0361.9230191 $3.00 + .OO
Brain Research Bulletin,Vol. 27, PP. 145-149. 0 Pergamon Press plc, 1991. Printed in the U.S.A
LABORATORY
INSTRUMENTATION
AND COMPUTING
Triggering Module for Waveform Digitization ROBERT
Department
of Physiology,
M. REINKING’
AND YIANNIS
College of Medicine, Received
LAOURIS
University of Arizona,
19 February
Tucson, AZ 85724
1991
REINKING, R. M. AND Y. LAOURIS. Triggering module for waveform digitizafion. BRAIN RES BULL 27(l) 145-149, 1991.-A full circuit description is provided for a triggering module used to assist a small laboratory computer in digitizing muscle force- and EMG waveforms. During the stimulation of individual motor units using a standard fatigue test, a train of 13 pulses are delivered at a rate of 40 pps either intracellularly to a motor neuron, or extracellularly to functionally isolated single motor axons from among divided ventral-root nerve filaments. Trains are delivered at a rate of l/s for the duration of the test, which may range from 120 to 3600 s. Both the force and EMG profiles undergo changes during such tests and the quantification of parameters associated with their waveforms are of interest to neurobiologists. The triggering module allows a typical small laboratory computer to capture user-selected waveforms and thereby reduces the programming problems, timing constraints, storage requirements and analysis time associated with obtaining these parameters. The versatile circuit may be easily adapted to solve similar data-acquisition problems. The method was implemented on an Apple Macintosh II computer but can also be applied to
other systems equipped with appropriate software and a data-acquisition card. Motor units
Laboratory
Data sampling
A/D conversion
computer
power Schottky transistor-transistor-logic (TTL) devices (Texas Instruments Corp., Dallas, TX). To clarify the following discussion, the names of the logic devices (e.g., AND gate, counter) used in the circuit are in italic, and signal names are shown in boldface caps (input and output signals) and boldface lower case (internal signals). Note, however, that devices contained on a commercial analog-to-digital converter board (i.e., a counter, used in conjunction with the triggering module) are not so marked. The complete circuit diagram is shown in Fig. 2. It requires two trigger inputs (Fig. 1) from the experimental setup, a system trigger (SYSTEM TRIGGER) and an EMG trigger (EMG TRIGGER), and two inputs from the computer, a clock signal (50 KHZ CLOCK) and a cycle-select pulse (CYCLE-SELECT PULSE). The system trigger is used to identify the beginning of each cycle. For on-line use of the trigger module, it is derived from the train-trigger output of a physiological stimulator. During off-line analysis, a window discriminator (Frederick Haer, Brunswick, ME) is used to provide the trigger from data stored on an analog tape recorder. The system trigger occurs just prior to each train. It marks one cycle of thirteen impulses delivered to the animal preparation. The EMG trigger signal identifies the onset of each individual stimulus impulse (thirteen impulses/cycle), and is similarly derived from the stimulator output or analog tape. The clock signal is used to synchronize digitization of selected EMG waveforms and may be obtained from any stable pulse- or square-wave generator. For our purposes, the programcontrolled, counter/timer circuit (a counter) built into the dataacquisition (DAQ) board proved to be a convenient and flexible
THIS paper describes the operation of a simple triggering module designed to assist in the acquisition of user-selected muscle force profiles utilizing a sample-and-hold technique (4), and electromyographic (EMG) waveforms during a standard fatigue test (1). The test, which lasts 120-3600 s, consists of 40 Hz stimulus trains of 300 ms duration, delivered at a rate of l/s to single muscles or motor units. This stimulus paradigm produces force- and EMG waveforms like those shown in Fig. 1. These waveforms change relatively slowly over time, i.e., in s and min rather than ms. This relatively slow rate of change permits periodic sampling of waveform parameters such that the datastorage and analysis-time requirements are reduced without compromising accurate measurements of neuromuscular performance (2). A complete description of typical parameter-extraction methods is available in a recent report by this laboratory (3). The data processing system consists of an Apple Macintosh II computer (Apple Computer Inc., Cupertino, CA), laboratorydeveloped programs written in the LabView programming language (National Instruments Corp., Austin, TX) and a library of subroutines (LIB I; Arizona Technology Development Corp.). The circuit to be described receives logic pulses derived from an electronic physiological stimulator (Grass Instrument Co., Quincy , MA) and produces timing signals (i.e., analog-to-digital conversion pulses) which control a Macintosh data-acquisition board (Model MIO-16, National Instruments Corp., Austin TX). CIRCUIT DESCRIPTION
The triggering ,_
module was constructed
‘Kequests for reprints should be addressed University of Arizona, Tucson, AZ 85724.
using standard,
low-
to Robert M. Reinking,
EMG
Senior Research
145
Engineer,
Department
of Physiology,
College of Medicine,
146
REINKING AND LAOURIS
EQUIPMENT 1. 2.
