EPA-670/4-75-002
February 1975
INTERFACING A 24-POINT ANALOG RECORDER
TO A COMPUTER CONTROLLED TELEMETRY LINE
By
John M. Teuschler
Methods Development and Quality Assurance
Research Laboratory
Program Element No. 1HA327
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI OHIO 45268
-------
REVIEW NOTICE
The National Environmental Research Center -- Cincinnati
has reviewed this report and approved its publication.
Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
11
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FOREWORD
Man and his environment must be protected from the adverse effects
of pesticides, radiation, noise and other forms of pollution, and
the unwise management of solid waste. Efforts to protect the
environment require a focus that recognizes the interplay between
the components of our physical environment--air, water, and land.
The National Environmental Research Centers provide this multi-
disciplinary focus through programs engaged in
studies on the effects of environmental
contaminants on man and the biosphere, and
a search for ways to prevent contamination
and to recycle valuable resources.
This report is part of a continued effort by the Instrumentation
Development Branch, Methods Development and Quality Assurance
Research Laboratory, NERC, Cincinnati, to evaluate instruments
and provide information to both users and suppliers. It is also
intended that instrumentation be upgraded and that a choice of
the most suitable instrument can be made for a particular appli-
cation.
A. W. Breidenbach, Ph.D.
Director
National Environmental
Research Center, Cincinnati
111
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ABSTRACT
Interface circuitry was designed so that telemetered data origina-
ting from various remote stations could be recorded by both a
digital computer and an analog recorder. The entire interface
circuitry is mounted on a 3-1/2 x 2-1/2 inch printed circuit card
and installed in the receiver. Data from the two methods of col-
lection can, therefore, be collected and a comparison can be made.
A switching network also permits computer control with computer
and recorder logging; or computer logging only; or recorder
logging only.
IV
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CONTENTS
Abstract iv
List of Figures vi
Sections
I Summary 1
II Introduction 2
III Design Objectives 3
IV Interface Theory 4
V Mathematical Analysis 7
VI Fabrication and Installation 14
VII Testing 17
VIII Appendices 18
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FIGURES
No. Page
1 Data Collected on Analog Strip Chart Recorder
With Computer Employed in Network and No
Interface Between Computer and Recorder 19
2 The Interface Block Diagram Showing the
Directional Control Between the Blocks 20
3 Interface Circuit Schematic 21
4 Interface Synchrogram 22
5 Data Collected in Unison Employing the
Interface Circuitry 23
6 Top View Showing Component Locations 14
7 Interface Shown Installed in Receiver/Recorder 15
8 Schematic of Power Supply on Interface Card 15
9 Photographs of the Interface Circuit 24
VI
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SECTION I
SUMMARY
An ASR-33 teletype printer and an analog strip chart recorder can
be used simultaneously to collect water quality data. The inter-
face circuitry has been functioning properly ever since the Sprague
capacitors and G.E. thyristor were substituted for the previous
types.
The only function which could be added to increase the effective-
ness of the interface would be a logic circuit used to decide if
the print wheel is in the correct position for that particular
station. This logic circuit could easily be added if the need
would ever arise.
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SECTION II
INTRODUCTION
Before the installation of the interface circuitry, water quality
data were collected by either the digital computer or the receiver/
recorder. Figure 1 shows the results of data collected with both
systems in operation and no interface circuitry present. The main
obstruction that prevented the simultaneous collection of data was
the difference in time base between the two systems. The computer
operates on a 10-minute-per-station time base and the receiver/
recorder operates on an 8-minute-per-station time base.
The circuitry was designed to provide the capability of collecting
data on both systems in unison. Not only can the data acquired by
the two methods be compared, but by applying a switching network,
either can be used as a back-up logging device.
The control pulses of the interface circuit are limited to an
initiating pulse and a finalizing pulse. The initiating pulse
could have been derived from either the direct current loop of the
network or from the computer. Employing the direct current loop
requires a buffer stage to modify the pulse to a transistor switch-
ing level. The computer, however, provides a clean "on/off" 5-volt
pulse. The design required that the finalizing pulse be taken from
the recorder.
