Contract J0177020 

Arthur D. Little, Inc. 

BUREAU OF MINES * UNITED STATES DEPARTMENT OF THE INTERIOR 

Minerals Health and Safety Technology 
The views and conclusions contained in this document 

are those of the authors and should not be interpreted as necessarily representing the official 

policies of the Interior Department's Bureau of Mines 

or of the U.S. Government. 

Arthur D Little Inc. 
I 4. TltIm and Subtitle 

Development of a Visual Display and 

50m - MI 

Control System - - - -- -- 

7. Author(~) 

1 . 

- - - . - -. - -- - - - -- . 9. Pmdormlng O~anizatlon Name and Addre~r 

3. Recipient's Access~on No 

Arthur D. Little, Inc. 

Acorn Park 

Cambridge, MA 02140 

2. REPORT DOCUMENTATION 

PAGE 

5. Report Date ( October 1980 

1. 00. 

8. Perform~ng Orarnlzat~on Rept. No 

C-80956 . . 

10. Pm)ect/Task/Work Unit No. 

I I. ContractfC) or Crrnt(C) No. 

50177020 

(C) I it. Sponsorin# Or~anization Name and Address ' 13. Type of Report 6 Period Covered I I I Pittsburgh Mining and Safety Research Center 

United States Bureau of Mines 

A - Pittsburgh - - - 9 - - Pennsy - - lvania_-- -- - - -- -- . - -- 

15. Supplementary Notes 

Final / 

. - - - - -- - - - - - - - - - 16. Abstract (Llmlt. 100 words) 

The object of this program was to extend the capabilities of visual 

paging systems both in size and function. The system developed uses 

the mine pager phone line for communicat~ion, monitoring, and remote 

control. A central station, using a mini-computer, transmits signals 

by phase inversion modulation of a 16,384 Hz carrier to as many as 

999 remote units. The remote unit addressed returns an echo to 

indicate that it is functioning and, in the case of monitor units, 

to convey the monitor data. Magnetically latched display discs, 

which can be seen from both sides when posted, are used for visual 

paging. Lists of the addresses of posted pages, unanswered pages, 

and non-echoing units are prepared by the central station computer. 

The system was designed to be intrinsically safe and should operate 

for three months on a 12-volt lantern battery. 

Mine communicators 

Visual paging 

Phone line 

Mine monitoring and control 

Supervisory feedback 

b Identlfl+n/Opon Ended Terms 

c COSATl f~eld/Group 

- - - - - .. - 1 

18. Availablllty Statement / IS). Secur~ty Class lTh~r Report) 

Unclassified Release unlimited )D. security class (T~I. r) 1 22. Price 

~nclassifiea I 

(S.0 ANSI-239.18) See Inrtructlonr on Reversa OPTIONAL FORM 272 (4-771 

(Formerly NTlE35) 

Ueprrtnrent of Commerce 

Arthur D Little Inc 
FOREWORD 

This report was prepared by Arthur D. Little, Inc., 

Cambridge, MA 02140 under USIBM Contract Number 

50177020. The contract was initiated under the Coal 

Mine Health and Safety Program. It was administered 

under the technical direction of the Pittsburgh 

Mining and Safety Research Center with Mr. Harry 

Dobroski acting as Technical Project Officer. Mr. 

Doyne W. Teets was the contract administrator for the 

Bureau of Mines. This report is a summary of the work 

recently completed as a part of this contract during 

the period August 1977 to Octobler 1980. This report 

was submitted by the authors in October 1980. 

Arthur D Little, Inc. 
TABLE OF CONTENTS 

I Item 

List of Illustrations 

List of Tables 

INTRODUCTION 

PHASE I -- ESTABLISH THE SYSTEM FUNCTIONAL REQUIREMENTS 

System Architecture 

System Size 

Special Requirements for Remote Units 

Signal Transmission 

PHASE I1 -- DESIGN THE SYSTEM 

PHASE 111 -- FABRICATE THE SYSTEM 

PHASE IV -- EVALUATE THE SYSTEM PERFORMANCE 

CONCLUSIONS 

INVENTIONS 

Page 

Arthur D Little, Inc. 
LIST OF ILLUSTRATIONS 

Number 

Number 

Title 

Wireline Transceiver Timing 

Visual Display and Control System Block Diagram 

Control Console 

Console, Ten Disk Display Unit, Monitor 

and Control Unit 

Display Unit 

Separate Disk Module 

LIST OF TABLES 

Title 

Address Assignments for TEMS and VDCS 

VDCS Test Results 

Monitor and Control Unit A/D Converter Test 

(Address 045) 