-i _ 4.
5. 6. 7.
SOURCES
Advanced Micro Devices. Inc.. P.O. Box 3453, Sunnyvale, CA 94088. Apple Computer, Inc., 20525 Mariani Ave., Cupertino, CA 95014. Arizona Technology Development Corp.. 1430 E. Fort Lowell Rd.. Suite 200, Tucson AZ 85719. Frederick Haer, P.O. Box 337, Brunswick, ME 04011. Grass Instrument Co., 101 Old Colony Rd., Quincy MA 02169. National Instruments Corp.. 6504 Bridge Point Parkway, Austin, TX 78730-5039. Texas Instruments, P.O. Box 1444, Houston, TX 77001.
D
the clock signal. The cycle-select pulse determines which cycles are to be digitized, as each occurrence of the pulse marks the period just prior to the desired data. This pulse can be derived from an operator-controlled switch, an interval timer or any other such source. Again, we found that a DAQ-board counter was the best choice for our needs. The circuit produces two outputs, a cycle-select pulse (CYCLE TRIGGER) which permits triggering on selected force and EMG waveforms, and gated digitizing pulses (GATED DIGITIZING TRIGGER) for digitizing selected EMG waves. For description, the circuit may be conveniently divided into two parts, the Cycle-Selection Circuitry and the EMG Digitizingpulse Circuitry. source
for
Cycle-Selection
Circuitry
The cycle-selection circuitry works in conjunction with the DAQ board to inform a laboratory-developed LabView program when a user-selected cycle is to be digitized. Each cycle-select trigger delivered to the DAQ board causes one force waveform to be digitized. The software is arranged to record each waveform using 500 samples (with 1Zbit precision), internally timed at 1.25 m&le (625 ms total). This epoch is sufficient to completely capture the profile of force elicited by the stimulus regime in a fatiguing muscle or motor unit. Only ICs la, lb and 2a (Fig. 2) are used for this part of the circuit. A SYSTEM TRIGGER pulse is conditioned by a oneshot multivibrator (IC la) to provide a brief (ca. 200 ns) pulse, the conditioned system trigger. System triggers supplied to counter 1 of the DAQ board (configured as an event counter). are used by the program to decide if the current count just precedes the user-selected one. When this condition is met, counter 2 of the DAQ board issues a 1.2-s pulse (CYCLE-SELECT PULSE). A flip-flop integrated circuit, IC 2a, is set by CYCLESELECT PULSE and reset by each conditioned system trigger pulse (IC la output). The resultant output is a pulse (next cycle), beginning shortly after the system trigger (with ca. 1 ms of program-induced jitter or uncertainty as to the exact starting time, i.e., “software uncertainty” in Fig. 3C) and ending exactly at the next SYSTEM TRIGGER, the user-selected cycle. The onset-time jitter is not important in the present system because the ending time is the signal that triggers data acquisition. A one-shot multivibrator integrated circuit, IC lb, conditions the output and provides a brief pulse (CYCLE TRIGGER, 200 ns) to signal the data acquisition card that A/D conversion is to begin. The software produces a pulse just preceding each user-selected cycle, and the data acquisition begins each time the CYCLE TRIGGER occurs.
FIG. 1. The waveforms to be analyzed and their trigger pulses. Shown top to bottom are, A: ryP&zl muscle-force waveform produced by stimulation of cat tibialis posterior muscle. B: EMG recorded using a silverball electrode placed on the surface of the muscle. The animal preparation is stimulated at a rate of l/s with trains of thirteen stimuli at 40 Hz (25-ms intervals). The evoked EMG waveforms consume about 10 ms of each interpulse interval. The remaining dead-time is not sampled when the data acquisition is controlled by the triggering module. C: EMG TRIGGER derived from the output of a physiological stimulator. D: SYSTEM TRIGGER PULSE derived from the train-synchronization trigger output of a physiological stimulator. These signals are available on-line during an experiment, as well as off-line when played back from an FM analog tape recorder. Calibrations: time. 50 ms; force. 500 mN; EMG, 25 mV.