To minimize overall wiring and signal sources, the control pulses
were derived from the receiver/recorder circuitry. The computer,
however, actuates the initiating pulse which is transformed by the
receiver into a 110-volt alternating current (AC) signal. This
110-volt signal is used to pulse the interface circuitry.
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SECTION III
DESIGN OBJECTIVES
The main functions to be accomplished were to:
1. Start the recorder print wheel within a sufficient time after
a station call is initiated by- the computer.
2. Stop the recorder print wheel after the eighth point is plotted
on the strip chart recorder.
3. Provide the capability of starting the print wheel at the ini-
tiation of another station call. (A reset function.)
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SECTION IV
INTERFACE THEORY
Figure 2 is a block diagram of a method to accomplish the main
objectives. Figure 3 displays the circuitry necessary to perform
the functions described in the blocks.
To turn the print wheel motor on at the specified time, blocks #1
through #6 are used. These six blocks accomplish the first main
objective which is to turn the recorder print wheel on when a
station call is initiated.
Block #2 (isolation and buffer) isolates the 110-volt signal and
also steps the voltage down to 6.3 volts and rectifies the alter-
nating current into direct current. Block #3 (station call locator)
is a resistor-capacitor (RC) time constant introduced in the circuit
to locate the station calls. Since data are transmitted over the
line at a much higher frequency than is a station call, a long RC
time constant will separate data from a station call. The RC cir-
cuit in block #3 will discharge only after a 4-second interval.
Because all of the station calls are longer than 4 seco-nds, each
time a station call is made, the "station call locator" will pass
a zero pulse to the "interface initiator" (block #4). The interface
initiator is a transistor that "turns off" when a zero pulse is
applied to the base, thereby turning off relay RY and applying a
positive pulse to block #5 (master control). Block #5's transistor
is "turned on" by the positive pulse, and relay RY is activated.
RY performs three functions when it is activated:
1. The input (A) to the station call locator is removed,
thereby allowing the master control to remain in
control.
-------
2. Power is applied to the recorder print wheel motor (block
#6) that plots the data.
3. The gate circuit to the silicon controlled rectifier (SCR)
(block #9) is opened, which allows the silicon controlled
rectifier to "turn off" when the positive voltage is re-
moved from its anode.
The second main objective, which is to stop the recorder print wheel
after the eighth point is plotted, is accomplished by blocks #7
through #11. The functions of these blocks are described in the
following paragraphs.
The master control allows the print wheel (by means of a synchro-
nous motor) to rotate through eight points. On the second point,
voltage is removed from block #7 (recorder voltage), thereby
"turning off" the SCR in block #9 (reset). The recorder will then
rotate through the points in sequence until it gets to the ninth
point, which applies a positive 6-volt pulse (indicated by block
#7). This pulse passes through the isolation diode (block #8)
that keeps any positive pulse from going back into the recorder
from the interface, thereby protecting the recorder from an inter-
ference by the interface.
The positive pulse also passes through block #9, which is the SCR
in the "off state." The pulse then charges the time delay circuit
in block #10. This charged timing circuit saturates the transistor
in block #11, thereby shorting the base of transistor T to ground.
J
This action "turns off" relay RY^ which then performs the following
functions:
1. The voltage is again applied to the station call locator
that removes the positive pulse from the base of transistor
-------
T , thereby" keeping the master control in the "off state."
J
This action is ensured by the timing circuit in block #10,
which keeps transistor T "turned off" for at least 68
milliseconds.
2. The power is removed from the print wheel motor (block
#6) after the eighth point is plotted. Therefore, the
second main interface function is accomplished.
