Schiller AT-2plus User manual

i
SCHILLER AT-2plus 6-Channel ECG Unit
AT-2plus
6-Channel
ECG unit
SCHILLER AG
Altgasse 68
CH-6341Baar,Switzerland
Phone: + 41 41 766 42 42
Fax: + 41 41 761 08 80
Homepage:http://www.schiller.ch/

vii
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
What's in this book
The service philosophy for the AT-2plus Memory is fault finding to module level. The purpose of
this book is to provide all the information necessary to enable the service engineer to efficiently
locate and replace a faulty module. This book assumes no detailed knowledge of the AT-2plus
Memory but does require that the service engineer is familiar with standard workshop practices.
The book is divided into the following chapters:
Chapter 1 - Operating Elements
The purpose of this chapter is to provide an easy reference for all the main operator functions and
to give a basic introduction to the AT-2plus Memory. This chapter gives details of the operator
controls with the operation and function of each key briefly explained. The information in this
chapter provides a background to the operating functions only. Complete operating information
is provided in the SCHILLER AT-2plus User Guide.
Chapter 2 - Functional Overview
This chapter provides a functional overview of the AT-2plus Memory. The description is supported
by functional block diagrams.
Chapter 3 - Fault Diagnosis
This chapter provides a guide to locate a fault to module level. The diagnostics are presented in
a logical sequence of fault finding algorithms and procedures. Illustrations are provided to support
the text where needed.
Chapter 4 - Module Removal and Replacement
This chapter gives an overview of the physical construction of the AT-2plus Memory with the
main physical attributes of the unit briefly described. The physical description is supported by
illustrations showing the internal location of all modules. Removal and replacement instructions
for all removable modules are also provided in this chapter. Each procedure is autonomous with
details of tools, jumper settings, adjustments and settings or special requirements that are required
before and after replacement. Functional checks that must be carried out after replacing a module
are also provided.
Chapter 5 - Adjustments
This chapter provides all adjustments and settings. Also detailed in this chapter are basic functional
test procedures that can be performed to check the functioning of the unit.
Chapter 6 - Spare Parts
This chapter provides the part numbers and reordering information for all replaceable modules.
Also included in this chapter are details of any special test equipment or special tools required for
adjustment or fault finding procedures.

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SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
What's in this book
Chapter7-Technical Data
The full technical specification of the AT-2plus Memory is given in this chapter.
Chapter8-Glossary
This chapter explains all the acronyms and signal titles used in this book and in the AT-2plus
Memory circuit diagrams.
CircuitDiagrams&Board layouts
The circuit diagrams and component layouts are provided for all boards. These details are provided
for information only.

Page 1.1
Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
Contents
Introduction 1.2
List of Symbols 1.2
Location & Power 1.3
Switching On and Off 1.3
The Keyboard 1.4
LCD Screen 1.6
AT-2plus Short Form Operating Instructions 1.7
Modes of Operation 1.8
Automatic Mode 1.9
Manual Mode 1.10
Settings 1.12
Automatic Mode (ECG) Settings 1.20
Memory and Data Transmission Option 1.24
Erasing Memory Files 1.25
Transmitting Stored Files 1.25
Transmission Settings 1.25
Modem Transmission 1.26
Line Transmission 1.27
Software Updating via RS-232 1.28
Care & Maintenance 1.29
Replacing the Recording Paper 1.31
Thermal Paper Handling 1.32

Page 1.2
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
Introduction
TheCARDIOVITAT-2plus is a 6-channel ECG recorder with all (12) ECG signals simultaneously
processed to provide instant ECG recordings. Two automatic recording modes can be individually
preset to enable one button ECG recording of preferred print formats.
The AT-2plus includes the following features:
•Low weight and compact dimensions
•Large A4 size printout from integrated quality thermal printer
•Built-in rechargeable battery for mains-independent use - 4hrs normal use or 300 printouts
on one battery charge
•Large, clear LCD for ECG preview prior to printing
•Simple one key operation for main functions
•Automatic or manual recording modes
•Selectable printing formats
•ECG memory for easy copying
•Interpretation program option (including measurements) for children and adults
•Alphanumeric keyboard for patient data entry and clinical comments
List of Symbols