EMG Digitizing-Pulse
Circuit?
During a typical fatigue test, stimuli within a train (cycle) are delivered at 25ms intervals, with each elicited EMG waveform lasting less than 10 ms. To avoid collecting unnecessary data, we only digitize 12 ms of the EMG recording channel after each EMG trigger. Using this technique, 156 ms of data completely describes the thirteen waveforms associated with a selected cycle. This results in significant savings in data storage and subsequent processing time. The digitizing pulse circuitry provides a simple method by which this task may be accomplished. A LabView program developed in our laboratory configures counter 5 of the data-acquisition board to provide a 50kHz clock signal and also configures the A/D converter to accept external clock pulses. Thus a IZbit analog-to-digital conversion is made each time a digitizing pulse is supplied by the timing module. The circuit is arranged to provide a 12-ms record of each EMG waveform within a selected cycle. The EMG digitizing pulse circuitry also works in conjunction with the DAQ board and uses the Cycle-Select Circuitry and a laboratory-developed LabView program to acquire EMG waveforms each time a user-selected cycle is to be digitized. The circuitry underlying this process is outlined in Fig. 2. Each EMG TRIGGER is conditioned by a one-shot multivibrator (IC 7b) to provide a 200-ns wide pulse, the conditioned EMG trigger. The program configures DAQ counter 5 to provide a clock signal (50 kHz CLOCK) that is gated by the EMG circuit and used by the A/D converter to acquire data. The digitizing pulses (GATED TRIGGER) are controlled by AND gates IC 4a and
WAVEFORM
147
DIGITIZATION
txdkioned
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SYSdem
e!Jger
SYSTEM TRlGGER+
0 A
> 4LS74
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IC2aT
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PRESET
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,
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FIG. 2. Circuit diagram of the triggering module which is constructed using standard transistor-transistor-logic (TlX) chips. The module works in conjunction with the counter/timer portion of a data-acquisition board to control the A/D converter portion of the same instrument, which plugs into one of the six NUBUS slots in an Apple Macintosh II computer. Integrated circuit type designations as well as their relevant input and output terminals (e.g., clock and clear inputs) are shown within each circuit symbol. Devices with dual functions (two or more circuits/package) are appropriately lettered (e.g., IC la, IC lb). Power supply connections and one-shot multivibrator timing components (all these devices produce noncritical pulse widths) are omitted for simplicity. Letters A-K correspond to logic pulses shown in the timing diagram, Fig. 3. The reader is referred to the text for a complete description of the circuit. IC 8a. Flip-flop IC 3a is set by each conditioned EMG trigger and reset by flip-flop IC 7a (end count) to produce one 12 ms clock gate for each EMG TRIGGER. AND gate IC 4a is open
for a time determined by the clock gate. The clock gate time is determined by a series of counters (described below) that use the clock signal as input. The conditioned EMG trigger pulse
REINKING
n
I
0
LAOURIS
clears the count of all counters (IC 5a, 5b and 6) so the count begins from zero for each trigger. When gate IC 4a opens. counter IC 5a begins counting. AND gate IC 4b produces an output when counter IC 5a reaches a count of IO. In a similar manner, counter IC 5b (count of IO) in conjunction with ANU gate IC 4c, and counter IC 6 (count of 6) in conjunction with AND gate IC 4d, form a divide-by-600 function. The result is that AND gate IC 4a is open for 12 ms, as determined by the product of the clock interval (20 ps) and the count (600). Thus. for every EMG TRIGGER supplied, AND gate IC 4a allow\ 600 pulses to pass. These pulses are then gated by 1C Xa on selected cycles (GATED TRIGGER) and are used by the MIO-16 board to control the A/D converter that enables the digitizing of each EMG waveform in the selected train.