3. A gate pulse is applied to the SCR in block #9 (reset),
which activates the SCR, dropping most of the 6-volt
input pulse across resistor R?. This action allows
transistor T to again be turned on when the station
locator (block #3) is activated. Therefore, the third
main interface function is accomplished by allowing the
entire system to reset itself so it can be activated by
another station call.
These functions are shown in the synchrogram in Figure 4. In Figure
5 is the final output on the strip chart recorder with the interface
circuitry incorporated.
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SECTION V
MATHEMATICAL ANALYSIS
The following mathematical analysis refers to the blocks in the
schematic and block diagram, Figures 2 and 3.
Block #2 (Isolation and Buffer)
Output voltage of full wave rectifier D
V . = V /T - V,
out ac d
V =6.3 volts Root Mean Squared (RMS)
3-C
V = 0.7 volts (diode drop)
V = 6.3 /T - 0.7
out
V . = 8.2 volts
out
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Block #3 (Station Call Locator)
This is the 4 second delay designed into the circuit to determine
station calls from data transmission.
V *
. ,, , csus
-t, = RC In
d
RC
= (capacitor leakage) = R, + h. , C,
-L 1C j L J. -L
RCf = (6800 ft) (250 yf) = 1.7 seconds
h. . = Short circuit input impedance
ie,tl
V = Voltage of capacitor necessary to sustain relay RY..
in operation = dbsusD (Rx) + vd
I, = Sustaining base current of T. = I /hf .
bsus 1 csus fe,ti
I = Sustaining collector current T1
csus 6 1
T _ Relay drop voltage _ 8.1 volts
csus ~ Relay resistance 180 n
hf > = Short circuit forward current gain
I, = I /h. .. (of T.) = 45 ma/100 = 450 ya
bsus csus fe,tl v V
V = (I, )(R-) + V, = (450 ya) (1000 fl) + 0.7 volts
csus * bsus^ ^ 1J d ^ J ^ J
V
-t. = RC In ..CSUS =1.7 seconds (-2.39)
d V
out
+t, = 4.06 seconds
d
This formula is derived from the capacitor discharge formula:
V = V ^ (e~t/RC).
csus out
tThis value for R is an average of the dynamic characteristics.
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Block #5 (Master Control)
Triggering of master control relay RY
I = Current necessary to turn relay on
RY2
IDV = 45 ma (from specifications)
RY2
'c.tS
b,t3 1000
= 11.3 ma
Therefore,
I , = (100) 11.3 ma
Cw y L O
I ._ = Collector current of transistor T,
c,t3 3
I, ., = Base current of transistor T_
b ,t3 3
V - V ,
T _ cc d
b,t3 ~ R4
V = Power supply voltage
12.0 v. - 0.7 v.
*The collector current is therefore more than enough to turn on
relay RY2> Because of the 180 ft resistance of relay RY2, the
collector current is limited to a 65.5 ma value.
-------
Block #9 (Reset)
Triggering and resetting of SCR
= 200 ya (maximum from specifications for SCR)
V - V ,
T cc d
R5 = R5
I
V = Power supply voltage
ID_ = Current through R_
Kb b
12.0 v. - 0.7 v.
R5 100K ft
= 113 ya
The current loc is of a sufficient magnitude to gate the SCR into
Kb
conduction. The saturation voltage (V ) of the SCR is 0.7 volts,
o o t-i
This is not even enough voltage to overcome the two diode voltages
that are in series with transistor T , This is shown, by the fol-
lowing equation.
V I fRl+V+V
trig = Lb,t2LK3J d4 Vd,t2
I, = base current of transistor T0
b ,tz 2.
v*. = !u ^ifR?) +0.7 volts + 0.7 volts
trig b,t2v V
V . has to be at least greater than 1.4 volts in order to
trig &
activate transistor T_. Since V = 0.7 v., transistor
2 sscr '
T is not triggered.
V = V -fl IfRl-V -V
trig sscr ^ b,t2J ^ 3 d4 d,t2
10
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Block #10 (Time Delay)
Delay of one RC Time Constant
D. - Charges C~ and then prevents the capacitor from discharging
when the SCR is fired.