Page 

Page 

Arthur D Little, Inc. 
INTRODUCTION 

The original objective of this program was to obtain a mineworthy 

optical/electromagnetic pager system that will alert supervisory, 

maintenance or other mine personnel that they are being paged on the 

mine pager phone system. The program goal was subsequently broadened to 

add monitor and control capability to the paging system, which was then 

renamed Visual Display and Control System (VDCS). Those objectives were 

achieved in a four-phase program: 

Phase I -- Establish the system functional requirements 

Phase I1 -- Design the system 

Phase I11 -- Fabricate the system 

Phase IV -- Evaluate the system performance. 

The resulting Visual Display and Control System has a total capacity of 

1000 addresses and is under the control of a central unit at which 

commands are dispatched and responses received. The VDCS as delivered 

has 23 addresses assigned to visual pager disks, three addresses 

assigned to relay control functions, and three addresses assigned to 

monitor functions. Since the central unit is microprocessor-controlled, 

the system has great flexibility and changes are easily incorporated. 

Arthur D Little, Inc. 
PHASE I - ESTABLISH THE SYSTEM FUNCTIONAL REQUIREMENTS 

The original purpose of the zone pager development program was to 

expand the capabilities of the visual pager developed under Contract 

H0252093. Early in the study it became apparent that the twisted pair 

pager phone line is a valuable resource, since it reaches most of the 

mine workings and could therefore be used for remote monitoring and 

control as well as paging. By adapting this broader view of the system 

capabilities at the beginning of the design process, it was possible to 

accommodate those additional functions at a later date. 

The system functional requirements were arrived at through discussions 

with mine operators and Bureau of Mines personnel. The genesis and 

rationale of those requirements are disczussed in detail in the Arthur 

D. Little Phase I report (1) - and are sunnmarized here. 

System Architecture 

The main guiding principle in choosing a system architecture was 

preservation of flexibility to modify functions or add new ones. To 

that end, the system components and functions were specified as follows: 

1. A central control unit to be located at the pit head or mine 

office. This unit will transmit signals to remote units on 

the phone line and receive answering signals from the remote 

units. The central control unit will use a microprocessor and 

be programmed to perform the desired functions. System 

flexibility will be achieved through programming the central 

unit. The system is to contain a supervisory control loop 

(1) - Arthur D. Little, Inc., Development of a Zone-Type and Single-Unit 

Visual Display System. Phase I report under Bureau of Mines Contract 

50177020, ADL Case 80956. 

Arthur D Little, Jnc 
whereby each remote unit, when interrogated, echoes its 

address and the command it received. This echo is checked 

against the transmitted data to detect malfunctioning remote 

units. The addressing system must include global addresses 

which permit activation of multiple display modules with a 

single command. Monitor unit outputs are to be logged on a 

hard copy device. Communications channel errors will be 

indicated on the console and their details will be logged on 

the hard copy device. The console will provide a means to 

display a list of all remote units which have been posted 

during the last five minutes, a list of all units which have 

been posted since system reset, and a list of all units which 

have had communication errors. 

Visual pager remote units located in the mine. These will be 

made up from single disk display modules, each of which has a 

unique address. Each module colntains a two-inch diameter 

numbered or lettered disk which can be rotated to either a 

horizontal or vertical position so that the disk is either 

invisible or visibie. Multiple display units will consist of 

the required number of single disk modules mounted in a 

common housing. Each module will have a unique address so 

that it can be individually selected by the central controller. All communications will be initiated by the central 

controller. Upon receiving its address and a coded operation 

instruction, the visual pager unit will echo back the address 

and the operation code. These units will respond to three 

operation codes: post, which will set the display disk to 

post a page; clear, which will reset the display disk to 

clear a page; and test, which echoes the address and command 

but does not change the state of the display. 

Arthur D Little. Inc. 
3. Monitor units located in the mine. These will have their 

own individual addresses like the visual pager units and 

will respond to calls initiated by the central control 

unit. The echo response from ia monitor unit will consist of 

the unit address with measurement data replacing the 

operation code. The input voltage range for monitor units 

is 0 to 4.92 volts. With a six bit A/D converter, the least 

significant bit represents an increment of 0.078125 volts. 