Page 1.3
Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Location & Power
Location
Do not keep or operate the apparatus in a wet, moist, or dusty environment.Also, avoid exposure
to direct sunlight or heat from other sources. Do not allow the unit to come into contact with
acidic vapours or liquids, as such contact may cause irreparable damage. The unit should not be
placed near X-ray or diathermy units, large transformers or motors. The unit must be placed on a
flat surface and must not be operated in areas where there is any danger of explosion.
Power Supply
The mains connection is on the rear of the unit. The mains indicator lamp on the keyboard is
always lit when the unit is connected to the mains supply. The unit can either be operated from
the mains supply or from the built-in rechargeable battery. The power source isa indicted on the
top line of the LCD.
R L F C1 C2
C3 C4 C5 C6
HR: 76/min Wed 20-AUG-96 18:20:21
Power Indication
When mains is connected a mains symbol is displayed (as shown above). When the unit is
running on battery power a battery symbol is displayed:
When battery capacity is limited, the battery symbol flashes on and off.
To recharge the battery, connect the apparatus to the mains supply by means of the supplied
power cable. A totally discharged battery needs less than 15 hours to be fully recharged (60% in
less than 3 hours, 90% in less than 7 hours). A fully charged battery gives approximately 4 hours
of normal use. The unit can remain connected to the mains supply without any danger of damage
to either the battery or the unit.
Switching On and Off
The CARDIOVIT AT-2plus is switched on with the green ON key
and off by means of the red OFF key
The unit is automatically switched off after 5 minutes (30 seconds if battery capacity is limited) if
no key is pressed and the patient cable is not connected.
Potential Equalisation
If the AT-2plus is used in conjunction with other patient connected equipment, we recommend
that the potential equalisation stud on the rear of the unit is connected to the hospital/ building
common ground with the yellow/green ground cable (Part-no. 2.310005). When working from
an emergency vehicle, the vehicle common ground can be used.

Page 1.4
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
The Keyboard

Page 1.5
Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
The Keyboard (cont.)
1Print extra copy - of Auto mode recording currently in memory. Press the ALT key first
followed by this key to obtain a copy in Auto format 2.
2Display/enter patient data. When the patient data is displayed, pressing this key again
returns to the ECG. Use the up/down arrows to go to the next data entry field.
In the `Born` (date of birth field), only the patients year of birth need be entered if desired. When
two digits are entered (patients year of birth), the AT-2plus calculates the age of the patient
according to the year entered. When the full DOB is entered, the age is calculated precisely.
3Myogram filter ON / OFF. The cutoff frequency can be defined and is detailed in
`Settings`.
4. The top figures on the number keys designated > and < changes the lead group displayed
on the screen.
5. Auto sensitivity key - automatically sets the ECG printout sensitivity ( in AUTO mode
only) to the best setting for the signal strength (5mm/mV or 10mm/mV)
6. The top figures on the number keys designated 5, 10, and 20 set the sensitivity of the ECG
both on the screen and on the (manual) printout. The sensitivity is 5, 10 or 20 mm / mV.
7. The top figures on the number keys designated 5/10, 25, and 50 set the speed of the ECG
both on the screen and on the (manual) printout. The speed on the screen can only be set to
25 or 50 mm / s. The speed of the manual printout can be 5, 10, 25 or 50 mm/s. The 5 and
10 mm/s settings are both on the same key which toggles the two speeds.
8. The top character `QRS` toggles the QRS beeper ON/ OFF
9. Delete last typed character.
10.Switch the unit OFF.
11.Switch the unit ON.
12.Manual mode recording - start continuous printout of ECG - until STOP key pressed
13.Auto Mode recording (in Auto mode 1). Press ALT followed by the AUTO key for auto
mode 2.
14.STOP printout / confirm (new) setting
15.ALT key - key for initiation of setups and selection of second format for printout and auto
mode recording
16.In ECG mode use the UP/DOWN arrows to adjust screen contrast. When entering patient
data use the LEFT/RIGHT arrow keys to move cursor in data field. Use the UP/DOWN
arrow keys to go up/down to the next data entry
17.Mains Indicator - lit when mains connected.
Second letters on the keyboard - è, é, ç, ø are reached by holding the ALT key pressed
before the letter key. Accents on a letter e.g. ô, ñ etc. are reached by pressing <SHIFT>
and then the letter. In addition the following special characters are available (AT-2plus
Memory only !):
Key combination: SHIFT + 1 234567890
Character ! @# $ %&/*"=