RC time delay = (R_ + h. .9) C
O 16 9^^ ^
hie,t2=
m =1.4 for Si transistors
V = 25 mv. @ 72ฐF
\f
V - V
cc sat,t2
eq = R4
Vsat,t2 = ฐ'2 V0lt
I = 1.8 ma
eq
, - (1.4) (100) (.025)
ie,t2 " (.0118)
h. .. = 296 fl
ie,t2
RC time delay* = (R + h. 9) C9
J 16 y \,ฃ ฃ
RC time delay = (Ik Q + 296 n) 68 yf
RC time delay = 88 msec (for one time constant @ 1 =11.8 ma)
eq
This delay allows the contacts to block "A" to give a strong
voltage pulse to Block #3 (station call locator in Figure 3).
11
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Block #11 (Master Stop)
Saturation of Transistor
I .- = Collector current of transistor T.
c,t2 2
h_ 2 = Short circuit forward current gain of
I, ._ = Base current of transistor !
b,t2 2
Rcdr.* Volt. - V,. - V,. - V, .
T _ _ d2 d4 d,t2
b,t2 ~ R2 " R3
6 - 0.7 - 0.7 - 0.7
b,t2 IK + IK
I, = 1.95 ma
D,t2
IC)t2 = (100) (1.95 ma)
I _- = 195 ma
c,t2
I . _ = Sufficient current to saturate transistor Tป
c,t2 2
V + = 0.2 v with R. connected in series with the
collector-emitter terminals
* Rcdr. = Recorder
+ Since transistor T_ needs 0.745 volt to operate relay RY , the satura-
tion of transistor T drops this voltage to 0.2 volt, thereby turning
off transistor T_ and deactivating relay RY2.
12
-------
SCR in conducting state
T - cc d2 dscr
scr R
12-0.7-0.7
scr IK
I = 10.6 ma (well within limiting characteristics of 4 amperes)
SCR in nonconducting "state
SCR acts as an open circuit
I = 0.0
scr
13
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SECTION VI
FABRICATION AND INSTALLATION
A prototype engineering model was assembled on a 3-1/2 x 2-1/2
inch printed card. Etching techniques were used to make connec-
tions among the components. All components are shown in their
respective positions on the printed circuit card layout (Figure
6).
PRINTED CjIRCUIT CARD
LAYiOUT
HEAT SINK
D
R*
9 9 $!
F i cr]H/L
c,
r
D|
,
czu '
RI
RY,
i i (n 1
N
C2
n
Djj
R2
RYg
t
0
c
D
Re
DC
Figure 6. Top view showing component locations
14
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The reason for keeping the interface compact can be seen from the
lack of space in the receiver (Figure 7).
Interface
"Circuit
Figure 7. Interface shown installed in receiver/recorder
The interface's total support comes from a 10-pin cinch connector
mounted in the side of the receiver; therefore, space and weight
had to be kept to a minimum. This was accomplished easily using
the printed circuit card techniques mentioned above.
Power for the printed circuit card comes in the form of a 12-volt
regulated power supply (Figure 8).
HEAT SINK
12.7 Volt
Zener Diode
12 v.D.C.
15
-------
This power supply is mounted in the printed circuit card with the
other components.
Total cost of the entire interface less labor was approximately
$18. This includes connector, printed circuit board, relays,
and all other electronic components.
16
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SECTION VII
TESTING
The interface circuit was operated for 6 months in the receiver
before a final design was fabricated, the failures that occurred
resulted from the electronic components, which were used within
their specifications.
Most failures resulted from the Centralab capacitors, C.. and C,
(Figure 3) rated at 15 volts for 250 yf, opening their circuit
after a few months of operation. The problem was corrected by
using Sprague capacitors rated at 25 volts for 250 yf.