4. Remote control units located in the mine. These will 

provide for remote control of pumps, fans, regulators or 

other equipment in the mine. Each will have its own 

individual address and will echo this address and operation 

code when called by the centr(a1 control unit. The post 

command will close a pair of relay contacts in the remote 

unit, the clear command will open the relay contacts, and 

the test command operates as in the visual pager units. The 

relay contacts are rated at 120 volts AC and five amperes, 

noninductive. 

The actual system which was delivered clonsists of three single disk 

display units, two five-disk display units, one ten-disk display 

unit, a central control unit with hard copy device, and a 

monitor/control unit having three monitlor and three control modules. 

System Size 

The system is to have the capability of addressing 1000 remote units, 

including global addresses. 

Arthur D Little, Jnc 
S~ecial Reauirements for Remote Units 

The remote units of all types located underground must be designed to 

meet some rather restrictive requirements. In addition to the rugged 

construction needed to survive in a mine environment, these include: 

Underground units must be intrinsically safe. Therefore, these 

units must use battery power and draw very little current in the 

standby state (that is, when not responding to signals from the 

central control unit). Typical twelve-volt batteries have about 

ten ampere-hour capacity and will give three months1 service at 

a two milliampere rate. This problem was solved in the previous 

visual pager by using a dc "wake-up" signal on the phone line to 

turn on the receiver power. A similar technique will be used in 

this application, except a carrier frequency tone burst will be 

used as the "wake-up" signal. 

The underground units must be connected to the phone line all 

the time and therefore must not load the line. The characteristic impedance of a pager phone line is about 135 ohms. 

The VDCS units must have an input impedance much higher than 

this. The visual pager receiver had a 40 kilohm impedance. 

The VDCS receiver impedance will be about as high. 

VDCS receivers both above and below ground must accept signals 

over a wide range of power. The attenuation of the phone line 

can be as high as 3 dB per mile and, in addition, there are 

losses because of impedance misrnatch (low impedance pager 

phones; in particular, the MSA pager I and I1 phones, and 

branches or taps on the line). These reflective losses could 

total 20 dB. Since noise on the phone line is expected to be 

about 35 millivolts in a 500 Hz bandwidth (2), the receivers - 

(2) - N.B.S. Technical Note 654, Electromagnetic Noise in Robena No. 4 

Coal Mine. April 1974. 

Arthur D Little, Inc. 
should be designed to detect signals as small as about 

100 millivolts. Maximum transmittler power will be about 1 watt 

or about 10 volts rms on a 135-ohm impedance line. Therefore, 

the receivers must work well over at least a 40 dB range and 

be protected against transients. 

Signal Transmission 

The signals for the zone pager are transmitted by phase inversion 

modulation of a carrier frequency. This method of modulation has the 

advantages that it is easy to modulate and demodulate the signal and 

that it is highly efficient. Most of the transmitted power is 

contained in the sidebands which convey the information. Only a small 

amount of power remains at the carrier frequency. 

The carrier frequency was chosen in the range of 10 to 20 kilohertz. 

The spectral density of noise on the pager phone line has a broad 

minimum at about 16 kilohertz (2). - Noise increases significantly below 

this frequency. Attenuation increases with frequency by about 

0.25 d~/mile in this octave. The exact frequency does not seem to be a 

critical parameter, and the selection of this frequency was made 

during the detailed design phase, based on the availability of 

suitable oscillator crystals. The standard electronic watch crystal 

(32,768 Hz) is used to produce a 16,384--Hz carrier frequency and a 

128 bit per second data rate. The signal format is shown in Figure 1. 

An initial tone burst is used to "wake up" the remote units. This is 

followed by two bytes, each containing a start bit, eight data bits, a 

parity bit, and two stop bits. The remotte unit echoes back two bytes. 

Display units and control units echo the address and command code for 

verification of the communication path. Monitor units echo the address 

and the six bits of measured data. 