;oftwar~rcertanty’
D
AND
RELEVANCE TO OTHER DATA~ACQUISITION PROBLbMS
G
lUllIll
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II
II
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KK FIG. 3. Timing diagram of triggering module. Integrated circuits (ICs) reported here are those shown in the circuit diagram in Fig. 2. Waveforms A-E show the method for digitizing user-selected cycles. A: SYSTEM TRIGGER marks onset of each cycle. B: signal A after conditioning by IC la, a one-shor mulrivibraror. C: computer program logic pulse (CYCLE-SELECT PULSE) issued prior to the user-selected cycle. A time uncertainty of up to 1 ms exists at the onset of this pulse. and of the pulse shown in 3D, which is due to the language and operating system used. The onset-time uncertainty is unimportant in the present
system because the “off” time of flip-flop IC 2a is the signal for starting data acquisition. D: output offliplflop IC 2a (next cycle; “set” by software and “reset” by the conditioned system trigger). The falling edge of next cycle is conditioned by IC lb and provides the DAQ trigger (CYCLE TRIGGER) when internally timed data acquisition is required. E: output of one-shor multivibrutor IC lb, a 200-ns pulse used to gate selected cycles during externally timed data acquisition. Waveforms F-K show the method for digitizing user-selected EMG waveforms. F: cycle gate; pulse that begins when the user-selected cycle starts and ends with the next SYSTEM TRIGGER. G: EMG TRIGGER marks onset of each stimulus. H: conditioned EMG trigger, a 200~ns pulse derived from the input, I: output (end count) of one-shot multivibrator IC 7a at the end of the divide-by-600 counter group (ICs 5a, 5b, and 6). .I: trigger gate that results from flip-flop IC 3a being “set” by conditioned EMG trigger and “reset” by end count. K: schematic representation of the pulses (GATED TRIGGER) gated by IC 8a that provide the trigger when internally timed data acquisition is required for EMG digitization.
The circuit arrangement and timing parameters given are appropriate for the standard fatigue test used by physiologists to classify motor units (1). However, it can be easily changed to accommodate other tasks. For example, in the EMG DigitizingPulse Circuitry, the overall operation is controlled by the two external trigger pulses SYSTEM TRIGGER and EMG TRIGGER. By changing the interval between SYSTEM TRIGGERS, one could accommodate higher or lower cycle rates. Changing the number, interval and timing (there is no requirement for regular timing) of EMG TRIGGERS would allow very flexible data acquisition. The duration of each epoch sampled in response to an EMG TRIGGER may be altered by rearranging the connections of counters (IC 5a. 5b and 6) and the AND gates (IC 4b, 4c and 4d). to provide a wide range of counts and hence sample durations. An even more versatile arrangement, not required in the present application, would be to obtain the counter function from a computer-programmable circuit such as the AM95 13 System Timing Controller (Advanced Micro Devices. Inc., Sunnyvale. CA). To alter the digitizing rate one need only to adjust the frequency of the externally supplied clock, 50 kHz CLOCK, in the present arrangement. The basic circuit design presented here can provide a wide range of data capture by simply altering internal connections or externally supplied trigger pulses. OPERATION The circuit requires no operator intervention during its function. Once enabled, all operations take place automatically. Trigger signals supplied to the module reset or clear all circuit elements such that proper operation is ensured without adjustment or manipulation of controls. In summary, this simple module assists in the acquisition of user-selected muscle force and EMG profiles and also provides significant savings in data storage and subsequent processing time. ACKNOWLEDGEMENTS The authors wish to thank Dr. Douglas G. Stuart for his interest and support and Drs. Roger Enoka, Thomas Hamm and Michael Nordstrom for reading a draft of this manuscript. The work was supported in part by USPHS grants HL 07249 (Department of Physiology), NS 07309 (Motor-Control Program), NS 25077 (to D. G. Stuart). and NS 20544 (to R. M. Enoka and D. G. Stuart).
REFERENCES
I. Burke, R. E.; Levine, D. N.; Tsairis, P.: Zajac, F. E. Physiological types and histochemical
profiles
in motor units of the cat gas-
trocnemius. J. Physiol. (Land.) 234:723-748; 1973. 2. Enoka, R. M.; Rankin, L. L.; Stuart, D. G.; Voltz. K. A. Fatiga-
WAVEFORM
DIGITIZATION
bility of rat hindlimb muscle: associations between electromyogram and force during a fatigue test. J. Physiol. (Land.) 408:251-270; 1989. 3. Laouris, Y.; Reinking, R. M.; Stuart, D. G. Computer-aided extraction of the EMG of single motor units. Brain Res. Bull. 26:997-
149
1002; 1991. 4. Reinking, R. M.; Stephens, J. A. Interface unit for on-line measurements of motor unit properties with a small laboratory computer. Am. J. Phys. Med. 54:181-192; 1975.