Page 1.6
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
LCD Screen
1. Current Heart Rate (averaged over 4 beats and refreshed every 2 seconds). The HR is also
given on a manual printout. Note that with an auto mode printout the HR is averaged over
the full 10 seconds of the recording.
2. Top line - Current Day, Date and Time
Bottom Line - System messages
3. Top Line - Current power source - mains or battery. When battery capacity is limited the
battery symbol flashes.
Bottom line - `ALT` in this box indicates that the ALT key has been pressed.
4. Electrode connections - when a lead flashes it indicates that the electrode resistance is too
high. The electrode must be reapplied
5. Lead indication (leads currently displayed on the screen). Change the lead group with the
keys `1` and `2`.
6. Myogram Filter indication - `Filter` = filter ON; no indication = filter OFF. Switch the
filter on or off with the Filter key.
7. An `A` in this box indicates that automatic sensitivity is selected (auto mode printout
only). Switch automatic sensitivity on or off with key `3`.
8. Sensitivity - 5, 10 or 20 mm/mV. Change the sensitivity with the keys `4`, `5`, and `6`.

Page 1.7
Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Automatic ECG Recording
•Prepare skin, hook up patient.
•Switch unit on, press ON .
•Press and enter patient data.
•Press again and wait for at least 10 seconds until
a clear and stable trace is displayed.
•Press AUTO to record and print.
•Press COPY for additional copies.
AT-2plus Short Form Operating Instructions
Manual ECG Recording (Rhythm Strip)
·Prepare skin, hook up patient.
•Switch unit on, press ON .
•Press MAN START .
•Change lead group with 1and 2.
•Press STOP to stop the printout.
Electrode hook-up check
·Press ALT 0
1mV 3
AUTO 3 AUTO for electrode check.
Best results are obtained when the electrode voltage readings
(right column) are between ±50mV.
Filter On/Off
•Press to switch the (Myogram) filter On / Off.
System Configuration
•Press ALT 0
1mV 1 1 to print system settings.

Page 1.8
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
Modes of Operation
Automatic Mode
Automatic Mode provides a printout giving 10 seconds of ECG recording of all 12 leads with a
choice of 2 different formats.
Lead format and chart speed can be programmed freely for each of the 2 formats before recording.
With the optional interpretation program installed it is also possible to select the measurement
table, average cycles with optional markings and interpretation statements for the printout.
For further information see paragraph `Settings for Automatic Mode`.
Manual Mode
Manual Mode provides a real time printout of 6 leads that are selected and indicated on the
screen.
The following can be freely selected before or during recording:
•Lead Group •Chart Speed
•Sensitivity •Myogram Filter
For further information see paragraph `ECG Recording in Manual Mode` following.