Arthur D Little, Inc. 
K A om 

e hi 

ZZ I 

P'i 

I 

Figure! 1 

Wireline Transceiver Timing 

Arthur D Little, Inc. 
PHASE I1 - DESIGN THE SYSTEM 

Figure 2 shows a block diagram of the VDCS designed in response to the 

functional specifications from Phase I. The control console is shown 

in Figure 3. Figure 4 shows the control c:onsole and two of the six 

remote units, the monitor and control unit and the ten-disk visual 

display unit. Figures 5 and 6 show the display unit disks, one set of 

five disks attached directly to the display unit and the other set of 

five connected to the unit by cable. 

Table 1 shows the addresses currently assigned to the various remote 

units. To post, clear, or test a unit, one enters first the three 

digits of its address (e.g., 016), then the desired command. As each 

address digit is entered, it appears in the CURRENT display. When the 

command digit is entered, it appears briefly in the CURRENT display, 

which then blanks while the command is being executed. The command 

execution sequence is shown in Figure 1. If the address and command 

code in the echo match those transmitted, the system assumes that the 

remote unit has executed the command properly. The address and command 

will appear in the STATUS display and the NORMAL light will light. If 

there is a disagreement in either address or command, or an incomplete 

echo, or if there is no echo at all, the STATUS display will show the 

status of the remote unit prior to the command and the ERROR light 

will light. In addition, a detailed description of the error is logged 

on the printer. (See the TECHNICAL MANUAL for details.) In both cases, 

the contents of the CURRENT display are transferred to the PREVIOUS 

display. 

When one of the monitor units is posted, the six bit reading is stored 

in the controller memory and can be observed by entering the monitor 

unit address, followed by a DISPLAY command. The data value is typed 

on the printer in units of 7.8125 volts (e.g., 4.92 volts is printed 

as 00.63). For convenience in repeatedly addressing the same unit, the 

RE-ENTER button can be used to transfer the address shown in the 

Arthur D httle, Inc 
Figure 2 

Visual Display and Control System Block Diagram 

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Arthur D Little, Inc. 

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5 DISKS 

MONITOR AN0 "--------VISUAL OISPLAY UNITSCONTROL (6 

-+ 

ANALOG 

SIGNALS 

(3) 

CONTACT 

CLOSURES 

(31 
Figure 3. 

control Console 

Arthur D Little Inc 
CONSOLE, TEN DISK 
Figure 5. 

Display Unit 

Arthur D Little, Inc 
Figure 6. 

Separate Disk Module 

17 

Artt lur D Little Inc 
Address 

Table 1 

Address Assignments for TEMS and VDCS 

-- Function 

Unused 

Global, Addresses 16 - 46 

Global, Addresses 16, 21, 26 (All A disks) 

Global, Addresses 17, 22, 27 (All B disks) 

Global, Addresses 18, 23, 28 (All C disks) 

Global, Addresses 19, 24, 29 (All D disks) 

Global, Addresses 20, 25, 30 (All E disks) 

Global, Addresses 16, 17, 18, 19, 20 

Global, Addresses 21, 22, 23, 24, 25 

Global, Addresses 26, 27, 28, 29, 30 

Global, Addresses 31, 32, 33, 34, 35, 36, 37, 38 

(disks 1 to 8) 

Global, Addresses 3'9, 40, 41 

Global, Addresses 42, 43, 44 

Global, Addresses 16 - 25 

Global, Unused 

Display Unit Disks 

Monitor and Control Unit Relays 

Unused 

Monitor and Control Unit Analog Voltages 

Unused 

Monitor Unit (TEMS only) 

Unused 

(VDCS 

only) 

Arthur D Little, Inc. 
PREVIOUS display to the CURRENT display. CLEAR ENTRY clears the 

CURRENT display to permit correction of an error in entry. The ILLEGAL 

ENTRY light comes on if a command digit is entered in the address 

field or vice versa. 

The system controller maintains three lists on which it keeps track of 

the status of the remote units. The CURRENT list is a record of all 

units which have been successfully posted in the last five minutes, 

and the OLD list is a record of all units posted since the most recent 

system reset, except for those on the CURRENT list. Units are automatically moved from the CURRENT to OLD llist after five minutes have 

elapsed since posting. The ERROR list is a record of all units which 

have experienced communication errors. These lists can be displayed by 

momentarily placing the STEPISCAN switch in the SCAN position. The 

KEYBOARD light will go out, indicating that the system is now in the 

automatic list display mode. The contents of the list selected by the 

LIST SELECT switch will then be displayed in the STATUS display, 

showing the address of each unit on the llist for two seconds and 

repeating indefinitely. Alternatively, the list can be stepped through 

manually by repetitively placing the STEPISCAN switch in the STEP 

position. At any time that an address is in the STATUS display, either 

in the STEP or SCAN mode, it can be removed from the list by pressing 

the DELETE button. Addresses will automat:ically be removed from the 

CURRENT and OLD lists by a clear command to that unit. Addresses will 

automatically be removed from the ERROR list by a successful communication sequence to that unit. 