Brain Research Bulletin,Vol. 27, PP. 145-149. 0 Pergamon Press plc, 1991. Printed in the U.S.A
LABORATORY
INSTRUMENTATION
AND COMPUTING
Triggering Module for Waveform Digitization ROBERT
Department
of Physiology,
M. REINKING’
AND YIANNIS
College of Medicine, Received
LAOURIS
University of Arizona,
19 February
Tucson, AZ 85724
1991
REINKING, R. M. AND Y. LAOURIS. Triggering module for waveform digitizafion. BRAIN RES BULL 27(l) 145-149, 1991.-A full circuit description is provided for a triggering module used to assist a small laboratory computer in digitizing muscle force- and EMG waveforms. During the stimulation of individual motor units using a standard fatigue test, a train of 13 pulses are delivered at a rate of 40 pps either intracellularly to a motor neuron, or extracellularly to functionally isolated single motor axons from among divided ventral-root nerve filaments. Trains are delivered at a rate of l/s for the duration of the test, which may range from 120 to 3600 s. Both the force and EMG profiles undergo changes during such tests and the quantification of parameters associated with their waveforms are of interest to neurobiologists. The triggering module allows a typical small laboratory computer to capture user-selected waveforms and thereby reduces the programming problems, timing constraints, storage requirements and analysis time associated with obtaining these parameters. The versatile circuit may be easily adapted to solve similar data-acquisition problems. The method was implemented on an Apple Macintosh II computer but can also be applied to
other systems equipped with appropriate software and a data-acquisition card. Motor units
Laboratory
Data sampling
A/D conversion
computer
power Schottky transistor-transistor-logic (TTL) devices (Texas Instruments Corp., Dallas, TX). To clarify the following discussion, the names of the logic devices (e.g., AND gate, counter) used in the circuit are in italic, and signal names are shown in boldface caps (input and output signals) and boldface lower case (internal signals). Note, however, that devices contained on a commercial analog-to-digital converter board (i.e., a counter, used in conjunction with the triggering module) are not so marked. The complete circuit diagram is shown in Fig. 2. It requires two trigger inputs (Fig. 1) from the experimental setup, a system trigger (SYSTEM TRIGGER) and an EMG trigger (EMG TRIGGER), and two inputs from the computer, a clock signal (50 KHZ CLOCK) and a cycle-select pulse (CYCLE-SELECT PULSE). The system trigger is used to identify the beginning of each cycle. For on-line use of the trigger module, it is derived from the train-trigger output of a physiological stimulator. During off-line analysis, a window discriminator (Frederick Haer, Brunswick, ME) is used to provide the trigger from data stored on an analog tape recorder. The system trigger occurs just prior to each train. It marks one cycle of thirteen impulses delivered to the animal preparation. The EMG trigger signal identifies the onset of each individual stimulus impulse (thirteen impulses/cycle), and is similarly derived from the stimulator output or analog tape. The clock signal is used to synchronize digitization of selected EMG waveforms and may be obtained from any stable pulse- or square-wave generator. For our purposes, the programcontrolled, counter/timer circuit (a counter) built into the dataacquisition (DAQ) board proved to be a convenient and flexible
THIS paper describes the operation of a simple triggering module designed to assist in the acquisition of user-selected muscle force profiles utilizing a sample-and-hold technique (4), and electromyographic (EMG) waveforms during a standard fatigue test (1). The test, which lasts 120-3600 s, consists of 40 Hz stimulus trains of 300 ms duration, delivered at a rate of l/s to single muscles or motor units. This stimulus paradigm produces force- and EMG waveforms like those shown in Fig. 1. These waveforms change relatively slowly over time, i.e., in s and min rather than ms. This relatively slow rate of change permits periodic sampling of waveform parameters such that the datastorage and analysis-time requirements are reduced without compromising accurate measurements of neuromuscular performance (2). A complete description of typical parameter-extraction methods is available in a recent report by this laboratory (3). The data processing system consists of an Apple Macintosh II computer (Apple Computer Inc., Cupertino, CA), laboratorydeveloped programs written in the LabView programming language (National Instruments Corp., Austin, TX) and a library of subroutines (LIB I; Arizona Technology Development Corp.). The circuit to be described receives logic pulses derived from an electronic physiological stimulator (Grass Instrument Co., Quincy , MA) and produces timing signals (i.e., analog-to-digital conversion pulses) which control a Macintosh data-acquisition board (Model MIO-16, National Instruments Corp., Austin TX). CIRCUIT DESCRIPTION
The triggering ,_
module was constructed
‘Kequests for reprints should be addressed University of Arizona, Tucson, AZ 85724.
using standard,
low-
to Robert M. Reinking,
EMG
Senior Research
145
Engineer,
Department
of Physiology,
College of Medicine,
146
REINKING AND LAOURIS
EQUIPMENT 1. 2.