Page 1.9
Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Automatic Mode
Inautomaticmode, a full 12-lead ECG is printed in one of two predefined formats with a sensitivity
of 10 mm/mV. These two formats are selected by the user to suit his specific needs and
requirements.
When the AUTO SENSITIVITY key 3
AUTO is pressed before recording in automatic mode, the
unit detects very large waveform amplitudes and sets the sensitivity for the extremity and/or
precordial leads to 5 mm/mV to reduce the overlapping of traces. An `A` on the bottom line of the
LCD indicates that Auto sensitivity is set.
To start the automatic ECG recording in Format 1, press the AUTO key:
To start the automatic recording in the second format, press the ALT key followed by the
AUTO key:
ALT +
The printout gives the following:
•ECG recording of all leads in either Standard or Cabrera format according to selection
•Sensitivity
•Heart Rate
•Speed
•Filter Settings
•Time and Date
•Interpretation statements
•Average Cycles
•Intervals
•Axis
•Sokolow Index (ECG index for hypertrophy)
•Detailed Measurement Table
To obtain an extra printout of the ECG recording in Format 1, simply press the COPY key:
COPY
To obtain an extra printout of the second format, press the ALT key followed by the COPY key:
ALT - COPY
The Auto mode settings for the two formats are detailed in the paragraph entitled `Settings for
Automatic Mode` later in this book

Page 1.10
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
Manual Mode
Manual mode providesa direct printoutofthereal-timeECGwith full control of parameter selection.
Tostartthemanual recording ofareal-timeECG,presstheMANUALPrintoutkey
Tostopthemanual recording (printout) press the STOP key
The printout provides you with the following:
•Six (selected) leads with lead identification.
•On the lower edge, the chart speed, user identification and filter settings (if on).
•At the top, the heart rate as current average of 4 beats, trace sensitivity, and the time and
date
The following can be freely chosen during or before the recording:
Lead Group by means of the LEAD FORWARD and LEAD BACKWARD
key
12
The following lead groups are selectable:
•I, II, III aVR, aVL, aVF
(Cabrera: aVL, I, -aVR / II, aVF, III)
•V1, V2, V3 /V4, V5, V6
•II, aVF, III /V2, V4, V5
•V4, V5, V6 / V7, V8, V9
Note: The LCD only displays three leads at one time. When the lead forward or lead backward
key is pressed, the following /preceding three lead group is displayed
Leads
For software versions higher than 4.10 it is also
possible to chose further leads. The desired leads
are activated as shown in the table.
Select leads
Entry Key Sequence Lead group Confirm
ALT 7 9
0Rhythm ON
Press STOP
key
II, avF, III, V2, V4, V5
1Rhythm OFF
II, avF, III, V2, V4, V5
2Left posterior ON
V4, V5, V6, V7, V8, V9
3Left posterior OFF
V4, V5, V6, V7, V8, V9
4Right precordial up to V5r ON
V1, V2, V3, V3r, V4r, V5r
5Right precordial up to V5r OFF
V1, V2, V3, V3r, V4r, V5r
6Right precordial up to V6r ON
V1, V2, V3r, V4r, V5r, V6r
7Right precordial up to V6r OFF
V1, V2, V3r, V4r, V5r, V6r
8NEHB ON
D, A, J
9NEHB OFF
D, A, J

Page 1.11
Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Manual Mode (cont.)
ChartSpeed Select speed 5, 10, 25 or 50mm/s by means of the SPEED keys:
7
5/10 8
25 9 50
Notes: Key 7 is a toggle key -press once and 5 is selected, press a second
time and 10mm/s is selected.
When the 25 or 50mm/s key is pressed, the same speed is set on
both the screen and the (manual) printout. When 5 or 10 mm/s is
selected, this affects the manual printout speed only.
Sensitivity Select 5, 10 or 20 mm/mV by means of the SENSITIVITY keys:
4
55 10 6 20
Myogram Filter Switch the filter ON or OFF with the FILTER key:
`FILTER` is displayed on the bottom line of the LCD when the
filter is switched on.
Recentering To re-centre the ECG traces, press the 1mV key 0
1mV
WARNING
AFTER HEAVY ARTEFACTS OR LEAD OFF, THE INDICATION OF THE HEART RATE
MAY NOT BE RELIABLE.