The system will automatically return to the keyboard mode, ready to 

accept commands, when any of the sixteen keypad keys is operated. The 

SYSTEM RESET button does just that and is used only to recover from a 

system hang-up state where the normal keyboard functions are not 

operational. 

Arthur D Little Jnc 
Recognizing the experimental nature of the VDCS, it was designed with 

built-in capability to modify its ~oftw~are for test purposes. This 

facility is accessed via the RUNITEST switch and is described in the 

TECHNICAL MANUAL (3), - which also contains complete schematics and 

descriptions of all system components. The OPERATOR'S MANUAL (4) - gives 

complete operating instructions. 

(3) Arthur D. Little, Inc., Technical Mianual for the Through-the-Earth - 

Monitor System and the Visual Display and the Control System. USBM 

Contract No. 50177020. October 1980. 

(4) - Arthur D. Little, Inc., Operator's Manual for the Visual Display 

and Control System. USBM Contract No. 50177020. October 1980. 

Arthur D Little, Inc. 
PHASE I11 - FABRICATE THE SYSTEM 

During this phase, the following units were fabricated: 

- 1 ea. Control Console serial number 001 

- 1 ea. Zone Visual Display (10) serial number 001 

- 2 ea. Zone Visual Display (5) serial numbers 002, 003 

- 3 ea. Unit Visual Display (1) serial numbers 004, 005, 006 

- 1 ea. Monitor and Control serial number 001 

- 1 ea. Printer NCR260 serial number 02 (purchased) 

The down-mine units are enclosed in gaslseted housings. Most of the 

circuits in the remote units are CMOS to maximize battery life. 

Current drain of remote units in their quiescent state is two milliamps or less, except for the Monitor and Control Unit, whose current 

drain is 4.6 milliamps. The useful battery life will be greater than 

three months for the display units and greater than 2.3 months for the 

Monitor and Control Unit. 

The control console is also housed in a gasketed steel box. The microprocessor components are mounted in a standard Intel card cage along 

with the transceiver circuits. Keyboard and display circuits are 

mounted directly on the front panel. The printer is a standard 

30 character per second commercial unit. 

Arthur D Little, Jnc 
PHASE IV - EVALUATE THE SYSTEM PERFORMANCE - 

Following fabrication, each of the units was tested for transmitter 

output, sensitivity to received signal level and, in the case of 

remote units, for sensitivity to battery voltage. The signal was 

attenuated by a 135 ohm T pad unit until the unit's receiver just 

failed to work properly, and this value of minimum input signal was 

recorded. White noise, band-limited to 20 kHz, was also injected onto 

the line and the value recorded at which it began to cause errors in 

data reception. These values, normalized to the 3 kHz receiver bandwidth, vary between 12 and 34 millivolts~ Since the noise level in a 

3 kHz band is expected to be as high as 86 millivolts in the noisiest 

mine, the units will have to be operated at reduced gain in such an 

environment, so the operating range will be somewhat shorter in such a 

mine. Table 2 lists these results for all unit tests, together with 

transmitter output voltage and function tests for each disk. Battery 

current drain in the quiescent state was measured for each remote 

unit, as were the battery voltages required for a normal echo return 

and for proper disk operation. 

The Monitor and Control Unit analog data channel was tested at several 

voltages, with the results shown in Tables 2 and 3. Table 3 shows that 

the A/D converter linearity is preserved within its 6-bit resolution. 

Arthur DLittleInc 
Table 2. 