-i _ 4.
5. 6. 7.
SOURCES
Advanced Micro Devices. Inc.. P.O. Box 3453, Sunnyvale, CA 94088. Apple Computer, Inc., 20525 Mariani Ave., Cupertino, CA 95014. Arizona Technology Development Corp.. 1430 E. Fort Lowell Rd.. Suite 200, Tucson AZ 85719. Frederick Haer, P.O. Box 337, Brunswick, ME 04011. Grass Instrument Co., 101 Old Colony Rd., Quincy MA 02169. National Instruments Corp.. 6504 Bridge Point Parkway, Austin, TX 78730-5039. Texas Instruments, P.O. Box 1444, Houston, TX 77001.
D
the clock signal. The cycle-select pulse determines which cycles are to be digitized, as each occurrence of the pulse marks the period just prior to the desired data. This pulse can be derived from an operator-controlled switch, an interval timer or any other such source. Again, we found that a DAQ-board counter was the best choice for our needs. The circuit produces two outputs, a cycle-select pulse (CYCLE TRIGGER) which permits triggering on selected force and EMG waveforms, and gated digitizing pulses (GATED DIGITIZING TRIGGER) for digitizing selected EMG waves. For description, the circuit may be conveniently divided into two parts, the Cycle-Selection Circuitry and the EMG Digitizingpulse Circuitry. source
for
Cycle-Selection
Circuitry
The cycle-selection circuitry works in conjunction with the DAQ board to inform a laboratory-developed LabView program when a user-selected cycle is to be digitized. Each cycle-select trigger delivered to the DAQ board causes one force waveform to be digitized. The software is arranged to record each waveform using 500 samples (with 1Zbit precision), internally timed at 1.25 m&le (625 ms total). This epoch is sufficient to completely capture the profile of force elicited by the stimulus regime in a fatiguing muscle or motor unit. Only ICs la, lb and 2a (Fig. 2) are used for this part of the circuit. A SYSTEM TRIGGER pulse is conditioned by a oneshot multivibrator (IC la) to provide a brief (ca. 200 ns) pulse, the conditioned system trigger. System triggers supplied to counter 1 of the DAQ board (configured as an event counter). are used by the program to decide if the current count just precedes the user-selected one. When this condition is met, counter 2 of the DAQ board issues a 1.2-s pulse (CYCLE-SELECT PULSE). A flip-flop integrated circuit, IC 2a, is set by CYCLESELECT PULSE and reset by each conditioned system trigger pulse (IC la output). The resultant output is a pulse (next cycle), beginning shortly after the system trigger (with ca. 1 ms of program-induced jitter or uncertainty as to the exact starting time, i.e., “software uncertainty” in Fig. 3C) and ending exactly at the next SYSTEM TRIGGER, the user-selected cycle. The onset-time jitter is not important in the present system because the ending time is the signal that triggers data acquisition. A one-shot multivibrator integrated circuit, IC lb, conditions the output and provides a brief pulse (CYCLE TRIGGER, 200 ns) to signal the data acquisition card that A/D conversion is to begin. The software produces a pulse just preceding each user-selected cycle, and the data acquisition begins each time the CYCLE TRIGGER occurs.
FIG. 1. The waveforms to be analyzed and their trigger pulses. Shown top to bottom are, A: ryP&zl muscle-force waveform produced by stimulation of cat tibialis posterior muscle. B: EMG recorded using a silverball electrode placed on the surface of the muscle. The animal preparation is stimulated at a rate of l/s with trains of thirteen stimuli at 40 Hz (25-ms intervals). The evoked EMG waveforms consume about 10 ms of each interpulse interval. The remaining dead-time is not sampled when the data acquisition is controlled by the triggering module. C: EMG TRIGGER derived from the output of a physiological stimulator. D: SYSTEM TRIGGER PULSE derived from the train-synchronization trigger output of a physiological stimulator. These signals are available on-line during an experiment, as well as off-line when played back from an FM analog tape recorder. Calibrations: time. 50 ms; force. 500 mN; EMG, 25 mV.
