octave. In this manner, 

hari~lonics from any transmitter would fall into a frequency range above the 

highest channel . Thi s meant pl aci ng the cornmuni cations channel s from 340 kHz 

to 640 kHz. At these frequencies a 8-km (5-mile) range is practical without 

repeaters by utilizing a high qua1 ity twisted shielded pair (TSP) cable. This 

required a dynamic range of approximately 60 db, which was achieved. 

CONTROL 

The rnul tiplexed phone system is functionally siinilar to the commercial 

phone system; this means control of the system is automatic. Control is 

accornpl ished by a network of microprocessors (one in each phone), which share 

system decisionrnaking via a diyital communications chanriel centered at 30 kHz. 

This protocol minimizes the possibil ity of a single phone fail ing in a manner 

that would disrupt the enti re mu1 tip1 ex system. 
The rni croprocessor ci rcui try control s the generat ion of a1 1 audio cues 

(busy signal, dial tone, etc.). This is to provide the user with coinrnercial 

?hone system-type feedback signal s that are readi ly recoyni zabl e. The ri ngi ny 

signal is generated at the called phone and sent back to the caller via the 

audio channel. This also inforrns the user that the line between the two 

pllories is good. The use of these signal s combines to give a good overall 

indication of the system's status. 

RESIDENT PAGER PHONE 

In order to retain all-page capability, each Multiplex Phone4 has within 

it a resident payer phone that functions independently of the phone's 

microprocessor. Thi s syter~i uti 1 i zes baseband cornmuni cat i ons and dc si gndl i ng 

1 i ke a standard pager phone. In fact, the rnultiplexed phone system can be 

interfaced to an existing pager phones with the addition of a decoupling 

circuit to prevent tile payer phones from loading down the hi yher frequency 

channels of the rnultiplexed phone system. If the all-paye function is used 

when private conversations are in proy ress, the paye broadcast is superi~nposed 

upon the existing private rnessaye. Thus the listener cdn respond to whichever 

lnessaye is most important. It was felt that the abil ity to page all phones 

within the system (even those already involved in a conversation) was 

essential in the event of an erneryency. It should be noted that the page 

system can act as a backup if the rnul ti pl ex features fai 1. A compl ete 

spectrum of the rnultiplex system is shown in figure 1. 

SYSTEM POWER 

The rnultiplexed phone system was designed to meet the Mine Safety and 

Health Adr~ii ni stration's (MSHA) certification requi rernents for use in gassy 

mines. Thi s 1 imi ted powering a1 ternati ves, and battery power was ultimately 

sel ected. 

To minimize the power drain of the Mult'iplex Phone, a two-level power-up 

sequence was instal led. Each phone contains a low-power 30-kHz detector, 

which is continually active. If 30 kHz is noted, a phased locked loop 

dernodul ator and di gi tal address decoder are activated. This device then scans 

for its address and proceeds to power up the microprocessor only if it is 

being called. In this manner projected battery life has been extended to 4 

months (depending on use). 

SPECIAL FUNCTIONS 

Additional capacity is available with the addition of two optional pieces 

of equipment, a conference call unit and a dispatcher's phone. These units 

add much flexibility to the system. 

4Tradetnark of Catalyst Research Corp., Ba1 timore, MD; Use of company or 

t fade narnes is for identi f i cation purposes only and does not irnply endorsement 

by the Bureau of lulines. 
The conference call unit will enable a three-way private conversati on. 

This unit is expected to be useful in much the same manner as the conference 

call capabil i ty of an aboveground phone system. It complements the basic 

phone system but is not required. The di spatcherls phone provides a variety 

of functions. It is a Multiplex Phone with some additional programming 

capacity, and it can be useful in monitoring the system. It displays which 

channels are active and can, upon command, check the battery status of each 

Mu1 tiplex Phone. It can also preempt a conversation in progress so that a 

party involved in a private conversation may be reached without the use of an 

all-page call. 

The rnicroprocessor used in each Mu1 tiplex Phone has flags which can be 

either read or set. Two of these flags are not used and remain available for 

use as contact-type inputs or outputs. The dispatcher's phone can read and/or 

set flags to monitor and/or control parameters in the mine. One example would 

be the monitoring of a float switch. Application of this systern is flexible 

and depends on mine requi rements. 

SYSTEM STATUS 

In December 1981, an in-mine test was initiated to evaluate a system of 

17 phones. This included approxilnately 4.5 km (2.8 miles) of cable with 

another 4.5 km (2.8 miles) of loss artificially simulated. 

The cable used for this test is a 124-ohm low-loss TSP with #16 AWG solid 

conductors. A1 1 taps and spl ices were made with conventional spl icing 

techniques and enclosed in plastic junction boxes. Resi sti ve power dividers 

were used for a1 1 taps over 50 m (150 ft) in length, and a1 1 cable runs over 

50 m (150 ft) were terminated resistively. 

To date, the system has performed we1 1 with only one failure attributable 

to the phones. This was a fail ure of a rnicroprocessor board. The only other 

failures have been breakage of conductors in the TSP. Battery life has been 

as long as 4 months depending upon phone usage. The Multiplex Phone is shown 

in figure 2 and its specifications are contained in the appendix. 

FUTURE DEVELOPMENT 

Plans exist to extend the capabil ity of the multiplexed phone system in 

the future. These plans currently call for the development of a data 

interface that will enable Be1 1 103-type equipment to function over the 

system, the development of a unit to interface with the commercial phone 

system, and the development of a small phone branch exchange (PBX) which will 

provide a subscriber service drop to a system of up to eight phones. Work in 

these areas has been completed through breadboards. 
CONCLUSION 

The 8ureau of Mines, in conjunction with Catalyst Research Corp., has 

developed a cost effective rnu1 tiplexed phone system that provides eight full 

duplex voice grade channels at a distance of up to 8 km (5 miles), which is 

control led by a distributed microprocessor systern. This distribution enhances 

system re1 iabil ity. Each phone also contains a resident pager phonem for all 

page and erneryency call s. 

Special features are available by the dddition of specialized pieces of 

equipment. A di spatchert s phone has been developed and a conference call unit 

is currently under development. A small PBX and a commercial phone systern 

interface are planned in the future. 

APPENDIX 

1Yu1 ti pl ex Phonem Speci fications 

Channel s : 1 all-pagelemergency baseband 

1 30-kHz FSK digital control 

8 FM full duplex voice grade 

Range: 8 km (5 miles) minimum with 16-km (10-mile) cable plant 

Power: 12 Vdc battery (NEDA 926) 

Dimensi ons: 12 in x 10-1/2 in x 5 in 

Transrni t Power: All-page: 10 V p-p 

Digital: 2.5 V p-p 

Voi ce : .1 V rms 

Receive Sensitivity: All page: .25 V rms 

Digital: 10 mV 

Voice: 100 PI 

Receive Selectivity: Digital : - + 9 kHz, -40 db 

Voi ce : - + 15 kHz, -70 db 

Adjacent Channel 

Rejection: 80 db 

Cable: 124 ohms low loss TSP 

#16 AWG conductors 

(.Be1 den 9860 or equi val ent) 
IOK l00K 

FREQUENCY kHz 

FIGURE 1. - Multiplex system spectrum 
FIGURE 2. - Multiplex Phone