VDCS Test Results 

10 Display Unit Serial Number 001 

Disc Address 

016 

01 7 

018 

019 

020 

03 1 

03 2 

03 3 

034 

03 5 

Post 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

Clear 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

Transmitter output into 135 ohms: 

Minimum input signal : 

Minimum input to saturate amplifier: 

Maximum noise (3 kHz band): 

Battery current, quiescent: 

Minimum battery voltage for disk operation: 

Minimum battery voltage for Echo normal: 

5 Dis~lav Unit Serial Number 002 

Test 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

9.2 volts 

68 mV 

195 mV 

28 mV 

1.83 mA 

10.56 V 

9.11 V 

Disc Address 

A 02 1 

B 022 

C 023 

D 024 

E 025 

Post 

X 

X 

X 

X 

X 

Clear Test 

X X 

X X 

X X 

X X 

X X 

Transmitter output into 135 ohms: 

Minimum input signal: 

Minimum input to saturate amplifier: 

Maximum noise (3 kHz band): 

Battery current, quiescent: 

Minimum battery voltage for disk operation: 

Minimum battery voltage for Echo normal: 

Arthur DLittle,Inc 
Table 2 (continued) 

VDCS Test Results 

5 Dis~lav Unit Serial Number 003 

Disc Address 

A 026 

B 027 

C 028 

D 029 

E 030 

Post Clear Test 

X X X 

X X X 

X X X 

X X X 

X X X 

Transmitter output into 135 ohms: 8.8 V 

Minimum input signal : 73 mV 

Minimum input to saturate amplifier: 230 mV 

Maximum noise (3 kHz band): 29 mV 

Battery current quiescent: 1.73 mA 

Minimum battery voltage for disk operation: 10.40 V 

Minimum battery voltage for Echo normal: 6.50 V 

Sinale Display Unit Serial Number 004 

Disc Address 

6 03 6 

Post Clear Test 

X X X 

Transmitter output into 135 ohms: 8.8 V 

Minimum input signal: 80 mV 

Minimum input to saturate amplifier: 210 mV 

Maximum noise (3 kHz band): 32 mV 

Battery current, quiescent: 1.82 mA 

Minimum battery voltage for disc operation: 9.60 V 

Minimum battery voltage for Echo normal: 5.08 V 

Single Display Unit Serial Number 005 

Disc Address 

7 03 7 

Post Clear Test 

X X X 

Transmitter output into 135 ohms: 8.7 V 

Minimum input signal: 74 mV 

Minimum input to saturate amplifier: 190 mV 

Maximum noise (3 kHz band): 33 mV 

Battery current, quiescent: 2.00 mA 

Minimum battery voltage for disk operation: 10.10 V 

Minimum battery voltage for Echo normal: 9.25 V 

Arthur D Little, Inc. 
Table 2 (continued) 

VDCS Test Results 

Single Display Unit Serial Number 006 

Disc Address 

8 038 

Post Clear 

X X 

Transmitter output into 135 ohms: 

Minimum input signal: 

Minimum input to saturate amplifier: 

Maximum noise (3 kHz band): 

Battery current, quiescent: 

Minimum battery voltage for disk operation: 

Minimum battery voltage for Echo normal: 

Monitor and Control Unit Serial Number 001 

Relay Address 

1 41 

2 42 

3 43 

Post Clear 

X X 

X X 

X X 

Transmitter output into 135 ohms: 

Minimum input signal: 

Minimum input to saturate amplifier: 

Maximum noise (3 kHz band): 

Battery current, quiescent: 

Minimum battery voltage for relay operation: 

Minimum battery voltage for Echo normal: 

Control Console Serial Number 001 

Transmitter output into 135 ohms: 

Minimum input signal: 

Minimum input to saturate amplifier: 

Maximum noise (3 kHz band): 

Test 

X 

Test 

X 

X 

X 

Arthur D Little, lnc. 
Table 3. 

Monitor and Control Unit 

AID Converter Test (~ddress 045) 

Input Readout 

Volts Volts/0.078125 

Arthur D Little, Inc. 
CONCLUSIONS 

The Visual Display and Control System achieves the objective of a 

mineworthy paging, monitor, and control system. Two aspects of system 

performance should receive further attention: The noise immunity of 

the receivers should be improved in order to permit operation to 

maximum ranges in the noisiest mines, and the threshold battery 

voltage for disk operation should be reduced so that the echo function 

fails first. Both improvements are achievable by a modest amount of 

redesign. 

Arthur D Little, Inc. 
INVENTIONS 

There were no inventions made in conjunction with this contract. 

Arthur D Little, Inc.