EMG Digitizing-Pulse
Circuit?
During a typical fatigue test, stimuli within a train (cycle) are delivered at 25ms intervals, with each elicited EMG waveform lasting less than 10 ms. To avoid collecting unnecessary data, we only digitize 12 ms of the EMG recording channel after each EMG trigger. Using this technique, 156 ms of data completely describes the thirteen waveforms associated with a selected cycle. This results in significant savings in data storage and subsequent processing time. The digitizing pulse circuitry provides a simple method by which this task may be accomplished. A LabView program developed in our laboratory configures counter 5 of the data-acquisition board to provide a 50kHz clock signal and also configures the A/D converter to accept external clock pulses. Thus a IZbit analog-to-digital conversion is made each time a digitizing pulse is supplied by the timing module. The circuit is arranged to provide a 12-ms record of each EMG waveform within a selected cycle. The EMG digitizing pulse circuitry also works in conjunction with the DAQ board and uses the Cycle-Select Circuitry and a laboratory-developed LabView program to acquire EMG waveforms each time a user-selected cycle is to be digitized. The circuitry underlying this process is outlined in Fig. 2. Each EMG TRIGGER is conditioned by a one-shot multivibrator (IC 7b) to provide a 200-ns wide pulse, the conditioned EMG trigger. The program configures DAQ counter 5 to provide a clock signal (50 kHz CLOCK) that is gated by the EMG circuit and used by the A/D converter to acquire data. The digitizing pulses (GATED TRIGGER) are controlled by AND gates IC 4a and
WAVEFORM
147
DIGITIZATION
txdkioned
U-Up
SYSdem
e!Jger
SYSTEM TRlGGER+
0 A
> 4LS74
74Ls22,
IC2aT
A
CLR
AD
B 0
D -cm 0next
CYCLE TRIGGER
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CLOCK CLR
IC 1b I
PRESET
0
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+
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L
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74LS90 A
GATED -,-.-,l-ll
74LSO8 QD
IC6QA
IC 7a 74LS221
QB QC -
,
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FIG. 2. Circuit diagram of the triggering module which is constructed using standard transistor-transistor-logic (TlX) chips. The module works in conjunction with the counter/timer portion of a data-acquisition board to control the A/D converter portion of the same instrument, which plugs into one of the six NUBUS slots in an Apple Macintosh II computer. Integrated circuit type designations as well as their relevant input and output terminals (e.g., clock and clear inputs) are shown within each circuit symbol. Devices with dual functions (two or more circuits/package) are appropriately lettered (e.g., IC la, IC lb). Power supply connections and one-shot multivibrator timing components (all these devices produce noncritical pulse widths) are omitted for simplicity. Letters A-K correspond to logic pulses shown in the timing diagram, Fig. 3. The reader is referred to the text for a complete description of the circuit. IC 8a. Flip-flop IC 3a is set by each conditioned EMG trigger and reset by flip-flop IC 7a (end count) to produce one 12 ms clock gate for each EMG TRIGGER. AND gate IC 4a is open
for a time determined by the clock gate. The clock gate time is determined by a series of counters (described below) that use the clock signal as input. The conditioned EMG trigger pulse
REINKING
n
I
0
LAOURIS
clears the count of all counters (IC 5a, 5b and 6) so the count begins from zero for each trigger. When gate IC 4a opens. counter IC 5a begins counting. AND gate IC 4b produces an output when counter IC 5a reaches a count of IO. In a similar manner, counter IC 5b (count of IO) in conjunction with ANU gate IC 4c, and counter IC 6 (count of 6) in conjunction with AND gate IC 4d, form a divide-by-600 function. The result is that AND gate IC 4a is open for 12 ms, as determined by the product of the clock interval (20 ps) and the count (600). Thus. for every EMG TRIGGER supplied, AND gate IC 4a allow\ 600 pulses to pass. These pulses are then gated by 1C Xa on selected cycles (GATED TRIGGER) and are used by the MIO-16 board to control the A/D converter that enables the digitizing of each EMG waveform in the selected train.