Page 1.12
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
Settings
Each parameter is set by means of a code. This code comprises a combination of keys starting with
the ALT key followed by two or three numbers. The setting is confirmed with the STOP key. As
soon as the ALT key is pressed, the keyboard is dedicated to the programming function.
Note: When the ALT key is pressed `ALT` appears on the LCD (see previous page)
Note: The Alternative (ALT) function is only active for 4 seconds. If a programming key is
not pressed within 4 seconds, the unit reverts to standard mode. The ALT key must
again be pressed to activate the programming mode
The setting is remembered and the keyboard released for other functions when the STOP key is
pressed. Once a setting has been confirmed, it is stored in the memory even when the unit is
switched off.
Example
If you want to reset your AT-2plus to the basic default settings, the key sequence given on page
14 is ALT;0; 6; 6.STOP.
Onthe following pages the programmable parameters and the programming sequences are described
indetail.

Page 1.13
Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Settings (cont.)
The defined formats and settings for your unit can be checked as follows:
ALT - 0 - 1 - 1
A printout of the defined settings will be produced and gives the following information, depending
on the installed software:
Unit designation Software version, Software option installed (C = Interpretation)
and interpretation version
Serial number Serial number of the unit
Leads Standard (S) or Cabrera (C)
ECG Format Long (ooo), Short (o) or Suppressed (-)
MECG Average cycles as defined in auto ECG recording setup (e.g. 4 *
3 (25 mm/s) + 2)
Measurements Enabled (+) or Suppressed (-)
Marks Enabled (+) or Suppressed (-)
Interpretation Enabled (+) or Suppressed (-)
Selected Rhythm leads Leads selected for R1, R2 resp.
Automatic Centering Enabled (+) or Suppressed (-)
Printout of signals Sequential or Simultaneous
Baseline Filter 0.05, 0.15 or 0.30 Hz
Mains Filter 50, 60 Hz or OFF (-)
Myogram Filter 25 or 35 Hz, ON (+) or OFF (-)

Page 1.14
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
Settings (cont.)
Interpretationsettings: N/A:+/- ‘normal/abnormal’ is written (+) or suppressed (-)
U:+/- ‘unconfirmed report’ is written (+) or suppressed (-)
A30:+/- patient age is assumed to be < 30 (-) or >30 (+)
S:+/- low (-) or high (+) sensitivity
For AT-2plus Memory only:
AutomaticSave Enabled (+) or Suppressed (-)
AutomaticErase Enabled (+) or Suppressed (-)
Baud Data transmission speed (9600 - 115200 bit/s, see page 1.25)
Com-Type Communication mode, Line or Modem (see page 1.25)
Default Settings
To reset the unit to the basic default settings, proceed as follows:
ALT - 0 - 6 - 6
SETTINGS STANDARD WITH INTERPRETATION
LANGUAGE AS SET AS SET
LEADS STANDARD (S) STANDARD (S)
AUTO FORMAT 1 ECG: 25MM/S, SHORT (O)
ECG : 25MM/S, SHORT (O)
MECG: 2*6 (50MM/S + 1)
MEASUREMENTS:
SUPRESSED (-)
INTERPRETATION:
ENABLED (+)
MARKS: ENABLED (+)
AUTO FORMAT 2 ECG: 25MM/S, LONG (OOO)
ECG : 25MM/S, LONG
(OOO)
MECG: NONE
MEASUREMENTS:
SUPRESSED (-)
INTERPRETATION:
DISABLED (-)
MARKS: ENABLED (+)
RHYTHM LEADS V1 V1, II
AUTOM.
CENTERING ENABLED (+) ENABLED (+)
PRINTOUT OF
SIGNALS SEQUENTIAL SEQUENTIAL
BASELINE FILTER
SETTING 0.05HZ 0.05HZ
MAINS FILTER
SETTINGS 50HZ (60HZ) 50HZ (60HZ)
MYOGRAM
FILTER SETTING 35HZ, OFF 35HZ, OFF
MEMORY AND
SERIAL
COMMUNICATIO-
N INTERFACE
OPTION
BAUD RATE 115200 BPS BAUD RATE 115200 BPS
AUTO STORAGE ON (+)
AUTO DELETETION OFF (-) AUTO STORAGE ON (+) AUTO DELETETION OFF (-)
TRANS. MODE: LINE TRANS. MODE: LINE
MEMORY AUTO SAVE ENABLED (+)
AUTO ERASE DISABLED (-) AUTO SAVE ENABLED (+) AUTO ERASE DISABLED (-)
INTERPRETATION
SETTINGS
N/A: SUPRESSED (-)
U: ENABLED (+)
A30: UNDER THIRTY (-)
S: LOW (-)