Another failure was caused by a Motorola thyristor (HEP 320)
short-circuiting after a few months of operation; the device
was rated at 800 ma. This problem was corrected by using a GE
thyristor (C106B2) rated at 4000 ma.
The final design was fabricated after the testing (Figure 9).
The final design incorporated the described changes, i.e., Sprague
capacitors and a G.E. thyristor.
17
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SECTION VIII
APPENDICES
Page
A. Figure 1. Collection of Data before Interface was
Incorporated 19
B. Figure 2. Functional Block Diagram 20
C. Figure 3. Schematic in Functional Block Diagram
Form 21
D. Figure 4. Synchrogram of Interface Functions 22
E. Figure 5. Collection of Data after Interface was
Incorporated 23
F. Figure 9. Finalized Design of Interface Circuit 24
G. Parts List 25
18
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APPENDIX A
STATION LOCATION
PARAMETER
Test
PH
Cond.
Blank
D.O.
Temp.
SRI
Blank
LEGEND
1 2 3
LMR LAB GMR
NUMERICAL IDENTIFICATION
1
2
3
4
5
6
7
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Figure 1. Data collected on analog strip chart recorder with
computer employed in network and no interface between computer
and recorder. Note that a data word is logged with varying
numerical identification.
19
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APPENDIX B
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20
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APPENDIX C
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21
-------
Figure 4. Interface synchrogram.
22
-------
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APPENDIX F
Top view showing physical layout of components
Side view showing relative heights of components
Bottom view showing etching techniques incorporated
Figure 9. Photographs of the interface circuit.
24
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APPENDIX G
PARTS LIST
R - IK OHMS (part of an RC 4 second delay circuit)
R2 - IK OHMS (Current limiter for SCR)
R - IK OHMS (Bias resistor for T )
3 ^
R4 - IK OHMS (Bias resistor for T^
R - 100K OHMS (Bias resistor for SCR trigger)
R6 - IK OHMS (Bias resistor for T4)
C - 250 yf (part of an R C_ 4 second delay circuit)
C- - 68 yf (Delay capacitor)
C - 250 yf (Filter capacitor)
DI - 6941 Mallory (Full wave rectifier bridge)
D? - (Insolation diode)
D3 - C106B2 (SCR)
D. - 1N1521A (part of RqC- time constant circuit)
D - 6941 Mallory (Full wave rectifier bridge)
D, - Motorola (12.7 volt zener diode)
T - 40346 (Relay RY control)
T - 2N697 (Short circuits T to ground)
T3 - 2N697 (Relay RY2 control)
T - 2N697 (Voltage regulator)
TR _ (110V. to 6.3V. isolation transformer)
RY - 2P2T 12 volt Potter Brumfield relay
RY - 4P4T 12 volt Potter Brumfield relay
25
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-670/4-75-002
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
INTERFACING A 24-POINT ANALOG RECORDER TO A
COMPUTER CONTROLLED TELEMETRY LINE
5. REPORT DATE
February 1975;
Issuing Date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
John M. Teuschler
8. PERFORMING ORGANIZATION REPORT NO.
3. PERFORMING ORGANIZATION NAME AND ADDRESS
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
10. PROGRAM ELEMENT NO.
1HA327; ROAP 01AAD; TASK 10
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Same as above
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Interface circuitry was designed so that telemetered data originating from
various remote stations could be recorded by both a digital computer and an
analog recorder. The entire interface circuitry is mounted on a 3-1/2 x
2-1/2 inch printed circuit card and installed in the receiver. Data from
the two methods of collection can, therefore, be collected and a comparison
can be made. A switching network also permits computer control with computer
and recorder logging; or computer logging only; or recorder logging only.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
COSATl Field/Group
Logic design
Logic circuits
Control circuits
Timing circuits
Controllers
Data processing equipment
Telemetering data
13B
13. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
32
20. SECURITY CLASS (This page I
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
26
U.S. GOVERNMENT PRINTING OFFICE: 1975-657-591/53M Region No, 5-11
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