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RELEVANCE TO OTHER DATA~ACQUISITION PROBLbMS
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KK FIG. 3. Timing diagram of triggering module. Integrated circuits (ICs) reported here are those shown in the circuit diagram in Fig. 2. Waveforms A-E show the method for digitizing user-selected cycles. A: SYSTEM TRIGGER marks onset of each cycle. B: signal A after conditioning by IC la, a one-shor mulrivibraror. C: computer program logic pulse (CYCLE-SELECT PULSE) issued prior to the user-selected cycle. A time uncertainty of up to 1 ms exists at the onset of this pulse. and of the pulse shown in 3D, which is due to the language and operating system used. The onset-time uncertainty is unimportant in the present
system because the “off” time of flip-flop IC 2a is the signal for starting data acquisition. D: output offliplflop IC 2a (next cycle; “set” by software and “reset” by the conditioned system trigger). The falling edge of next cycle is conditioned by IC lb and provides the DAQ trigger (CYCLE TRIGGER) when internally timed data acquisition is required. E: output of one-shor multivibrutor IC lb, a 200-ns pulse used to gate selected cycles during externally timed data acquisition. Waveforms F-K show the method for digitizing user-selected EMG waveforms. F: cycle gate; pulse that begins when the user-selected cycle starts and ends with the next SYSTEM TRIGGER. G: EMG TRIGGER marks onset of each stimulus. H: conditioned EMG trigger, a 200~ns pulse derived from the input, I: output (end count) of one-shot multivibrator IC 7a at the end of the divide-by-600 counter group (ICs 5a, 5b, and 6). .I: trigger gate that results from flip-flop IC 3a being “set” by conditioned EMG trigger and “reset” by end count. K: schematic representation of the pulses (GATED TRIGGER) gated by IC 8a that provide the trigger when internally timed data acquisition is required for EMG digitization.
The circuit arrangement and timing parameters given are appropriate for the standard fatigue test used by physiologists to classify motor units (1). However, it can be easily changed to accommodate other tasks. For example, in the EMG DigitizingPulse Circuitry, the overall operation is controlled by the two external trigger pulses SYSTEM TRIGGER and EMG TRIGGER. By changing the interval between SYSTEM TRIGGERS, one could accommodate higher or lower cycle rates. Changing the number, interval and timing (there is no requirement for regular timing) of EMG TRIGGERS would allow very flexible data acquisition. The duration of each epoch sampled in response to an EMG TRIGGER may be altered by rearranging the connections of counters (IC 5a. 5b and 6) and the AND gates (IC 4b, 4c and 4d). to provide a wide range of counts and hence sample durations. An even more versatile arrangement, not required in the present application, would be to obtain the counter function from a computer-programmable circuit such as the AM95 13 System Timing Controller (Advanced Micro Devices. Inc., Sunnyvale. CA). To alter the digitizing rate one need only to adjust the frequency of the externally supplied clock, 50 kHz CLOCK, in the present arrangement. The basic circuit design presented here can provide a wide range of data capture by simply altering internal connections or externally supplied trigger pulses. OPERATION The circuit requires no operator intervention during its function. Once enabled, all operations take place automatically. Trigger signals supplied to the module reset or clear all circuit elements such that proper operation is ensured without adjustment or manipulation of controls. In summary, this simple module assists in the acquisition of user-selected muscle force and EMG profiles and also provides significant savings in data storage and subsequent processing time. ACKNOWLEDGEMENTS The authors wish to thank Dr. Douglas G. Stuart for his interest and support and Drs. Roger Enoka, Thomas Hamm and Michael Nordstrom for reading a draft of this manuscript. The work was supported in part by USPHS grants HL 07249 (Department of Physiology), NS 07309 (Motor-Control Program), NS 25077 (to D. G. Stuart). and NS 20544 (to R. M. Enoka and D. G. Stuart).
REFERENCES
I. Burke, R. E.; Levine, D. N.; Tsairis, P.: Zajac, F. E. Physiological types and histochemical
profiles
in motor units of the cat gas-
trocnemius. J. Physiol. (Land.) 234:723-748; 1973. 2. Enoka, R. M.; Rankin, L. L.; Stuart, D. G.; Voltz. K. A. Fatiga-
WAVEFORM
DIGITIZATION
bility of rat hindlimb muscle: associations between electromyogram and force during a fatigue test. J. Physiol. (Land.) 408:251-270; 1989. 3. Laouris, Y.; Reinking, R. M.; Stuart, D. G. Computer-aided extraction of the EMG of single motor units. Brain Res. Bull. 26:997-
149
1002; 1991. 4. Reinking, R. M.; Stephens, J. A. Interface unit for on-line measurements of motor unit properties with a small laboratory computer. Am. J. Phys. Med. 54:181-192; 1975.