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Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Settings (cont.)
Language
The language can only be set with the SWUP program.
UserIdentification
The user identification is printed on all recordings. The user ID can be the department, doctor or
hospital etc. Enter the user ID as follows:
Press the ALT key followed by key 9, 3, 3
ALT - 9 - 3 - 3
The user entry field is displayed on the LCD. Enter up to 30 characters via the keyboard.
Confirm the new user ID by pressing the ENTER key.

Page 1.16
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Chapter 1
Operating Elements
Settings (cont.)
Filters
There are three different filters which can be set individually as follows:
•Baselinefilter
•Mains filter
•Myogram filter
BaselineFilter
The digital Baseline filter suppresses excessive baseline drifts. The setting options are as follows:
Baseline Filter
Entry Key
Sequence Filter Setting Confirm
ALT 5
00.05 Hz
(default) Press
STOP
key
10.15 Hz
30.30 Hz
Confirm the selection by pressing
STOP
Note: The set value is the lower limit of the frequency range and is normally set to 0.05 Hz.
The settings 0.15 and 0.30 Hz should only be used when absolutely necessary, as the
possibility exists that they could affect the original ECG signal, especially the ST
segments.
Mains Filter
The Mains filter is an adaptive digital interference filter designed to suppress AC interference
without attenuating or distorting the ECG.
Set the mains filter in accordance with the frequency of your local mains supply as follows:
Mains Filter
Entry Key
Sequence Filter Setting Confirm
ALT 8
5Mains Filter 50 Hz Press
STOP
key
6Mains Filter 60 Hz
9Mains Filter Off

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Chapter 1
Operating Elements
SCHILLER AT-2plus 6-Channel ECG Unit
SERVICE HANDBOOK Issue d May 2002
Settings (cont.)
MyogramFilter
The Myogram filter suppresses disturbances caused by strong muscle tremor. The set value will
be the new upper limit of the frequency range as soon as the FILTER key is pressed on or
programmed as default when the unit is switched on. When the Myogram filter is on `Filter` is
displayed on the bottom line of the LCD.
Myogram Filter
Entry Key
Sequence Setting Confirm
ALT 8
2Myogram Filter 25 Hz
Press
STOP
key
3Myogram Filter 35 Hz
1
Myogram Filter active
when the unit is first
switched on (marked
on printout with +)
8
Myogram Filter off
when the unit is first
switched on (marked
on printout with -)
Confirm the selection by pressing the STOP key
STOP
The myogram filter is switched on and off manually with the FILTERKEY
Note: An ECG recorded in auto mode is stored unfiltered. It is therefore possible to print the
stored ECG either with or without passing the myogram filter. Filter ON is indicated
in the bottom information line of the LCD. When the FILTER key is pressed again, the
filter is switched off and the `Filter` indication on the bottom information line of the
LCD is removed. The cutoff frequency of the myogram filter is set to either 25 or 35
Hz.